DECEMBER1974 HEWLETT-PACKARDJOURN 1 I © Copr. 1949-1998 Hewlett-Packard Co. Improved Accuracy and Convenience in Oscilloscope Timing and Voltage Measurements Timing measurements are made more easily and accurately with the dual-delayed sweep of a new oscilloscope. An internal microprocessor gives direct readout of time or voltage, greatly simplifying measurement procedures. by Walter A. Fischer and William B. Risley MANY ELECTRONIC ENGINEERS would agree that the oscilloscope is the most useful of test instruments. They do not customarily expect a high degree of precision in an oscilloscope, however, and accept the 3 to 5% accuracy that most oscilloscopes provide. The exception has been timing measurements. Engineers concerned with measurement of very short time intervals such as rise times, propagation delay, clock phasing and other high-speed digital events depend on the oscilloscope for their timing measurements. To get accuracy in these measure ments they have had to order instruments with spe cial CRTs and specially linearized sweeps. But even with the best of conventional oscilloscopes, a major source of errors still remains in the measurement technique. The engineer either has to count graticule lines from one point on a waveform to another or, for better accuracy, he has to position the starting point at center screen with the sweep delay control, write down the control setting, position the stopping point at center screen, write down the new control setting, take the difference between the two readings, and multiply the result by the main time base setting to get the answer. Although 1% accuracy can be ob tained this way, the procedure obviously has the po tential for many errors. To eliminate this bother and at the same time to im prove accuracy, a new technique has been developed for the new HP 1722A Oscilloscope (Fig. 1). This os cilloscope displays two intensified markers on the waveform (Fig. 2). The operator positions the first marker at the point where the time interval measure ment is to start and the second marker at the stopping point. A LED digital readout, automatically scaled to the time base setting, then displays the time lapse be tween the markers directly. The technique is fast and accurate, and it considerably reduces the chance for human error â€” there is no need to count graticule lines or calculate results from readings. Voltage Readings Too Better timing measurements are only one of the new capabilities of this instrument. It also makes fre quency measurements and does so quickly by auto matically converting a period measurement, made with the use of the markers, to frequency (f = 1/t). In addition, it makes measurements on the CRT vertical Cover: The LED numeric display on this oscilloscope is an essential part of a new way of measuring very short time intervals, such as the propagation delay of a flipflop breadboarded here on the HP logic lab. A descrip tion of the oscilloscope and the new technique begins on this page; the logic lab was described last month. In this Issue: Improved Accuracy and Convenience in Oscilloscope Timing and Voltage Measurements, by Walter A. Fischer and William B. Fiisley . .... page 2 Laboratory Notebookâ€” An Active Loop-Holding Device page 11 A Supersystem for BASIC Time sharing, by Nealon Mack and Leonard page 12 Deriving and Reporting Chromatograph Data with a MicroprocessorControlled Integrator, by Andrew S t e f a n s k i . p a g e 1 8 Adapting a Calculator Microprocessor to Instrumentation, by Hal Barraclough, page 22. Â£ Hewlett-Packard Company 1974 Printed m U S A © Copr. 1949-1998 Hewlett-Packard Co. V -JbÂ¿ A t 'Â«*â€¢.â€¢;Â«â€¢'!"?>*. 'LÃ;Â¡vlÃ¯iÃ¼ / axis, presenting a digital reading of the average dc val ue of the displayed waveform or the voltage differ ence between any two selected points on the wave form, such as the overshoot on a pulse. It can also de rive the percentage of a part of a waveform with re spect to the whole, as in measuring modulation on a carrier. Several developments combined to achieve these capabilities. The first development is the basic oscil- Fig. 1. Model 1722 A Oscil loscope is a high-performance dual-channel instrument with 1.3 ns rise time, 50ilor 1 Mil/1 1pF input impedance, sweep times to 1 ns/div, and the dual-delayed sweep that provides higher ac curacy, resolution, and conve nience in time-interval measure ments. Its LED display gives direct readout of time intervals, frequen cy (t TIME/, and voltage. loscope, which is the same as the laboratory-grade 275-MHz Model 1720A Oscilloscope, an advance in cost-effectiveness described in the September issue of the Hewlett-Packard Journal1. The second develop ment is a proprietary technique known as "dual-de layed sweep", which gives the capability for more accurate determination of time intervals. The third is the microprocessor used in the HP hand-heW calcula tors2, which is built into this instrument to derive answers from the information the instrument provides. Dual-Delayed Sweep Fig. 2. Two markers are positioned to indicate the start and stop regions of a time-interval measurement and the digital readout shows the time interval between the markers. The example here shows the pulse width to be 18.80 Â¿is The basics of the dual-delayed sweep are shown in Fig. 3. The delayed sweep circuit itself is conven tional but it can be started by either of two compara tors. These are enabled alternately such that the de layed sweep starts on one main sweep when the main sweep ramp reaches the El level, and on the next sweep when the ramp reaches a level equal to E, + EAt. To make measurements using the dual-delayed sweep, the oscilloscope is operated in the MAIN INTEN SIFIED mode in which the main sweep drives the hori zontal deflection system and the delayed sweep merely intensifies the trace. The operator sets the delayed sweep to intensify short segments of the main sweep. Ej and EAt are adjusted to place the two intensified segments on the points of interest, as shown in Fig. 4a. Ej is set by the DELAY control and EA, by the © Copr. 1949-1998 Hewlett-Packard Co. Comparators Fig. 3. The dual delayed sweep uses two comparators that are enabled alternately. Comparator A enables the mam sweep ramp to trigger the delayed sweep when the ramp reaches the E, level. On the next main sweep, com parator B enables the main sweep to trigger the delayed sweep at a later time when it reaches the E, DEC-INC switches (Fig. 5) which, when held to one side or the other, cause EAt to decrease or increase, moving the right-hand marker to the left or right along the waveform. The microprocessor reads the value of EAt, converts it to the equivalent time interval scaled according to the time base setting, and dis plays the result. Once the segments are positioned, higher accur acy can be obtained by switching the oscilloscope to the DELAYED SWEEP mode, which expands the intensi fied segments to full screen width, displaying the two segments overlapped as shown in Fig. 4b. The operator can then adjust EAt to superimpose the two waveform segments exactly, as shown in Fig. 4c. The digital readout displays the time interval between the two segments with 4-digit resolution which, on the 20 ns/div sweep time range, can give 20-ps resolution. Accuracy The accuracy achievable by the Model 1722 A Os cilloscope in time interval measurements is speci fied conservatively as Â±0.5% of reading Â±0.05% of full scale (full scale is 10 CRT divisions) on main time-base settings between 100 ns/div and 20 ms/div. When the time interval is equivalent to less than one CRT division, however, the microprocessor automati cally downranges, giving 10x greater resolution in the reading. Accuracy then improves to Â±0.5% of reading Â±0.02% of full scale (10 divisions). It is in the measurement of very short time intervals that the Model 1722A makes its greatest contribution to mea surement accuracy. Comparisons of the accuracy of the Model 1722 A with that of a high-quality conventional oscilloscope are shown in Fig. 6. Whereas the percent error is about the same as a conventional high-quality scope for time intervals approaching the full display width of the CRT, the Model 1722A is superior for very short time intervals. Measurement accuracy is enhanced by the fact that the start and stop waveform segments are displayed simultaneously. With the segments overlapped as in Fig. 4c, it would immediately become apparent if Fig. the Oscilloscope. for making a time-interval measurement with the Model 1 722A Oscilloscope. With the to set to operate in the VAIN INTENSIFIED mode, the DELAY control is used to posi tion the DEC-INC brightened segment of the trace to cover the starting point and then the DEC-INC switches are used to place the second segment over the stopping point, as in "a" (main sweep time = 0.5 the and delayed sweep time = 20 ns/div). The instrument is then switched to the delayed digital mode (b) and the DEC-INC switches used to superimpose the traces (c). The digital readout in gives the time interval between the pulse leading edges with 4-digit resolution, in this case 1.65 Â¿Â¿s © Copr. 1949-1998 Hewlett-Packard Co. EXT TRIG 1 MO 1 - TIME INTERVALD E L A Y D E C K I N O COARSE DLY'D TRIG'G CHANI A Q | (REF SET> POSN INPUT Conventional Oscilloscope â€¢fi I N T E R V A L !4i /TIME r IN E 1 0 2 0 5 0 1 0 0 200 (a) Time Interval (ns) (.AI : VOLTS DC UNCAL % V')LTS ; E C / S/ SEI C TRIG LEVEL Conventional Oscilloscope E 9 . 5 D - B DELAYED TIME/DIV o! ' 0 . 1 1 (b) Time Interval in Div Fig. 5. The microprocessor is activated by pressing one of the function buttons <POSN. INPUT, TIME. ITMEÂ¡. li any oi the oscilloscope controls are not set appropriately for the measurement selected, the digital readout displays ".0". sweep triggering had been affected by drift in the sig nal. And, because the operator makes his measure ment by superimposing the waveforms rather than by noting where the waveform crosses graticule lines, the CRT serves simply as a null indicator so non-linearities and drift in the vertical and horizon tal amplifiers do not affect measurement accuracy. Accuracy is determined primarily by the sweep ramp generator, which is accurate within 0.02%. The delay potentiometer, which largely determines the accuracy of measurements made by the conventional differential delayed time base technique, does not enter into the measurement. The accuracy with which EAt is derived is better than 0.005% of full scale, so EAt does not introduce significant errors into the measurement. Other Uses The dual-delayed sweep gives added measure ment flexibility to the oscilloscope by making it pos sible to view two separate expanded portions of a dis play simultaneously. In conjunction with the micro processor, it can also be used as an indicator for ad justing, say, a clock repetition rate to an exact value. In this case, EAt is adjusted to cause the digital readout to display the desired frequency in the i/TIME Fig. 6. Specified measurement accuracy of the Model 1 722 A Oscilloscope compared to a conventional high-quality oscil loscope using the differential delay technique. Plot "a" is for absolute values of time and plot "b11 is in terms of horizontal deflection (in a range of 100 ns/div to 20 ms/div). The upward discontinuity at 5 ns is where the accuracy specification changes for sweep times shorter than 100 ns/div. The dis continuity shown at 1 CRT division is where the micro processor down-ranges to give ÃOx better resolution. mode, which is interpreted internally as the desired clock period. The clock repetition rate is then ad justed to cause the two waveform segments to be superimposed. The dual-delayed sweep can also be used for mea surements between points on two waveforms, such as measurements of propagation delay. When the in strument is displaying two waveforms in the ALTER NATE SWEEP mode, the delayed sweep is started by Ej when channel A is displayed and by E, + EAt when channel B is displayed, giving the time interval be tween the points selected on the two waveforms (Fig. 7). The phase delay of a two-phase clock can be ad justed, for example, by displaying the master clock on one channel and the delayed clock on the other. EAt is adjusted to cause the readout to display the exact value of phase delay desired. The clock phase delay is then adjusted to align the waveforms. Measurements on the Vertical Axis When the button labeled INPUT DC VOLTS (Fig. 5) is pressed, the digital readout displays the average © Copr. 1949-1998 Hewlett-Packard Co. Fig. 7. In the ALTERNATE sweep mode, the oscilloscope measures the time interval between points on two waveforms. Alignment ot the two points on the same vertical graticule, in this case the 50% amplitude points as in the photo at right, gives a precise measure ment of time interval. (Main sweep: 0.1 fis/div; delayed sweep: 10ns/ div: time interval: 45.3 ns). value of the input to channel A. The instrument then functions as a 3V2-digit voltmeter with full-scale ranges from 100 mV to 50V. If a 10:1 divider probe is used, a front-panel switch compensates the reading, giving full-scale readings from IV to 500V. Pressing the REF SET button stores a reading as a reference. The display will then show the difference between the reference and a new voltage at the chan nel A input. Normally, the REF SET button is pressed while the input is grounded so subsequent readings give absolute values. Since another voltage may be used as the reference, differential readings are easily made. The accuracy of dc voltage measurement, speci fied conservatively as Â±0.5% of reading Â±0.5% of full scale (full scale corresponds to 10 divisions even though only 6 divisions are displayed), is dia grammed in Fig. 8. As in the case of measurements on the horizontal axis, the operator does not have to count graticule lines nor multiply by range factors. Accuracy is enhanced by the fact that unlike volt- 1 2 3 4 5 6 7 meters with decade ranges, the vertical deflection fac tor ranges are in a 1 , 2 , 5 , 1 0 sequence, which makes it possible to measure most voltages near full scale. Pomt-to-Point Voltage Measurements When the POSN (position) button is pressed, the DVM circuits read the level of the position control voltage. This makes it possible to measure the instan taneous voltage of any part of a waveform through dc substitution. To do this, a reference point on the waveform is selected and brought to a convenient horizontal graticule line (Fig. 9). The REF SET level is pressed to establish this graticule line as the zero lev el, then the position control is used to bring the point to be measured to the same line. The digital readout then displays the voltage level between this point and the reference. Since the reference can be set to any level, the tech nique can be used to measure point-to-point voltages on any part of a waveform. Here again, the CRT serves simply as a null indicator with the reference and measurement point both positioned to the same graticule line, so vertical channel non-linearities, a common source of oscilloscope measurement errors, do not enter into these measurements. 9 10 Amplitude (div deflection) Fig. 8. Curves show specified accuracy of the Model 1 Oscilloscope in voltage measurements (a and b) as com pared to a conventional oscilloscope (c). Curve "a" is for dc voltage measurements. Curve "b" is for point-to-point measurements. © Copr. 1949-1998 Hewlett-Packard Co. Fig. 9. Double-exposure photo shows how point-to-point voltage measurements are made. The reference point is first cen to a horizontal graticule line, in this case the cen ter line (upper trace), with the vertical position control. The REF SET button is pressed, and the other point is brought to the same line (lower trace). The digital readout displays the voltage difference between the two points . Microprocessor Output Interface Buffer Storage and OAC Dâ‚¬L*Y (E,) E, - EÂ¿, (To delayed sweep comparators) Fig. 10. Microprocessor related circuits. Measuring Percent Measurements of a voltage level as a percent of a waveform are made by switching the channel A atten uator vernier out of the GAL position. The vernier is then used to establish a five-division separation be tween the desired zero and 100% points of the wave form on the CRT graticule. Next, the 7ern percent level is positioned to a reference horizontal graticule line, and the REF SET button is pressed. Positioning any other part of the waveform to the reference line then gives a reading of that waveform level in percent. Besides quickly measuring such quantities as the percent overshoot on a pulse, this technique is also useful for defining percentage levels. For example, it can show exactly where the 50% level is on a pulse for consistent measurements of pulse width, or it can define the 10% and 90% pulse levels for risetime measurements. Enter the Microprocessor There are a number of ways that logic may be im plemented to perform these various functions. The use of a microprocessor, however, turned out to be the most efficient way in terms of hardware and costs. It also provided a convenient means for broad ening the capabilities of the instrument, such as en abling the i TIME calculation. The microprocessor developed for the HP hand held calculators was an appropriate choice for this in strument, primarily because it already had the means for driving the digital readout. The decimal addersubtractor lends itself easily to the scaling problem, and the internal flags of the calculator permit separat ing and controlling the programs. The microproces sor consists of the calculator's arithmetic-and-register and control-and-timing MOS/LSI circuits2 working with two ROMs designed expressly for the programs used in this instrument. The two ROMs contain a to tal of 512 words. A block diagram of the circuits related to the micro processor is shown in Fig. 10. The initial problem was to interface the calculator circuits to the oscil loscope controls and to the digital-to-analog conver ter that derives EAt. The front-panel controls serve as the calculator "keyboard" with the controls encoded and multiplexed to appear as keystrokes. As in the hand-held calculators, the microprocessor con tinuously scans the control settings to see what task is called for (TIME, I/TIME, DC VOLTS, POSN, %) and what range factors should enter into the calculations. The input interface encodes the appropriate front-panel control settings and these are presented to the micro processor as particular memory addresses. Programs stored at these addresses perform the indicated func tions (compute time, increment, decrement, etc.) The output interface converts the serial data to par allel data for the digital-to-analog converter (DAC), and retains it temporarily in buffer storage (the micro processor uses words consisting of 14 BCD digits pre sented serially on the data bus). During a time- interval measurement, EAt (Fig. 3) is stored as a digital number in the microprocessor. The DEC-INC switches cause this number to be incre mented or decremented, the size of the increment or decrement being determined by which of the three switches is activated. The digital number is con verted to the equivalent dc voltage by the DAC. The scaled value of EAt is presented in units of seconds on the display in scientific notation (A x 10B) where 10 is implied and only the exponent is given. For example, 3.514 fj.s is displayed as 3.514 -6. However, to simplify interpretation, only the values 9, 6, 3, and 0 are used for the exponent. With this ar rangement, 128.6 ms would not be displayed as 1.286 -1, as it would be in pure scientific notation, but as 128.6 -3, which is easily interpreted as milli seconds. The same scheme is used for the display of © Copr. 1949-1998 Hewlett-Packard Co. Fig. 11. Flow diagram of a voltage measurement. frequency and also for voltage, where the exponent â€” 3 denotes millivolts. A block diagram of the circuits involved in a timeinterval measurement was shown earlier in Fig. 3. Not shown were the interfaces and the entries from front-panel controls other than the time-base setting. These other entries blank the digital readout if the control settings are not appropriate for the measure ment. This prevents the display of such ambiguous information as would occur, for example, if the sweep vernier were out of the GAL position, or if MIXED SWEEP had been selected, or if the delayed sweep TRIG LEVEL control were not in the STARTS AFTER DELAY position. Voltage Measurements Voltage measurements are made by comparing the input voltage Vin to a voltage derived by the micro processor. The result of the comparison is reported back to the calculator, closing the loop. The derived voltage is stored as a digital number in the microprocessor and converted to a dc voltage in the DAC. The analog amplifier assembly provides two pieces of information: (1) the polarity of the in put voltage; and (2) whether the derived voltage is greater or less than the input. In response to this in formation, the derived voltage is incremented or decremented until it is within one least significant bit of the input. The value previously stored as the zero reference is then subtracted from this value and the result displayed. To simplify the program and reduce the number of processing steps, the derived voltage is obtained by a successive approximation procedure. As shown by the logic flow diagram of Fig. 11, at the start of a mea surement the most significant digit is set to 5. If the comparison shows this to be greater than the input, the digit is decremented to 4 and the comparison re peated. This process continues until the comparison shows the most significant digit to be less than the un known. This digit is retained and now the next most significant digit is set to 5 and comparisons made un til the correct value for this digit is found. The pro cess repeats for each digit until finally the derived voltage is within one least significant bit of the input. At most, only 20 iterations are required. The instru ment makes about two readings per second. If in the initial comparison the result shows the most significant digit to be less than the unknown, it is incremented upwards until it exceeds the un known. It is then decremented one count before the comparison switches to the next most significant digit. In a percent measurement, the microprocessor is instructed to scale the measurement as 20V/div re gardless of the attenuator setting. Thus, a voltage equal to 5-cm vertical deflection is displayed as 100.0. A 5-cm deflection is thus equivalent to 100.0% and all other voltage levels are displayed as a percent of the 5-cm level. High-Resolution DAC The digital-to-analog converter obviously is a key element in this system. Since measurement accuracy depends upon its output, it needs superior resolu tion and stability, but not necessarily fast response. Available DACs that have the requisite resolution and stability are quite fast, and also very expensive. An alternate solution therefore was sought. The DAC that evolved from this search is built around a "rate multiplier", a device that outputs pulses in proportion to the BCD number at its input.3 For example, if the number were 6, a rate multiplier would output 6 pulses for every 10 input clock pulses. © Copr. 1949-1998 Hewlett-Packard Co. Clock Generator (140 kHz) Buffer Storage Rate Multipliers rate multiplier, which outputs 30 pulses. In the same way, the rate multiplier for the least significant digit outputs 2 pulses for every 10,000 clock pulses. The pulses are interleaved such that 6432 discrete pulses are supplied to the integrator, which outputs a dc voltage proportional to the number of pulses. Resolution is 1 part in 10,000. Careful attention was paid to the design variables that affect stability. For example, it was found that an increase in ambient temperature slowed the pulse transition times while at the same time slightly in creasing pulse height. The clock repetition rate was selected so these effects compensate each other, maintaining the area under each pulse constant. Overall stability of the DAC is 0.005%/Â°C, eliminat ing it as a significant source of errors. Total cost of the components, on the other hand, is of a very low order (<$15). Acknowledgments The dual-delayed sweep concept was developed by William Mordan. Product design was by George Blinn, who rearranged the Model 1720A front panel neatly to incorporate the added capabilities of the Model 1722A. and industrial design was by Bill Fischer. The authors also wish to acknowledge the contributions of all those who developed the Model 1720A Oscilloscope1 used as the basis for the Model 1722A, including CRT designers Henry Ragsdale and Ronald Larson and hybrid circuit production ex pert Jay Cederleaf, and the many people in engineer ing, marketing, manufacturing, and quality assur ance who contributed valuable suggestions on what form the 1722 A should take. 2 Transfer References E.J, or Vin (derived) Fig. 12. Digital-to-analog converter achieves high resolution and stability with inexpensive components. 1. P.K. Hardage, S.R. Kushnir, and T.J. Zamborelli, "Opti mizing the Design of a High-Performance Oscilloscope", Hewlett-Packard Journal, September 1974. 2. T.M. Whitney, F. RodÃ©, and C.C. Tung, "The 'Powerful Pocketful': an Electronic Calculator Challenges the Slide Rule", Hewlett-Packard Journal, June 1972. 3. See for example "Operation of the Digital Programma ble Frequency Generator", Hewlett-Packard Journal, No vember 1973, p. 14. A block diagram is shown in Fig. 12. This includes the storage buffers that store the parallel data derived from the microprocessor's serial data (in BCD). If the number stored in the buffer were 6432, then, for every 10,000 clock pulses, the rate multiplier for the most significant digit would output 6000 pulses. This multiplier also gates the multiplier for the next most significant digit so this multiplier accepts clock pulses for 1/10 the time, thus outputting 400 pulses for every 10,000 clock pulses. It in turn gates the next © Copr. 1949-1998 Hewlett-Packard Co. TRIGGERING INTERNAL: dc to 100 MHz on signals causing 0.5 division or more vertical de flection, increasing to 1 division of vertical deflection at 300 MHz in all display modes. Triggering on line frequency is also selectable. EXTERNAL: dcto 1 00 MHz on signals of 50 mV p-p or more increasing to 100 mV p-p at 300 MHz. EXTERNAL INPUT RC: approx 1 megohm shunted by approx 15 pF. TRIGGER LEVEL AND SLOPE INTERNAL: at any point on vertical waveform displayed. EXTERNAL: continuously variable from + 1 .0V to 1 .0V on either slope of trigger signal, -10V to -10V in divide by 10 mode(^10). COUPLING: AC. DC, LF REJ. or HF REJ. AC: attenuates signals below approx 10 Hz. LF REJ: attenuates signals below approx 15 kHz. HF REJ: attenuates signals above approx 15 kHz. TRIGGER HOLDOFF: time between sweeps continuously variable, exceeding one full sweep from 10 ns/div to 50 ms/div. MAIN expanded Intensifies that part of main time base to be expanded to full screen adjust delayed time base mode. Delay and time interval controls adjust position of intensified portions of sweep. ABRIDGED SPECIFICATIONS Model 1722A Oscilloscope Complete specifications available on request Vertical Display Modes Channel A: channel B: channels A and B displayed alternately on successive sweeps chan or by switching between channels at 1 MHz rate (CHOP): chan nel A plus channel B (algebraic addition). Vertical Amplifiers (2) BANDWIDTH: DC-COUPLED: dcto 275 MHz in both 50 ohm and high impedance input modes. AC-COUPLED: approx 10 Hz to 275 MHz. RISE TIME: -,1.3 ns BANDWIDTH LIMIT: limits upper bandwidth to approx 20 MHz. DEFLECTION FACTOR: RANGES: 10 mV/div to 5 V/div in 1,2,5 sequence. VERNIER: continuously variable between all ranges INPUT RC (selectable) AC AND DC: 1 megohm shunted by approx 1 1 pp. 50 OHM: 50 ohms Â±2%. MAXIMUM INPUT: AC AND DC: Â±250V (dc + peak ac) 50 OHM: 5V rms or Â±250V peak whichever is less. A - B OPERATION Bandwidth and deflection factors are unchanged. Channel B maybe inverted for A-B operation. TRIGGER SOURCE: Selectable from channel A, channel B. or Composite. Delayed Time Base SWEEP RANGES: 10 ns/div to 20 ms/div (20 ranges) in 1.2.5 sequence MAGNIFIER: (0 to 55Â°C): same as main time base. TRIGGERING INTERNAL: same as main time base except there is no Line Frequency triggering. STARTS AFTER DELAY: delayed sweep automatically starts at end of delay period. TRIGGER: with delayed trigger level control out of detent (Starts After Delay) delayed sweep is triggerable at end of delay period. TIME two MEASUREMENTS: measures time interval between two events on channel A (channel A display); between two events on channel B (channel B display); or between two events starting from an event on channel A and ending with an event on channel B (Alternate display). Input - DC Volts (Channel A) DISPLAY: light emitting diodes (LED). NUMBER OF DIGITS: 3Vi. â€¢ 1 RANGE: 100 mV to 50 V full scale vertical deflection (10 mV/div to 5 V/div). â€¢ 10 RANGE: 1V to 500V full scale vertical deflection (100 mV/div to 50 V/div with '10 probe). ACCURACY: Â±0.5% reading Â±0.5% full scale (full scale = 10 cm), 20Â° C to 30 C. STABILITY: temperature coefficient, <Â±0.02%/Â°C. SAMPLE RATE: approx 21s. RESPONSE TIME: 1 s REFERENCE SET: voltmeter circuits may be zeroed permitting dc voltage measurements with respect to any voltage within selected range. OVERRANGE: flashing display indicates overrange condition. ACCURACY M a i n T i m e A c c u r a c y Base Setting (-20Â°C to Â±30Â°C) 100 ns/div to 20 Â±0.5% of measurement Â±0.02% of full scale for ms/div measurements <1 cm. For measurements >1 cm, Â±0.5% of measurement Â±0.05% of full scale. 50 ns/div Â±0.5% of measurement Â±0.6% of full scale. 20 ns/div and 50 Â±0.5% of measurement Â±0.15% of full scale, ms/div to 0.5 s/div Position - Volts (Channel A) "Starting after 3 cm of sweep. (Channel A vernier in CAL detent.) With the following exceptions, specifications are the same as Input - DC Volts. MEASUREMENT: dc substitution method using channel A position control to determine voltage of any point on displayed waveform using any graticule line as reference. DYNAMIC RANGE: Â±6 cm from ground referenced to center screen. REFERENCE SET: meter may be zeroed; permits instantaneous voltage measure ments with respect to any voltage within selected range. ACCURACY: Â±1%reading Â±0.5%offull scale(10 ' the volts/div range) measured at dc. RESOLUTION: intervals â€¢ 1 cm, -0.01% of full scale; intervals >1 cm, >0.1%of full scale; maximum display resolution, 20 ps. STABILITY: (0Â°C to +55"C): short term, <0.01%. Temperature, Â±0.03%/Â°C deviation from calibration temperature range. 1/TIME interval. calculates and displays reciprocal of measured time interval. ACCURACY: same as Time Interval Measurements. Mixed Time Base Dual sweep base in which main time base drives first portion of sweep and delayed time base completes sweep at the faster rate. Position - % (Channel A) (Channel A vernier out of CAL detent.) MEASUREMENT: dc substitution method using channel A position control to determine percent of any waveform point with respect to user defined 0 and 1 00% points. RANGE: 0 to Â±140% (calibrated with vernier so that 100% equals 5 div). ACCURACY: Â±1% REFERENCE SET: voltmeter circuits may be zeroed to permit percent measurements with respect to any waveform point. Cathode Ray Tube and Controls TYPE: post accelerator, approx 20.5 kV accelerating potential, aluminized P31 phosphor GRATICULE: 6 * 10 div internal graticule, 0.2 subdivision markings on major horizontal and vertical axes. 1 div = 1 cm. Internal flood gun graticule illumination. INTENSITY MODULATION: -8V ^50 ns width pulse blanks trace of any inten sity, useable to 20 MHz for normal intensity. Input R. 1 kÃ¼ Â± 1 0%, Maximum input - 10V (dc - peakac). Horizontal Display Modes General SWEEP MODES: main, main intensified, mixed, delayed, and x 10. SWEEP RANGES: 10 ns/div to 0.5 s'div (24 ranges) in 1,2,5 sequence MAGNIFIER: expands all sweeps by a factor of 10, extends fastest sweep to 1 nsdiv. REAR PANEL OUTPUTS: main and delayed gates, vertical output. CALIBRATOR TYPE: 1 kHz ~ 10% square wave. VOLTAGE: 3V p-p Â± 1%. RISE TIME: <0.1 us. POWER: 100. 120, 220, 240, -10% -5%; 48 to 440 Hz; 1 10 VA max. WEIGHT: 29 Ib (13.2 kg). DIMENSIONS: 13-3 16 W - 7-3/4 H ' 20 in. D. (335 < 197 * 508 mm). PRICE IN U.S.A.: S4500 MANUFACTURING DIVISION: COLORADO SPRINGS DIVISION 1900 Garden of the Gods Road Colorado Springs. Colorado 80907 SWEEP TRIGGER MODE NORMAL: sweep is triggered by internal or external signal. AUTOMATIC: bright baseline displayed in absence of input signal. Triggering is same as normal above 40 Hz. SINGLE: in Normal mode, sweep occurs once with same triggering as normal. reset pushbutton arms sweep and lights indicator: in Auto mode, sweep occurs once each time Reset pushbutton is pressed 1C © Copr. 1949-1998 Hewlett-Packard Co. William B. Risley A native of Trinidad, Colorado, Bill Risley earned an AB in Physics at Princeton University in 1 968. He then went to work at the Army's Fort Monmouth laboratories as a physicist specializing in electronics but left two years later to do graduate work at Colorado State University. On getting his MSEE in 1972, he joined HP's Colorado Springs Division. Spare-time activities include fishing and gardening. Bill and his wife have a one-year-old son. Walter A. Fischer Walt Fischer joined the Boonton Radio Corp., then an affiliate of Hewlett-Packard, in 1961 and worked on the 202J FM/AM Signal Generator. He left the next year to fulfill his military obligations but returned in 1964 and contributed to the designs of the 3211 A Sweep Oscillator and the 3205A Telemetry Signal Generator. In 1968 he accepted a position as lab manager for an oscilloscope manufacturer but rejoined HP in 1972, this time at the Colorado Springs Division where he is now a group leader. Walt earned a BSEE at the Newark Pniiogp nf Epg!ncerinn in 1961 and an MSEE at the same institution in 1968. Free time activities include horseback riding and skiing with his wife and two children, ages 13 and 11. Laboratory Notebook An Active Loop-Holding Device For operation on switched telephone circuits, equipment that terminates a line-pair must provide a dc path for the holding current. The usuaJ holding device is an inductor, but for a wideband precision measuring instrument, a prohibi tively large value of inductance would be required. For use on switched networks, an option for the Model 3770A Amplitude/Delay Distortion Analyzer provides for the dc holding current without the use of any inductors (the instrument normally presents an approximate 600Ã1 resis tive impedance to inputs and outputs). A diagram of the loop-holding device is shown in the draw ing. Hi and Cl form a low-pass filter such that only the dc component of the signal can turn on the Darlington pair. When turned on, the Darlington pair can sink up to 100 mA dc. To the ac component, the Darlington pair in conjunction with impedance Z appears as a current source (Z may be a low-value resistance or. for higher impedance, another ac tive current sink). Thus, ac currents "see" a high impedance (50 kilj. HI, fl2, and Z were chosen to ensure that the transistors do not begin conduction until the dc voltage across the device is sufficient to allow linear transistor operation with the largest ac signal voltage expected. Thus, when there is no holding current, the transistors are turned off and the device may be left connected without causing signal distortion. Two cir cuits connected in parallel opposition enable currents of either polarity to be accommodated. â€”David H. Guest Hewlett-Packard Limited 11 © Copr. 1949-1998 Hewlett-Packard Co. A Supersystem for BASIC Timesharing This HP 3000 Computer System is optimized for BASIClanguage timesharing, but it also supports concurrent batch processing in BASIC, FORTRAN, COBOL, and SPL. by Nealon Mack and Leonard E. Shar THE HP 3000 COMPUTER SYSTEM is a low-cost general-purpose computer system capable of concurrent batch processing and on-line terminal processing. The system can be accessed by many users simultaneously using any of several program ming languages and applications library programs. Operation is under the control of the Multiprogram ming Executive (MPE/3000).1 To meet the needs of users who want a computer system primarily for BASIC-language timesharing, MPE has now been modified to emphasize the inter active capabilities of the system. The result, called Multiprogramming Executive for Timesharing (MPET), provides the BASIC/3000 timesharing user with the fastest possible response, yet retains the ability to support concurrent multilingual batch processing. In its most modest form, MPET supports 16 BASIC users and batch in the background (Fig. 1). Programs written in BASIC, FORTRAN, COBOL, or SPL (HP 3000 Systems Programming Language) can be run in batch mode. Calls to programs or subroutines that have been batch-compiled in FORTRAN, COBOL, or SPL can be included in BASIC user programs, a fea ture that can greatly increase the speed of execution of BASIC programs. Also unique among BASIC time sharing systems is the new system's ability to store and operate on integer, real, long-precision, and com plex numbers in the same program. File systems are identical for timesharing and batch processing, so all files can be made available to any user in either opera ting mode, as desired by the system or account manager. Other features of MPET and MPE are a simple command language, complete accounting of resources, logging facilities, file backup and security, dynamic resource allocation, and virtual memory. Two standard hardware configurations capable of running MPET are the HP 3000 Model 100CX and Model 200CX Systems. The HP 3000 Model 100CX includes an HP 3000 Computer with 48K memory, a line printer, two 4.7-megabyte disc drives, a card reader, and a magnetic tape drive. The HP 3000 Model 200CX consists of an HP 3000 with 64K mem ory, a larger line printer, a 2-megabyte fixed-head disc drive, a 47-megabyte mass-storage disc drive, a card reader, and a magnetic tape drive. What Was Done The MPET project started with performance eva luations of the HP 3000 running under the control of the Multiprogramming Executive. The evaluation was accomplished using special software and hard ware measurement aids (these aids will be discussed later) and some purely subjective reasoning. The re sult of this evaluation indicated that to become an op timal timesharing system, MPE would need im provement in the following areas: Log-on or session initiation BASIC subsystem access BASIC LIST command BASIC GET and SAVE commands BASIC run-time performance. The Multiprogramming Executive performs sys tem and user functions as a series of processes. It was found that when a session was initiated at the termin al or the BASIC subsystem loaded, several processes had to be created. These processes would in turn create other processes and transfer control of the sys tem to the newly created processes. The process switching was the main factor in the amount of time required to initiate a session or load the BASIC subsystem. Fig. 2 illustrates how a typical user process (BASIC) is created under the control of MPE. The processes at or near the root of the process tree in Fig. 2 are highpriority processes. High priority means that the exe cution of these processes takes precedence over all other system and user code. Thus high-priority pro cesses are executed rather quickly under MPE. The nodes of the process tree in Fig. 2 that are be- © Copr. 1949-1998 Hewlett-Packard Co. Functional View of MPET/3000 System Commands Basic/3000 Interpreter â€¢ User Compiled Programs â€¢ Languages Subsystem BASIC COMPILER i Data Management Subsystems File System Input/Output Fig. 1.MPE7", a modification of the Multiprogramming Executive (MPE) for HP 3000 Computer Systems, optimizes the system for B ASIC-language timesharing, yet retains the ability to support concurrent multilingual batch processing. Data and Program Storage entry into the BASIC subsystem. Normally, initiating BASIC requires a complete process creation with all the necessary linkage edit ing. In MPET, however, since BASIC is invoked so of ten, its creation can be speeded by permanently link ing it as a part of the operating system. When the first user requests B ASIC a "virgin" B ASIC process (which is never executed) is created and linked into the oper ating system's process structure as shown in Fig. 3. Thereafter, when a user requests BASIC the virgin process is merely copied and the copy on which he executes is linked as a son of his SMP. low the dotted line are low-priority processes. This means that these processes are executed on the gener al process queue with all other system and user code. When a user initiates a session a unique session main process (SMP) is created for him at high prior ity. However, the SMP itself executes in the general process queue. This queue is circular and is rotated in a "round-robin" fashion to allow each active process in turn to use the CPU for no more than one time slice. The number of processes on this queue will, of course, be large in a heavily loaded timeshared system. To improve session-initiation time it is necessary to force the newly created SMP to the head of the general process queue so it can initialize itself im mediately without having to wait for its turn. How ever, the currently executing timeshare process is allowed to complete its time slice to minimize thrash ing (excessive moving of code into and out of main memory), which could result from frequent pre emption. A queuing analysis was performed on this method of modifying the scheduling algorithm. This study showed that, in the restricted environment of singlelanguage timesharing, average response time could be improved by judicious use of this technique. It was felt that when a user interacts with the system he should immediately get enough CPU time to execute the majority of his requests. To achieve this the rele vant process is forced to the head of the general pro cess queue and is given a double-length time slice. Also, when control is transferred between processes on the general process queue, the newly active pro cess is similarly forced to the head of the queue. This improves response to commands that involve the initiation of a new process â€” in particular, log-on and The LIST Function On heavily loaded HP 3000 Systems it was ob served that, when listing BASIC programs or print- G 3 C 9 Session Main User Process (BASIC) Fig. 2. In the original MPE process structure, BASIC is a lowpriority process that executes in the general process queue. In MPET, the BASIC user's session main process (SMP) is forced to the head of the queue when it is first created. This im proves session-initiation time. 13 © Copr. 1949-1998 Hewlett-Packard Co. save user BASIC program files interactively. While the MPE file system is very general, BASIC program files are very specific. That is, all BASIC pro gram files have the same record width, 128 words, and are in all other respects identical in structure. It became obvious that a rather simple file system inter face could be written to fetch and save BASIC pro gram files. It could be simple because all the options and record sizes handled by the MPE file system would not have to be considered. This specialized in terface was implemented, and as a result, most opera tions on BASIC program files are completed within the user's first time slice. Response times to these commands are improved by a factor of five on a loaded system. In addition to the file system interface, certain other economies resulted by allowing two extra re cords for expansion of B ASIC program files. Previous ly the BASIC subsystem created a program file of the exact length of the program to be filed. When a pro gram was modified, it became necessary to purge the old file and create a new one of different length. This is no longer necessary. With the extra space allotted at the creation of the file, the program can (Never Executes) Interactive Processes Batch Processes Fig. 3. Since BASIC is invoked so often in MPET. it is perma nently linked into the operating system. A virgin BASIC pro cess, created when the first user requests BASIC, is simply copied for subsequent requests. ing data, the output would often be in spurts of four or five lines per time slice. Investigation showed that the MPE terminal buffers could hold 32 ASCII char acters each with a maximum of six buffers per termin al, and that only one data transfer was made to these terminal buffers per time slice, after which the user process would be inactive until it went around the general process queue. It's important to note that terminal buffer I/O con tinues after the process has lost its time slice. In other words, once the data has been buffered, it becomes a system process to output the data. With this in mind, it was felt that if the terminal buffers were arranged in a circular structure, then a process could continue to fill those buffers that had been emptied during its time slice. This guarantees that a process will have six full terminal buffers to be emptied under the con trol of the I/O system after it has lost its time slice. With this method it was found that approximately twice as much data could be output during and be tween a user process's time slices. Fig. 4 shows how this technique works. 1 2 3 Next Available Storage Location Unshaded Area Empty Buffer Locations Terminal Buffer 3 Terminal Buffer 4 Last - Character Printed Out Loading and Saving BASIC Programs The MPE file system is a highly generalized subsys tem capable of handling files of practically any type, size, or structure. A certain amount of overhead is the price paid for these conveniences. Although the flex ibility of the file system is one of the advantages of MPE (and of MPET), it was obvious that the existing file system was not the most efficient way to load and Fig. 4. To speed up the LIST function, MPET terminal buffers are arranged in a circular structure so they can be refilled as data is printed out. â€¢© Copr. 1949-1998 Hewlett-Packard Co. tive terminals so that terminal access no longer has to go through the file system, and placing critical sec tions of code in core when sufficient core is available. The batch mode of MPE was left unchanged, ex cept that jobs now run on a low-priority subqueue to minimize the effect on the timesharing user. User pro grams and any of the subsystems supported by MPE may be run in batch mode. expand and contract within the initial space without having to waste time purging and creating files. BASIC Interpreter The BASIC/3000 interpreter presented special problems. It had been well thought out by its de signers and coded in a very efficient manner. Yet its run-time performance had to be improved while maintaining all the flexibility that has given this in terpreter its outstanding reputation. (We feel that BASIC/3000 is the most powerful BASIC interpreter ever written.) With the aid of measurement tools, it was deter mined that the interpreter was spending large amounts of time in a relatively small routine called the expression evaluator. Further investigation showed that a number of procedures were being called by this routine and the amount of time spent executing these procedure calls was significant. These procedures, some of which were very small, proved to be excellent candidates for optimization and/or relocation. Many were placed in-line, thus eli minating the time-consuming procedure calls and providing a substantial increase in run-time perfor mance. Also, computed GOTO statements were re written in assembly code and this improved their exe cution time significantly. These modifications yield a 20% reduction of CPU load for the typical BASIC program. TEPE MPET was a relatively short project that could not have been successful without certain performance evaluation and measurement tools developed for in-house use at HP. The Timesharing Event Performance Evaluator (TEPE) is an HP 2100-Computer-based software sys tem that is capable of simulating up to 32 timeshar ing terminals simultaneously. To run the system, the user provides a script that describes each terminal's conversation with the system under test. TEPE transmits data from the script file to the sys tem under test and then collects data on response time. This information is written to magnetic tape and later analyzed by an off-line process. To create realistic models or scripts for the TEPE system and thus obtain reliable information, it was necessary to define what the typical user of a time sharing system does. The literature on this subject is sometimes ambiguous and inconclusive. However, there are a few studies on the subject that have made real contributions. 2'3'4'5'6 These studies indicate that the typical user loads a timeshare system as follows: â€¢ Approximately 30 to 35% of interactions result in CPU-bound jobs or tasks. The user requires an average think time of about 25 seconds between entries (a mixture of getting, running, modifying, and saving programs). It was felt that if good response times were ob tained from TEPE data using these two important quantifiers then there was a good chance that actual system performance would be good. In fact the typi cal user defined for TEPE is a bit more demanding. From Fig. 5, a typical TEPE user interaction, it can be seen that the models include most of the opera tions done at a BASIC timesharing terminal. For ex ample, all models contain operations that are charac terized as CPU-bound (e.g., running BASIC pro grams). However, some are more CPU-bound than others (e.g., shorter programs or more I/O). Fig. 5 also illustrates a typical program file run by the TEPE sys tem. By varying the loop parameter (N) these pro grams can be made to provide a variable CPU load that is in close agreement with statistics published in the literature. The degree of CPU-boundedness of the various simulated users was chosen to fit the curve shown in Fig. 6, which was derived from the pub- General Improvements Certain modifications to MPE and the BASIC/3000 interpreter were aimed at a general improvement of the timesharing environment, especially at increas ing the number of simultaneous users of the system. Some of these modifications were initially made to solve particular problems and were later discovered to have significant effects on total system perfor mance. Among the most important of these modifications was the decision to restrict the types of operations that could be done at a terminal, and to give higher priority to interactive access than to batch access. Allowing only one subsystem, BASIC, to run from the terminals improves system throughput by maxi mizing the code-sharing capability of MPET and minimizing memory traffic. BASIC/3000 maintains user data areas nicely in that it expands and contracts them as needed, thus leaving more memory available for code. It is also the most popular timesharing lan guage for small machines. In addition to BASIC, all the MPE commands that display system information, manipulate files, and perform general user and oper ator functions are still allowed from any terminal. Other general performance modifications include faster, more specialized routines to serve the interac 15 © Copr. 1949-1998 Hewlett-Packard Co. rHELLO TEPE.SHAR.BDATA SESSION NUMBER = JCS35 THU. OCT 31. 1974. 1 1 : 1 S AM HP320PI0C. X0.92 tBASIC B A S I C 3 . 0 > G E T P Z 0 7 5 > 1 0 N= 1200 >200 PRINT"XXXXXXXXXXXXXXXX" >2f)l PR!NT"XXXXXXXXXXXXXXXX" >2PI2 PRINT "XXXXXXXXXXXXXXXX" > L I S T 1 - 2 0 4 PZ075 1 Â¡-Â¡EM THIS IS P7.075 10 N = 1200 20 DIM At 3 0 M = 0 4 0 5 0 10. 10] J = 5 0 L = 0 51 T=TIM(- 1 ) 5 2 C = C P U ( 0 ) 60 7 0 L=L+1 I F J o L T H E N Fig. 6. TEPE interactions were adjusted to provide a CPU lo_ad in agreement with this curve, which is derived from pub lished statistics. 1 0 0 3 0 J = J + 5 0 9 0 G O T O 1 9 9 1 0 0 I F L < > N T H E N 6 0 110 At 10. 1) = 3. 1416 1 1 1 T = TIM(- 1 )-T 1 1 1 1 12 1 3 2 0 3 0 C = CPU(0)-C P R I N T " C P U T I M E " ; I F M = 4 T H E N 1 6 0 I N P U T K we feel that TEPE provides a realistic image of the typical user of MPET. C . " E L A S P E T I M E " l T Trace " " Another tool, the segment trace system, used a hardware trace facility to collect data pertaining to processes at the time of intersegment transfers. These transfers are the result of procedure calls and exits from procedures in user or system code. The system collects data that, when reduced, reveals the number of segment calls of the traced routines, tells whether the segment called was absent or present, and reveals the time spent in each segment. Trace is handy for determining resegmentation schemes to minimize segment faults, or absences, for both user and system code. 1 5 0 G O T O 5 0 160 STOP 1 9 9 G O T O 1 0 0 2 0 0 P R I N T ' X X X X X X X X X X X X X X X X " 2 0 1 2 0 2 P R I N T P R I N T 2 0 3 P R I N T 2 0 4 P R I N T >RUN P7075 ' X X X X X X X X X X X X X X X X " ' X X X X X X X X X X X X X X X X " ' ' X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X C P U T I M E 4 . 1 0 6 ?5 C P U T I M E 4 . 0 5 76 C P U 77 T I M E ELASPE TIME 4. 156 ELASPE TIME 4. 05=? 4 . 0 5 2 E L A S P E T I M E 4 . 0 5 8 T I M E 4 . 0 5 S C P U T I M E 4 . 0 5 2 E L A S P E 73 C P U T I M E 4 . 0 5 ELASPE TIME 4.05<? >NAME SW07SDUM >SAVE >PURGE SW175DUM >EXIT END OF SUBSYSTEM :BYE CPU C SEC) = 25 C O N N E C T ( M I N > = 5 T H U , O C T 3 1 , 1 9 7 4 , END OF 1 1 : 2 3 A M SESSION Fig. 5. TEPE, the Timesharing Event Performance Evaluator, was one of the tools used in the MPET project. It simulates up to 32 timesharing users simultaneously. This typical TEPE user interaction includes most of the operations commonly done at a BASIC terminal. lished statistics. The TEPE system uses a random think time be tween one second and 100 seconds. The mean think time is 23 seconds, which again is in close agreement with published statistics. The think time distribu tion is exponential, as shown in Fig. 7. In general, Sampler The software sampling system is a useful tool for measuring the relative time spent executing various sections of code. A special external clock interface is used to produce controlled random interrupts. The interrupt receiver for this clock gathers information about the environment prior to the interrupt and dumps this data to magnetic tape. A data reduction program provides reliable histograms of code execu tion times. Resolution is selectable and can be as fine as single instructions. With this information a pro grammer can easily determine those sections of code for which optimization will provide the greatest per formance improvements. The sampler was especially useful for fine-tuning the BASIC/3000 run-time ex pression evaluator. Results Because the goal of this project was superior inter active performance, that is, fast response times to the user, the results of the modifications as the user sees them are of great importance. On the 16-user HP 3000 © Copr. 1949-1998 Hewlett-Packard Co. MPET/3000 100 PRICES IN U.S.A.: 32010A MPET Operating System (ordered separately). S5000. Complete systems including MPET: HP 3000 Model 50CX, $99.500. HP 3000 Model 100CX, $129.500. HP 3000 Model 200CX, $171.000. HP 3000 Model 300CX. $203.500 MANUFACTURING DIVISION: DATA SYSTEMS DIVISION 11000 Wolfe Road Cupertino. California 95014 US. A. 10 20 30 40 50 60 T (seconds) 70 80 90 100 Fig. 7. TEPE simulates a random user think time between one second and TOO seconds. The think time distribution is ex ponential, and the mean value is 23 seconds, which agrees closely with published statistics. Model 100CX System, and using MPE as a standard for comparison, we find that under MPET the BASIC subsystem can be loaded approximately 14 times faster. All other interactions are from 10% to 450% faster than the same interactions under MPE. MPET on the HP 3000 Model 200CX System can support 24 or more simulated typical users with ap proximately these improvements. Although through put was not measured specifically on MPET, it is evident that it has increased greatly over MPE for a BASIC timesharing load. Of course, actual perfor mance will depend on the system load imposed by the particular user environment. Nealon Mack (right) Acknowledgments Neal Mack joined HP's Data Systems Division in 1973. He's worked on performance measurement, software quality assurance, and performance and human engineering im provements on MPE/3000. Born in Shreveport, Louisana, Neal served in the U.S. Air Force from 1963 to 1967, then attended California State University at Long Beach, gradu ating in 1971 with a BA degree in mathematics. In 1973 he received the MS degree in computer engineering from Stan ford University. He also holds community college teaching credentials in electrical engineering, computer science, and mathematics. A resident of Sunnyvale, California, Neal enjoys reading, sports car touring, and the active social life of a bachelor. We express our appreciation to the following peo ple for their assistance in the implementation of MPET/3000: Joel Harriett, Tom Blease, John Dieckman, and John Hawkes (TEPE).ff References 1. T.A. Blease and A. Hewer, "Single Operating System Serves All HP 3000 Users," Hewlett-Packard Journal, Janu ary 1973. 2. G.E. Bryan "Joss: 20,000 Hours at a Console," Pro ceedings of the 1967 Fall Joint Computer Conference, AFIPS Press. 3. A.L. Scherr, "Time-Sharing Measurement," Data mation, April 1966. 4. M. Parupudi and J. Winograd. "Interactive Task Be havior in a Time-Sharing Environment," Proceedings of the ACM 1972 Annual Conference, Association for Com puting Machinery. 5. H.D. Schwetman and J.R. DeLine, "An Operational Analysis of a Remote Console System," Proceedings of the 1969 Spring Joint Computer Conference, AFIPS Press. 6. S.J. Boies, "User Behavior on an Interactive Computer System," IBM System Journal, Vol. 13 No. 1, 1974. Leonard E. Shar (left) A native of Johannesburg, South Africa, Len Shar received his BSc degree in electrical engineering from the University of the Witwatersrand in 1968. He came to the U.S.A. in 1969 to study computer science at Stanford University and received his MS and PhD degrees in 1 970 and 1 972. Deciding that he liked the San Francisco bay area so much that he wanted to stay, Len joined HP's Data Systems Division in 1972. Now a project manager there, he's been heavily involved with HP 3000 performance measurements, diagnostics, and interface design. He's a member of IEEE. Bachelor Shar, who lives in Palo Alto, California, and enjoys hiking, reading, and music, is currently "trying" to teach himself to play guitar. 17 © Copr. 1949-1998 Hewlett-Packard Co. Deriving and Reporting Chromatograph Data with a Microprocessor-Controlled Integrator Printing retention times next to the peaks while plotting the chromatogram, a new integrator measures the chromatograph peak areas and, at the end of the run, derives concentrations and prints the analysis on the chromatogram. by Andrew Stefanski ALTHOUGH GAS AND LIQUID chromatography provides fast and convenient means for ana lyzing the chemical components of complicated mix tures, identifying and quantifying the raw chromato graph information requires a major effort. The result of a chromatograph procedure is a chro matogram (Fig. 1), usually made by a conventional strip-chart recorder that monitors the output of the chromatograph's detector. The substance to be ana lyzed is injected at the input to the chromatograph's column, a long tube packed with particles coated with a particular liquid. The sample is carried through the tube by a carrier gas or solvent and the chemical components become separated on the basis of the differences in their solubility in the liquid coat ing. The lighter molecules arrive at the end of the col umn first, the heavier molecules coming later. The detector responds to the presence of substances other than the carrier in the emerging stream, tracing a peak on the chromatogram for each chemical component detected. The time of occur rence of each peak corresponds to the travel time through the column and can be used to identify the corresponding chemical component. The area en closed by the peak corresponds to the concentration of that chemical. To calibrate the chromatogram. a known amount of a known substance is usually mixed with the sam ple. Reducing the data then requires the chromatographer to measure the retention times with a ruler, using the known substance as a reference, and to mea sure areas of the peaks by counting squares, using a planimeter, or cutting out the peaks and weighing the paper. Clearly, a lot of effort can go into reducing the data. Speeding Data Reduction This task was eased somewhat by the development Fig. 1. Typical gas chromatogram, this one resulting from a mixture of chlorinated benzenes. *See page Chromatography . Hewlett-Packard Journal. March 1973. page 4 18 © Copr. 1949-1998 Hewlett-Packard Co. while enabling more sophisticated recognition of peaks, but it soon became apparent that the availabil ity of an internal digital processor would present op portunities for new integrator capabilities. We there fore considered means of adding automatic calibra tion so the integrator could identify the peak belong ing to the calibrating sample and then scale results ac cordingly. The digital processor also provides means for reducing the effects of detector noise, and for let ting the instrument select the optimum slope sen sitivity automatically so it can be sensitive to small peaks while ignoring noise peaks. We also considered including the recorder as part of the integrator â€” combining the numerical data with the graphical data on one piece of paper would make it much easier for the chemist to relate the re duced data to the raw chromatogram. There was one major drawback to this idea: there was no suitablypriced recorder that could print as well as plot. There fore, we developed our own (see box, page 20). Fig. 2. Integrator printout with the corresponding chromatogram. The lower number of each pair gives the retention time in hundredths of minutes. The upper number gives the area count in fiV-seconds with the digit in the right-hand column representing a power of 10 multiplier, e.g. 3311 2 means 33 i i x Â¡O2. Communicating with the Processor Although the presence of a digital processor would allow all integrating parameters to be en tered through a calculator-type keyboard, it was re alized that the instrument would be easier to operate if certain parameters were entered by means of slide switches. The switch positions are encoded internal ly and the code is sent to the digital processor. By their setting, the switches provide continuous dis play of the integrating parameters. In the final design (Fig. 3), the instrument is oper ated entirely by the switches when end results are to be printed as "area %" (the percent area that each peak contributes to the total of all peak areas). The keyboard is used only when further computations are to be performed. At the end of a run, the recorder prints a report on the sample analysis. The analysis identifies each peak by its retention time and gives area count and amount or percent of concentration. It also lists the integration parameters used, such as slope sensitiv ity and the time between the sample injection and the start of integration (start delay). Thus, the chro matographer has on one chart a complete record that includes the raw chromatogram and the reduced data (Fig. 4). of integrators that automatically compute the areas under the peaks and print the area and retention time for each peak (Fig. 2). The early electronic integrators basically were voltage-to-frequency converters that monitored the output of the chromatograph detector and drove a counter activated by rather complex peak-recognition logic. The chromatographer was still required to scale the time base, however, and to compute percentage or absolute concentration from the area counts. The next step was to derive final results with the aid of a computer working directly from an analog-todigital converter. The cost of doing it this way, how ever, was usually justified only by time-sharing the computer with several chromatographs doing repeti tive analyses, such as those for checking pesticide re sidues or drugs. There was an obvious need for a modestly-priced single-channel instrument that incorporated digital processing. As large-scale integrated-circuit tech nology advanced during recent years, it was hoped that eventually the cost of digital processing circuits would become low enough to make the computing integrator economically feasible. This hope was re alized with the development of the digital processor for the HP pocket calculators.1 Processing the Chromatogram The new instrument's analog-to-digital converter uses integrating digital voltmeter circuits to measure the average amplitude of the chromatograph detec tor output five times per second. The dual-slope tech nique2 is used to convert the detector output voltage to digital form. The voltmeter output consists of bursts of 10-MHz pulses, the number of pulses in New Concepts In applying the digital processor to an integrator, the initial goal was to simplify hardware design 19 © Copr. 1949-1998 Hewlett-Packard Co. Fig. 3. Model 3380 A Integrator records chromatogram and analysis on the same sheet of paper. Recording and integration are controlled by the slide switches. The keyboard is used only for computations related to calibration procedures. A Printing Plotter Instead of a pen, the printer-plotter in the new Model 3380A Integrator uses a thermal print head. The print head, similar to those used in the printers for the HP 9800 series desk-top calcu lators,* has seven printing elements (heaters) in a row on a cer amic is For normal recorder operation, one element is left on continuously, tracing the chromatogram on heat-sensi- live ap To write characters, the elements are pulsed at ap propriate moments as the carriage is moved rapidly across the paper (Fig. A). When the integrator senses that the output of the chromatograph's detector has crested a peak, it commands the re corder to "steal" a little time from the chromatogram to write out the time of occurrence of the peak, or retention time as it is com monly called. Each peak is thus clearly identified by the reten tion time printed next to it. Thermal Print Head nftt Fig . B. Thermal print-head mounts on the recorder's carriage. At the end of a run, the plotter prints the analysis report, identi fying each peak by its retention time. The integrator retains the data until the next run so the plotter can be used to print addi tional copies, or it can print the results of further processing of the data by various calibration methods. Control Lines Fig. A. Heating elements are pulsed at appropriate times to write letters and numbers. To trace a chromatogram, one element is turned on continuously. â€¢D.B. Barney and J.R. Drehle. A Quiet. Low-Cost. High-Speed Line Pointer ". HewlettPackard Journal. May 1973 20 © Copr. 1949-1998 Hewlett-Packard Co. Once the processor has made the decision that a peak is being detected, a reversal of slope that contin ues for several consecutive samples indicates that the apex of the peak has been crossed. The processor then commands the plotter to print the time elapsed since the start of the run. Counts continue to be totaled until the sample-tosample difference indicates that the detector output has returned to the baseline. At this time the proces sor stores the accumulated count, commands the plotter to place a mark on the chart indicating the end of integration for that peak, and starts looking for a new peak. The processor memory is capable of holding counts obtained from 54 peaks in any one chromato graph run. Because there are times when a peak does not return to the starting baseline but returns to a drifting baseline or merges with a following peak, the processor also stores data pertinent to the slope re versal for later evaluation. Li ? : 4 ~ -; T 9 Fig. 4. Typical chromatogram and report generated by the Model 3380/4 Integrator. The retention time for each peak is printed alongside the peak. The analysis report is printed after the chromatograph run is completed. The final two lines give the settings of the integration controls. Automated Slope Sensitivity As with earlier integrators, the chromatographer can select the slope criterion (mV/min.). The use of a digital processor, however, provides a new conven ience: automatic slope sensitivity selection. To use this feature, the chromatographer depresses the SLOPE SENSITIVITY switch to the TEST position before starting the chromatograph run. This causes the in strument to monitor the detector output for 20 sec onds and to store the maximum sample-to-sample difference encountered during that time. This is representative of the maximum noise to be expected, and it becomes the threshold level of the peak-recog nition criterion. The processor compares the beginning and end of each peak to detect baseline drift. It then adjusts the readings to account for drift, if present. This results in more accurate measurements than those made by older instruments that assume a level baseline for each peak. Another convenience the digital processor offers is operation controlled by an Automatic Sampler. The Sampler starts the Integrator each time a new sample is injected into the Chromatograph. The Integrator's self-timer stops the integration at which time the re port for that run is printed. The report includes the identification number of the bottle from which the sample was drawn. Thus long, repetitive analysis may be made with the equipment unattended. each burst being proportional to the amplitude of the corresponding sample. To smooth noisy chromatograrns, a running aver age of consecutive samples is calculated by a weighted averaging method. This smooths the highfrequency noise without distorting true peaks. The system totals the counts obtained on succes sive samples but it discards the stored count if it de cides that it is not measuring a true peak. This deci sion has always presented a dilemma to integrators. Integrators universally select peaks on the basis of slope. If the slope threshold is set too low, noise on the baseline can trigger integration. If it is set too high, integration starts high on the peak and a signifi cant part of the peak is lost. The digital processor in the new integrator starts integration at the slightest hint of a peak but it discards the count if the peak pre sence is not confirmed. Thus, even with the slope threshold set high, total peak area is integrated. Peak Criteria The digital processor measures slope by contin uously comparing each new averaged value to the previous value. If the difference is positive and ex ceeds a certain minimum for several successive sam ples, the processor judges that a peak is being de tected. It then commands the plotter to mark the chart to indicate that a peak is being integrated, and continues to accumulate counts. If the sample-to-sample comparison indicates that the slope reverses before the threshold criterion is reached, then it is assumed that a noise peak had been encountered and the total count is discarded. Merged Peaks A particular problem for integrators is finding the true areas of peaks that overlap or merge on the chro matogram. Two merged peaks are diagrammed in Fig. 5. In the new Integrator, when the sample-to21 © Copr. 1949-1998 Hewlett-Packard Co. Adapting a Calculator Microprocessor to Instrumentation by Hal Barraclough While the advantages of digital implementation were being considered for the next generation of chromatograph integra tors, HP Labs was developing a microprocessor for a family of hand-held calculators. This microprocessor was being de signed specifically for the HP-35 and its descendants, and consequently was severely limited in several characteristics es sential for a complete processor structure. The cooperation of the microprocessor development team, in particular, Kenneth Peterson, was therefore most valuable in our effort at applying this microprocessor to an instrumentation problem. Because the chromatograph integrator receives its input da ta in a continuous stream, the processor must be capable of real-time operation. The HP-35 is, of course, designed for hu man use, which signifies two fundamental characteristics: (1) relatively slow speeds, and (2) closed-loop operation through the human's own processor, for its data input rate. The first task we faced, consequently, was to ensure that the microproces sor could keep up with the required rate of data delivered from any gas chromatograph detector. Fortunately that data rate proved slow enough. The next problem to solve was the instrument's requirement for long-term data storage. This arises in an integrator for sever al reasons, one of which is that several forms of relatively com plex data anomalies, primarily overlapping waveforms, are in herent in the raw measurements delivered to the integrator. Their resolution can be automated accurately only by choosing the most appropriate algorithm, and this choice must be post poned typ all data in the vicinity of the anomaly is received; typ ical examples of this are merged peaks requiring separation, drifting baselines to be distinguished from the onset of a real peak, and digital filtering of 1/f noise without loss of small peaks. Two additional reasons for having large data storage are the normalization of all peaks to percentage values at the end of the run, and the printing of a final report covering the entire run. Unfortunately, mass data storage is not a capability of calcu lators. In our chosen microprocessor, for example, mass me mory consists of one register, yielding a capacity of one data word. We added 16K bits of data memory, organized as 512 words of 32 bits each. The data format is BCD, for compatibility with the microprocessor; we interfaced to this at the most con venient place, the A/D converter. Next, moderate study indicated a data word of 8 digits would give us more than sufficient resolution, and 512 words gave us the capability to accept chromatograph runs considerably more complex than our original objective. After a good deal of trial designs for costing purposes, MOS shift registers were re jected very the storage medium, although their initial cost is very low and they fit nicely into the fully serial architecture of the mi croprocessor. Magnetic core is not really appropriate for this type of application, and the cost of static MOS was high while yielding no advantage at the cycle times required for this instru ment. The optimum choice for our integrator was dynamic MOS RAM. We designed the addressing of our 1K-by-1-bit chips to be counted sequentially through 32 steps, thereby per forming the necessary serialization directly. The most difficult problem remaining was the design of an addressing scheme for all this memory. The choices involved trade-offs between binary and BCD (our microprocessor is strictly BCD), various address computation schemes, the mi croprocessor's limitation of I/O to only one port, and the implica tions of all methods upon the execution times of the real-time program loop. Innumerable schemes were created, hardware designed on paper, and the result measured for effectiveness by microcoding the real-time procedures. At the end of all this we selected binary addressing, with the data addresses stored in ROM along with all the machine's instructions, and with a substantial dose of TTL interjected between the microproces sor and its program storage (i.e., ROM). The logic serves to oc casionally fool the microprocessor by intercepting pseudo-in structions and treating them as binary data addresses. For a touch of elegance we included an index register, made it con versant with BCD via the I/O port so its utilization by the pro grammers is easy, and permitted it to be duplexed so it doubles as a general-purpose register. Indexing is a very useful feature for our application because of the episodic nature of much chro matograph data, with repetitive kinds of data points common to most peaks. Finally, code conversion from the index register's BCD to our address register's binary, along with address com putation (performed mainly by addition with some concatena tion) was mixed with the address counter for serialization. We ul timately eliminated this counter and used the system state counter fora// timing. One can appreciate the number of timing diagrams that went to the Palo Alto paper recycling center. The limitation of only one I/O port was solved by assigning a unique storage address to each of the following: (1) the A/D converter, for input, (2) the printer-plotter, for output, (3) the front-panel switches, for human control inputs, and (4) the in dex register. The last design phase, performed while accommodating to the continuing changes made to the microprocessor by its development group, was an absolute logic minimization endea vor. All the digital processing was achieved on one 8" x 12" two-layer PC board requiring an 8-package microprocessor. 16 packages of RAM, test capability, and all the TTL and CMOS for the logic. sample comparison indicates that the slope of the chromatogram changes sense before it reaches the baseline, the processor stores the count accumulated up to that point, starts a new count, and draws a mark on the chromatogram to indicate that a new integra tion has been started. It also identifies the value of the first sample in the new count for later use- If the trace returns to the baseline on the next downslope, the two counts obtained are stored as the area counts for the two peaks. This is known as the "dropline" method of merged peak separation. Dur ing the final printout, the letter "VI" is printed on the line for the second peak to indicate that it was merged with the previous peak and that this method was used. 22 © Copr. 1949-1998 Hewlett-Packard Co. tude of the second peak is less than one-half that of the first peak, then the processor uses the tangent skim method. The end of the second peak is deter mined by continuously calculating the slope of a line drawn from the start of the second peak to the latest sample, and comparing the line's slope to the slope of the chromatograph curve. When the two slopes coincide, the end of the peak is indicated. The proces- Merged Peaks Tangent Skim Method Fig. 5. Areas of merged peaks (left) are separated by a line dropped from the valley to the baseline. The area of a small peak riding on a tailing peak (right) is computed using the tangent line as a baseline. The area in the trapezoid below the tangent line is allocated to the tailing peak. A common occurrence is that indicated in the right hand plot of Fig. 5 where a small peak rides on the tail of a larger peak. This calls for separation by a dif ferent method, known as the "tangent skim" method. The digital processor detects the presence of a tail ing peak by storing the time elapsed between the start of the peak and its apex, and comparing that to the time from the apex to the end of the peak. A large difference classifies the peak as "tailing". If the first peak is a tailing peak, and if the ampli- Andrew Stefanski Andy Stefanski received his Master's degree in EE from the Warsaw (Poland) Polytechnic Institute in 1962. He worked for a time at the Institute of Telecommunications in Poland designing TV oroadcast equipment but then came to the United States where he worked on an optical print reader and later on advanced consumer electronics while working towards an advanced degree at the University of Pennsyl vania. Obtaining his Ph.D. in EEin 1970, he came to work for Hewlett-Packard's Avondale Division. Andy flatly states that he has no hobbies to speak of. Hal Barraclough A sometime commercial pilot specializing in helicopters, Hal Barraclough joined HP Labs in 1 970 to work on computer architecture but he is presently on a leave of absence to teach computer design in the graduate school at Santa Clara Uni versity. Hal earned his BSEE degree at the University of Idaho in 1961 and his MSEE degree at Stanford in the HP Honors Co-op program. His most pleasant spare-time activity is the time spent with his two sons but he also derives satis faction from designing home electronics and tending a vege table garden at the Barraclough home in San Jose. Fig. 6. Typical calibration dialog. When the operator presses the CALIB key, the integrator asks for the method. In this case, the operator responds with ISTD (internal standard). The integrator then asks for the width of the retention time window (%fiTW) and the identification of the calibrating peaks, which are identified by their retention times (RT) and by the amount in the sample (AMT). The dialog is ended when the operator presses 0 in response to a request for another reference When the operator presses the LIST button, the integrator confirms the calibration by listing the parameters and the response factors, which it calculates. 23 © Copr. 1949-1998 Hewlett-Packard Co. sor then calculates the area. In the final printout, the letter "T" is placed on the line for the second peak. The tangent-skim method can also be invoked manu ally any time the chromatographer decides a more accurate integration would be achieved. By similar techniques, the processor derives counts for two or more merged peaks on the tail of a large peak by dropline to the tangent. The letters "TM" will appear in the printout for these peaks. When a run has been completed, the integrator pro cesses the stored data according to the method se lected. It retains the data until a new run is initiated, so the chromatographer can make additional copies of the analysis, or he can process the data again by an SPECIFICATIONS HP Model 3380A Integrator Input Characteristics VOLTAGE INPUT: -001 to 1.0V DYNAMIC RANGE: 10*10 1 Output Characteristics RESOLUTION: 1 area count - 1 nV sec INPUT EXCEEDED: Warning printed in report LOGIC MARKS: Peak recognition and termination marks RETENTION TIME: Printed at peak apex on chromatogram in 0.01 min units, maximum 330 mins INTEGRATION: Automatic tangent skim on tailing peaks with manual forced tangent skim possible; slope sensitivity may be selected manually or auto matically; compensation for up/down drifting baselines is automatic REPORT: Consists of a chromatogram, calculations, and listing of control settings. All stored peaks are reported, but only those identified as calibrated peaks are calculated to yield amountsReport is on 8!'2 - 11' sheets of Z-fold thermal writing paper. Area% calculation format peak of four columns: retention time (as printed at peak apex); peak type; Area%. Method calculation format consists of five columns: retention time (as printed at peak apex); peak type: area: calibrated peak identifier (ID#); amount. Controls ATTENUATOR: 1 to 1024 in binary steps, and log presentation SLOPE SENSITIVITY: Six settings from 0.01 to 30 mV min, and auto/test posi trons for automatic selection other method if he so chooses. When the slide switch labeled AREA%/METHOD is moved to the METHOD position, the digital processor initiates a dialog by way of the printer-plotter. This guides the user through the steps required to estab lish the calibrating parameters. An example is shown in Fig. 6.2 References 1. T.M. Whitney, F. RodÃ©, and C.T. Tung, "The 'Powerful Pocketful': an Electronic Calculator Challenges the Slide Rule", Hewlett-Packard Journal, June 1972. 2. See, for example, A. Gookin, "Compactness and Ver satility in a new Plug-Together Digital Multimeter", Hewlett-Packard Journal, April 1972. CHART: of and ON, OFF positions OFF position prevents plotting of chromatogram so each run is reported by a calculation only CHART SPEED: Four settings: 0.5-1-2-4 cm mm STOP TIMER: Off, and nine settings to 90 mins for automatic termination of run followed by report printout START DELAY: Off. and nine settings to 64 mins to delay start of integration METHOD: Selector for Area0Â» or Method calculation, keyboard is deactivated when switch is in AreaÂ°b position CALCULATIONS (keyboard controlled): Four are standard: Area%; Normali zation; Internal standard: External standard. Latter three use automatically determined or manually entered response factors. Single stored calibration shared run methods permitting any method calculation report for stored run data Special no limit to number of report copies, original or modified. Special key for entry of amount of internal standard added to sample and for dilution factor. Up to 54 peaks may be calibrated PEAK peak Calibrated peaks other than reference peak are auto matically identified by relative retention. In ESTO and NORM methods, identifica tion by absolute retention time occurs automatically if reference peak is not found. Analyst may deliberately select this alternate type of identification for all cali brated peaks in ESTD and NORM methods. General DIMENSIONS: 20.6 H t 43.5W < 57.2 D cm (8-1/8 h 17-1/8w â€¢ 11-1/2dins) WEIGHT: 17 kg (37 Ibs) POWER: 100-120-220-240 (-5. -10%), 50/60 Hz, 1 SOW max ENVIRONMENTAL: 10-50Â°, 0-95% rel. humidity up to45Â°C PRICE IN U.S.A.: S5200 MANUFACTURING DIVISION: AVONDALE DIVISION Route 41 and Starr Road Avondale, Pennsylvania 19311 Bulk Rate U.S. Postage Paid Hewlett-Packard Company Hewlett-Packard Company, 1501 Page Mill Road. Palo Alto, California 94304 DECEMBER 1974 Volume 26 â€¢ Number 4 Technical Information from the Laboratories of Hewlett-Packard Company Hewlett-Packard S.A.. CH-1217 Meyrin 2 Geneva. Switzerland Yokogawa-Hewlett-Packard Ltd.. Shibuya-Ku Tokyo 151 Japan Editorial Director â€¢ Howard L. Roberts Managing Editor â€¢ Richard P. Dolan Art Director. Photographer â€¢ Arvid A. Danielson Illustrator â€¢ Sue M. Perez Administrative Services. Typography â€¢ Anne S. LoPresti European Production Manager â€¢ Kurt Hungerbuhler M R 3 ' ' C I S O ? - - L nt .i â€¢â€ DATA .¢J Ã 5 0 1 F AÃ¼t Ã ‰ -n A PI Ã ‰ ^ Â«H CHANGEOFADDRESS:! 1 5 7 1F" b change your address or delete your name from our mailing list please send us your old address lable (it peels off) Send changes to Hewlett-Packard Journal. 1501 Page Mill Road. Palo Alto. California 94304 U.S.A. Allow 60 days. © Copr. 1949-1998 Hewlett-Packard Co.
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