Designing the DP8392 for Longer Cable Applications Designing the
National Semiconductor Application Note 621 Mohammed Rajabzadeh July 1989 The IEEE 802.3 standard is designed for 500 meters of Ethernet cable and 185 meters of Cheapernet (RG58A/U) cable. To extend such segments to 1000 meters of Ethernet cable and 300 meters of Cheapernet cable requires utilization of Transmit mode collision detection. This method is described below. Receive Mode: The Receive mode scheme has a very simple truth table. However, the tight threshold limits make the design of it difficult. The threshold in this case has to be between the maximum DC level of one station (b1300 mV) and the minimum DC level of two far stations (b1581 mV). Several factors such as the termination resistor variation, signal skew, and input bias current of non-transmitting nodes contribute to this tight margin. On top of the b 1300 mV minimum level, the impulse response of the internal low pass filter has to be added. The CTI incorporates a 4-pole Bessel filter in combination with a trimmed on board bandgap reference to provide this mode of collision detection. Transmit Mode: In this case, collision has to be detected only when the station is transmitting. Thus, collision caused by two other nodes may or may not be detected. This feature relaxes the upper limit of the threshold. As a result of this, longer cable segments can be used. With the CTI, a resistor divider can be used at the Collision Detection Sense (CDS) pin to lower the threshold from receive to transmit mode. COLLISION DETECTION SCHEMES The collision circuitry monitors the coaxial DC level. If the level is more negative than the collision threshold, the collision output is enabled. There are two different collision detection schemes that can be implemented with the CTI; Receive mode, and Transmit mode. The IEEE 802.3 standard allows the use of receive and transmit modes for non-repeater node applications. Repeaters are required to have to receive mode implementation. These different modes are defined as follows: Receive Mode: Detects a collision between any two stations on the network with certainty at all times. Transmit Mode: Detects a collision with certainty only when the station is transmitting. Table I summarizes the receive and transmit mode definitions: TABLE I Mode Receive COLLISION LEVELSÐTRANSMIT MODE Table II shows the parameter values that are used in calculating the collision levels in transmit mode. Transmit No. of Stations 0 1 2 l2 0 1 2 l2 Transmitting Non-Transmitting N N N N Y Y Y Y N N N N Y M Y Y Y e Detects Collision N e Does Not Detect Collision M e Might Detect Collision Designing the DP8392 for Longer Cable Applications Designing the DP8392 for Longer Cable Applications TABLE II. Assumptions and Definitions e Termination Resistor at 20§ C e Temp. Coef. of the Terminator e Maximum Segment Length RDC e Maximum Cable DC Res. at 20§ C tc Tm SR e Temp. Coef. of Copper e Maximum Cable Temp. e Step Response at Max Cable Length RC e Max. Connector Res./Station IB a IB b Imax Imin Ro N e e e e e e SK e Skew Factor, Effect of Encoder RS RL SEO e e e e Max. Positive Bias Current Max. Negative Bias Current Max. DC Drive Current Min. DC Drive Current Non Transmitting Output Impedance Max Nodes per Segment Skew on DC Level (SKEW c 4)/100 Max. DC Loop Res. of a Segment Load Resistance Seen by a Driver Sending End Overshoot C1995 National Semiconductor Corporation TL/F/10445 e e e e e e e e e e e e e e e e e e e 50 g 1§ %X 0.0001/Deg. 300m 1000m 0.0489X/m 0.0100X/m 0.004/§ C 50§ C 0.97 0.94 0.0034X 0.0001X 2 mA 25 mA 45 mA 37 mA 100 kX 100 100 Cheapernet Ethernet Cheapernet Ethernet Cheapernet Ethernet Cheapernet Ethernet Cheapernet Ethernet 802.3 ASSUMPTION 802.3 802.3 BELDEN BELDEN PHYSICS ASSUMPTION NATIONAL NATIONAL MIL SPEC ASSUMPTION 802.3 802.3 802.3 802.3 802.3 802.3 802.3 802.3 e 0.02 for 0.5 ns Skew e 0.10 e 0.14 Cheapernet Ethernet DEFINITION DEFINITION ASSUMPTION ASSUMPTION RRD-B30M105/Printed in U. S. A. AN-621 RT tT L minimum DC voltage generated by one minimum transmitting station and another minimum transmitting station at the other end of a maximum length cable. The calculations below explain how the values for the resistor divider in Figure 1 are obtained. First, collision levels Vmax and Vmin must be calculated. The Vmax or ‘‘no detect’’ level is the maximum DC voltage generated by one node. The worst case here occurs when the transmitting node is at the center of a maximum length cable, and the collision is being detected either by itself or by a station right next to it. On the other hand, the Vmin or ‘‘must detect’’ level is the The filter impulse response is not included in these calculation since it is mutually exclusive with the Sending End Overshoot. If the impulse response is larger than the Sending End Overshoot, the exceeding portion should be added on to the limits. Maximum Non Collision Level VMax (NO DETECT)ÐTransmit Mode TL/F/10445 – 1 RTmax RS RL VMax e RT c 1.01 c [(Tm b 20) c tT a 1] e RDC c L c [(Tm b 20) c tc a 1] a N c RC e (RTmax a RS/2)/2 RL VMax e [IMax c (1 a SK) a (N b 1) (IB b )] c RL c (1 a SEO) e (50.652 a 16.770/2)/2 e 29.519X e [45 c 1.02 a 99 c 0.025] c 29.519 c 1.10 e 1571 mV ETHERNET Cable, 1000 Meters, 100 Stations: RTmax e 50 c 1.01 c [(50 b 20) c 0.0001 a 1) e 50.652X e 0.01 c 1000 [(50 b 20) c 0.004 a 1] a 100 c RS e 11.21X 0.0001 e (50.652 a 11.21/2)/2 e 28.129X RL VMax e [45 c 1.02 a 99 c 0.025] c 28.129 c 1.14 e 1551 mV CHEAPERNET Cable, 300 Meters, 100 Stations: RTmax e 50 c 1.01 c [(50 b 20) c 0.0001 a 1] e 50.652X e 0.0489 c 300 [(50 b 20) c 0.004 a 1] a RS e 16.770X 100 c 0.0034 2 Minimum Collision Level VMin (MUST DETECT)ÐTransmit Mode TL/F/10445 – 2 RP e Near End Shunt Resistance e [Ro/(N b 2)]URTmin ETHERNET Cable, 1000 Meters, 100 Stations: e [100k/98]U(50 c 0.99) e 1020U49.5 RP e 47.209X VS1(1) e 37 c 0.98 c [47.209U(11.21 a 49.5)] e 963 mV VS2(2) e 37 c 0.98 c [49.5U(11.21 a 47.209)] e 972 mV VS2(1) e 972 c [47.209/(47.209 a 11.21)] c 0.94 e 738 mV e 1701 mV VMin e 963 a 738 RTMin e RT c 0.99 VS1(1) e Station 1’s DC Voltage at End 1 e IMin c (1 b SK) c [RPU(RS a RTmin)] VS2(2) e Station 2’s DC Voltage at End 2 e IMin c (1 b SK) c [RTminU(RS a RP)] VS2(1) e Station 2’s DC Voltage at End 1 e VS2(2) c [RP/(RS a RP)] c SR VMin e VS1(1) a VS2(1) CHEAPERNET Cable, 300 Meters, 100 Stations: e [100k/98]U(50 c 0.99) RP e 1020U49.5 e 47.209X VS1(1) e 37 c 0.98 c [47.209U(16.770 a 49.5)] e 1000 mV VS2(2) e 37 c 0.98 c [49.5U(16.770 a 47.209)] e 1012 mV VS2(1) e 1012 c [47.209/(47.209 a 16.770)] c 0.97 e 724 mV e 1724 mV VMin e 1000 a 724 CIRCUIT IMPLEMENTATION Table III summarizes the design parameters. TABLE III 3 Parameter ETHERNET CHEAPERNET L N VMin VMax R1 R2 1000 Meter 100 1701 mV 1551 mV 125X g 1% 10 kX g 1% 300 Meter 30 1724 mV 1571 mV 150X g 1% 10 kX g 1% Designing the DP8392 for Longer Cable Applications Circuit implementation is shown in Figure 1 TL/F/10445 – 3 FIGURE 1 To check the design, subtract the additional offset generated by the resistor divider from these levels (VMax and VMin) and make sure that the internal 8392 collision levels (1450 mV to 1580 mV) are within this window. The supply voltage is assumed to be 9V g 5%. Cheapernet 1571 mV b 8.55V (150X/(10 kX a 150X) e 1445 mV 1724 mV b 9.45V (150X/(10 kX a 150X) e 1584 mV These calculations show that the resistor values are properly selected. Ethernet 1551 mV b 8.55V (125X/(10 kX a 125X)) e 1445 mV 1701 mV b 9.45V (125X/10 kX a 125X)) e 1584 mV LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: AN-621 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform, when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. National Semiconductor Corporation 1111 West Bardin Road Arlington, TX 76017 Tel: 1(800) 272-9959 Fax: 1(800) 737-7018 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. National Semiconductor Europe Fax: (a49) 0-180-530 85 86 Email: cnjwge @ tevm2.nsc.com Deutsch Tel: (a49) 0-180-530 85 85 English Tel: (a49) 0-180-532 78 32 Fran3ais Tel: (a49) 0-180-532 93 58 Italiano Tel: (a49) 0-180-534 16 80 National Semiconductor Hong Kong Ltd. 13th Floor, Straight Block, Ocean Centre, 5 Canton Rd. Tsimshatsui, Kowloon Hong Kong Tel: (852) 2737-1600 Fax: (852) 2736-9960 National Semiconductor Japan Ltd. Tel: 81-043-299-2309 Fax: 81-043-299-2408 National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project