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Texas Instruments Designing Foundation Fieldbus and Profibus PA Devices with the DAC874xh Modem Fa Application notes
Designing Foundation Fieldbus and Profibus PA Devices
with the DAC874xH Modem Family
Garrett Satterfield
Introduction
Foundation™ Fieldbus H1 (FF) and Profibus PA (PA)
are fully digital communication protocols for factory
and process automation that offer many advantages
compared to the traditional 4-20mA current loop.
These protocols provide a real-time communication
network between many devices in an industrial
environment over a single cable. Similar to 4-20mA
loops, FF and PA can provide power for loop-powered
devices in addition to communication over the same
two wires, supporting intrinsic safety applications. In
addition to simple wiring, FF and PA networks benefit
from higher speed communication, higher current for
sensors/transmitters, and advanced diagnostic
capabilities.
The DAC874xH family of devices are standalone
modems supporting HART, Foundation Fieldbus, and
Profibus PA. These modems feature SPI/UART
interface options, an internal FF/PA input filter, low
quiescent current, and internal voltage reference. The
DAC874xH devices require few additional components
to implement an FF/PA physical layer compliant
medium attachment unit (MAU) making them ideal for
FF and PA transmitter applications.
FF/PA Overview
The FF and PA busses consist of devices connected
in parallel between two wires, a loop power supply with
power conditioning, and two bus terminators (T1, T2)
as shown in Figure 1. Devices on the bus
communicate via a 0.75Vpp-1Vpp voltage waveform
superimposed on the DC bus voltage representing
Manchester encoded digital data. Transmitters and
other devices produce this waveform by modulating
the current they draw from the bus, typically +/-9mA
from their DC value, allowing long distance
communication. The AC current waveform is allowed
to pass due to the combined impedance of the two
terminators, creating the voltage waveform on the bus
(+/-900mV). The bus power supply must provide
power conditioning to ensure the signal integrity of the
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waveform on the bus. Terminators are installed on
each end of the network, allowing a path for the AC
current to flow. The link-master on the network is
responsible for scheduling the communication between
devices.
Bus +
Supply ±
T1
FF/PA
Link-Master
FF/PA
Transmitter
Sensor
T2
Figure 1. FF/PA Fieldbus Network
Design Considerations
The DAC874xH family of modems modulate and
demodulate a low-speed 31.25kbit/s Manchester
encoded voltage waveform, and these devices are
compliant with both Foundation Fieldbus H1 and
Profibus PA physical layers. An external circuit is
required to convert this voltage modulation into the
current waveform used by devices on the bus. The
circuit is similar to the 2-wire 4-20mA loop powered
field transmitter topology used in analog systems.
Figure 2 shows a simplified schematic of the external
circuit required to implement an FF or PA device with
DAC874xH. The amplifier A1 regulates the current
through Q1 which sets the current through Q3 due to
the current mirror formed by Q2 and Q3. The current
mirror is used to force most of the loop current through
the shunt regulator (REG) producing a stable
regulated voltage that is not dependent on the loop
voltage. Since the loop voltage changes by ~1Vpp,
this configuration prevents the communication signal
from feeding through to the regulated voltage used by
the on-board circuitry. The ratio of R6:R7 should be
large to allow only a small current to flow in the path of
Q1. Additional start-up circuitry and filtering
components are required to ensure correct operation
when initially connected to loop power.
Designing Foundation Fieldbus and Profibus PA Devices with the DAC874xH
Modem Family Garrett Satterfield
Copyright © 2019, Texas Instruments Incorporated
1
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C2
Loop+
R7
Q3
R6
Q2
VREG
VREG
DAC
MODIN
REF
DAC874xH
MODOUT
REG
I DC
R1
R2
Q1
+
A1
C1
IMOD
R3
R5
R4
Loop-
Figure 2. Loop-Powered FF/PA Transmitter Circuit
(V-to-I Converter)
The loop current is set by the current injected into the
node connected to the non-inverting terminal of A1
from the reference voltage (REF) and modulated
voltage (MODOUT). The sum of the injected current is
gained to the output loop current by the ratio of R3:R4.
Since FF and PA are digital protocols high precision is
not required making component tolerance less critical
compared to analog current transmitters.
Equation 1 calculates the DC loop current based on
R1, the DAC874xH reference voltage (1.5V), and the
gain resistors R3/R4. The DC current for transmitters
is usually set between 10mA and 20mA depending on
the requirements of the on-board circuitry. Equation 2
calculates the loop current modulation based on R2,
the DAC874xH MODOUT voltage (800mVpp), and the
gain resistors R3/R4. Capacitors C1 and C2 AC
couple the voltage FF/PA waveforms in the transmit
and receive path of the modem.
§V
·§ R3
·
IDC ¨ REF ¸¨
1¸
© R1 ¹ © R4 ¹
(1)
IMOD
§ VMOD ·§ R3 ·
1¸
¨
¸¨
© R2 ¹ © R4 ¹
(2)
Figure 3 shows an example of the transmit waveform
measured on the fieldbus. This waveform was
measured with the FF/PA reference design using
DAC8742H connected to a bus with two terminators.
The current modulation of the reference design is +/9mA with a DC current of 16mA to allow for sensor
quiescent current. This corresponds to the 900mVpp
waveform on the bus that transmits the Manchester
encoded data.
2
Figure 3. Bus Voltage with FF/PA Signal
Transmitted
Physical Layer Compliance
FF and PA devices require a physical layer
compliance test to ensure interoperability with all other
FF/PA devices. The test requires transmitting a
request to the device under test and measuring
various characteristics of the DUT reply waveform.
Foundation Fieldbus and Profibus PA share a similar
physical layer but the test specifications differ slightly.
The transmitter must have an appropriate FF or PA
software stack in order to undergo the testing process.
Some of the tests include measuring the transmit
waveform amplitude, distortion, slew rate and bit rate.
Loop powered devices must also meet the start-up
current requirement defined in the specification. For
more information on physical layer testing refer to the
FF/PA reference design.
Conclusion
The DAC874xH devices are the only stand-alone
Foundation Fieldbus, Profibus PA, and HART modems
currently available in the industry. A single modem
supporting both HART and FF/PA allows for reduced
qualifying time for engineers designing both HART and
FF/PA hardware. A universal FF/PA + HART
transmitter can be designed that requires only a BOM
change to switch between the protocols, without
qualifying additional ICs. With DAC874xH devices and
simple, low-cost external circuitry, a physical layer
compliant FF or PA transmitter can be implemented,
offering greater functionality in automation control
applications than traditional 4-20mA current loops.
Designing Foundation Fieldbus and Profibus PA Devices with the DAC874xH
Modem Family Garrett Satterfield
Copyright © 2019, Texas Instruments Incorporated
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