How does adding a 250-ohm resistor make HART work? Sometimes there’s a problem getting a loop powered HART instrument to communicate when it’s on the benchtop, but it communicated fine when wired in the field. Why is that? It’s probably because when it’s on the test bench, the loop doesn’ t have enough resistance for the HART signal to be ‘seen’ by the HART handheld or HART modem. In a bench test situation, power is applied to the transmitter, so it fires up and runs fine. But there’s no analog input in the loop as there is when the transmitter is wired in the field. What’s missing is the dropping resistor on the analog input that supplies the loop resistance needed for the HART signal to develop. 4-20mA loops transmit the signal as electrical current (milliamps), but the receiver device with the analog input ‘reads’ the signal as a voltage, whether the signal to be read is analog or HART. The current signal becomes a voltage drop when the current passes through a resistor (Ohms Law). The HART signal is a 1200 baud signal superimposed on the loop’s 4-20mA DC signal. 1200 baud is a relatively high frequency signal compared to the 4-20mA DC current signal. Some minimum loop impedance is needed so that the HART signal can be ‘seen’ or ‘read’ by a HART master, like a HART communicator or HART modem. The analog inputs on the receiver device (DCS, PLC, RTU, PAC, controller, recorder, or indicator) have a precision shunt resistor installed at each analog input. It is usually 250 ohms, but not always. The voltage (IR) drop needed for HART communications is produced by the resistance at the loop receiver’s analog input. The loop’s wire resistance contributes very little resistance.
Sometimes theres a problem getting a loop powered HART
instrument to communicate when its on the benchtop, but it
communicated fine when wired in the field. Why is that? Its
probably because when its on the test bench, theloop doesn t have
enoughresistancefor the HART signal to be seen by the HART handheld
or HART modem.In a bench test situation, power is applied to the
transmitter,so it fires up and runsfine. But theres noanalog
inputin the loop as there is when the transmitter iswired in the
field. Whats missing is the dropping resistor on the analog input
that supplies the loop resistance needed for the HART signal to
develop.4-20mAloops transmit the signalas electrical current
(milliamps), but the receiver device with the analog inputreads the
signal as a voltage, whether the signal to be read is analog or
HART. The current signal becomes a voltage drop when the current
passes through a resistor (Ohms Law).The HART signal is a 1200
baudsignal superimposed on the loops 4-20mA DC signal.1200 baud is
a relatively high frequency signal compared to the 4-20mA DC
current signal.Some minimum loop impedance is needed so that the
HART signalcan be seen or read by a HART master, like a HART
communicator or HART modem. The analog inputs on the receiver
device (DCS, PLC, RTU, PAC, controller, recorder, or indicator)
have a precision shunt resistor installed at each analog input. It
is usually 250 ohms, but not always.The voltage (IR) drop neededfor
HART communications is produced by the resistance at the loop
receivers analog input. The loops wire resistance contributes very
littleresistance. Whena transmitter is powered up with just a DC
power supply there isnt enough voltage from the 1200 baud HART
signal for the forthe HART master (handheld communicator or a HART
modem) to see the signal. The internal resistance of a DC power
supply is insufficient to develop a HART voltage drop, and its
filter capacitors act as a low pass filter to squelch the
relatively high frequency 1200 baud HART signal.In the absence of a
receiver device with its analog input resistance being part of the
loop, there is insufficient loop resistance for a HART master
device to see the FSK HART signal andHART communicationsfails.The
good news is that aresistor can be inserted into the loop andthe
HART signal will develop a voltage drop andthe HART master can see
the HART signal.The HART Foundations website has a technical
specification that states that the minimum loop resistance needed
is 230 ohms. A 250 ohm resistor iscommonly used because 250 ohm
resistors work for that purpose and are typically available in the
instrument shops where these types of bench tests are done. The
resistor need not be a precision resistor.
Connect a 250 ohm resistor with alligator clips, like this one,
to a wiring terminal and a loose wire in a loop circuit to test
HART communications at the benchtop.HART handheld communicators
(275/375/475, Meriam 4150) usually have a pair of banana jacks on
the handheld for the purpose of connecting a plug adapter with a
249 or 250 ohm resistor to provide the needed loop resistance. If a
HART communicator or HART modem fails to establish communications,
it is advisable to insert a 250 ohm resistor in series in the loop
(or use the banana plug adapter with a communicator).
