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.   When a transmitter is powered up with just a DC power supply there isn’t enough voltage from the 1200 baud HART signal for the for the 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 and HART communications fails.

The good news is that a resistor can be inserted into the loop and the HART signal will develop a voltage drop and the HART master can ‘see’ the HART signal.

The HART Foundation’s website has a technical specification that states that the minimum loop resistance needed is 230 ohms.  A 250 ohm resistor is commonly 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.

250 Ohm resistor with alligator clips

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).

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