How does adding a 250-ohm resistor make HART work?

Technicians often report a problem getting a loop-powered HART instrument to communicate when it’s on the benchtop. You may run into this when a new, out-of-the box transmitter is wired up for initial commissioning and configuration, or when a transmitter is brought in from the field where HART communications had worked just fine. So why is there a problem when it’s on the shop bench? It’s not transmitter failure, it’s merely a missing signal. 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 modem.

In a bench test situation, DC 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 in 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 resistance of the loop’s copper wire contributes very little resistance; too little to enable HART. 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 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 filter out 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.

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 (Meriam 5150 or now obsolete Emerson 475) 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. The Honeywell Versatilis handheld communicator requires an external loop resistor for bench testing.

If a HART communicator or software using a 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).