Staying in the loop!

01 June 2020

You just received one of the new technology HART communicators that use your smart phone. You connect the modem to your HART instrument, launch the App and… nothing happens… ‘error connecting to device’. What’s going on? ProComSol offers some advice.

Loop resistor placement in the loop.
Loop resistor placement in the loop.

The problem being encountered is that the loop  is missing a vital element of HART - the loop resistor which, according to  HART specifications is an essential component. 

The resistor serves two purposes – it is needed to convert the current-based HART signal to a voltage signal and it prevents the loop power supply from cancelling out the HART signal. 

Resistance value
The HART specification defines the loop resistor value to be in the range of 230 ohms to 600 ohms. Typically, 250 ohms is used. This value provides an easy conversion from a current value to a voltage value used in some control systems. In this case 4mA through a 250 ohm resistor is 1Vdc and 20mA is 5 Vdc.

So where should you put the resistor? It could be anywhere in series in the loop. An obvious and convenient place is in the PLC I/O card itself. If the PLC does not have a built-in resistor, then it could be placed into the marshalling cabinet. Often, cable is run from the control room to the plant floor to a marshalling cabinet. 

The worse option for loops in service – because it means the loop must be interrupted – is to place the resistor at the HART instrument. Simply lift one of the loop connections from the instrument, connect a resistor in its place and the other end of resistor connects to the cable lead. There may be enough room in the instrument enclosure so you can refasten the cover, keeping the resistor in the loop for the next time you need to connect.

When working with a HART device on in the lab, more options are available. The first option is to simply connect a resistor to one of the instrument loop connections. Then connect the power supply to the instrument and to the other end of the resistor. 

Modern HART modems now have the ability to power an instrument and often also contain a built-in loop resistor. This makes powering a device and communicating with it very simple.

While the loop resistor is a HART specification requirement, loops with long cable runs may have enough loop resistance for HART communications to occur. The likelihood of successful HART communications in low resistance loops is increased when your HART modem output and sensitivity exceeds the HART specs. This means the modem can receive lower voltage signals and can transmit at higher voltage levels. Long cable runs will likely provide enough wire resistance so that a dedicated loop resistor is not necessary. However, modems with more sensitive inputs are more susceptible to noise which will reduce communication reliability.

Common mistakes
As simple as the loop resistor requirement is, connection mistakes still occur. A common one is put the modem connection in series with the loop. Another mistake is to put the resistor across the terminals of the instrument. This removes the instrument from the loop and creates a fixed current output. 

HART is always going to require a loop resistor. PSK (Phased Shift Keying, or high-speed HART will still need a loop resistor as there is no difference in the wiring between FSK (Frequency Shift Keying, or standard HART) and PSK.

The loop resistor is a HART requirement. While some loops may work without it, the tradeoff will be reliability. Noise will be more prevalent and command retries will occur more often, slowing down communications. So, install that loop resistor into the loop on your next shutdown. Or better still, install it the design phase! 

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