Troubleshooting current loop operation

The current value being read by the input module can be externally verified with the use of a digital multi-meter.

Set the multi-meter to the ‘Volts DC’ scale, and connect the probes across the positive and negative terminals of a single channel on the IM31.  The screws on the top of the terminal blocks are a convenient place to place the probe tips.  Read the voltage drop across the terminals and multiply by 10 for the current reading in milliamps.

For example, if the multi-meter reads 1.440 volts, the current flowing through the loop is 14.40 milliamps.

If the reading is zero, the circuit is not complete and current cannot flow.  Check wiring and test again.

It is useful to note that current loops use a ‘positive zero’, meaning that even when the sensor value is zero, the transmitter will output a positive value (4 mA).  This helps during troubleshooting because you can easily determine if the loop is broken (resulting in zero mA current flow), or if the loop is complete, but the sensor output is zero, resulting in 4mA current flow.