In the previous two issues of this newsletter, we looked at troubleshooting digital input modules. Basically, we looked for the presence or absence of a voltage. With analog modules, it's trickier because the level of that voltage now matters — unless it's a current loop, in which case the level of the current matters.

As with digital modules, there isn't a standard value. With current modules, it's typically 4mA to 20mA, and with voltage modules it's typically either 1V to 5V or 0V to 10V. However, the same system could use many other values.

From the loop drawings, determine if the loop is:

  • “Sinked” (power provided by field device) or “sourced” (power provided by I/O module).
  • Isolated or non-isolated.
Then, verify that's what you actually have. Next, determine the signal value being used (e.g., 4mA to 20mA). These steps allow you to correctly connect and set your loop simulation device.

On the PLC, call up the screen that shows where this input is used. For example, the field device is a level transmitter and the PLC depicts a vat. With 12mA (50% input) the vat shows 50% full. Change the simulated control signal, and watch the PLC for the expected response. If the PLC displays changes as expected, you've verified proper operation from the input module terminals to the PLC. If the response is wrong, then you have a problem between the input module terminals and the PLC.