I have a modicon PLC with various 4-20 ma inputs and outputs. By using PLC logic, engineering logic, and PLC configuration, what is the best or preferred way to setup the PLC so that it informs the user that a transmitter or output device has failed?

Thanks.
Chris Beutler

 

Generically, 4-20mA loop signal has its "live zero" at 4 mA.

If the signal drops to 0mA then there is high likelihood that the circuit has lost power, the wiring has been disrupted, or the field instrument has experienced a catastrophic failure. In any event, the PLC is not getting a valid signal from a field instrument and the 0mA input condition warrants attention.

On the output side, one of our process controllers can detect a 0mA condition and set an alarm flag. Don't know if Modicon has that or not.

Many field instruments have a 'diagnostic failure' setting, where the internal diagnostics detect a failure and set the 4-20mA output to a predetermined value, sometimes low at 3.8mA, sometimes high at 21.8mA, or some similar value outside the nominal 4-20 range, but still within the range of most inputs for detection purposes. Detecting this value and initiating the appropriate response would be specific to the instrument model.

John Titor