Micrologix 1400 input question

[Taterfarmer]

Hi, I am working with a Micrologix 1400 model 1766-L32BXB. With no input wires connected to the “in12” thru “in19”, I am getting 24 volts while the unit is powered on. Do I have a bad plc?

Thank you

 

[OkiePC]

If you have nothing landed on input terminal COM 3 (for that group of IN12 to IN19) and are measuring a voltage potential between any of those points and ... what is the other point of reference?

The PLC doesn't have a point of reference until you land a wire on the COM terminals and I don't think I've ever checked for the presence of voltage on a PLC input without the wiring landed on the input and its associated COM terminal, so I don't know what to expect.

 

[plvlce]

For that matter any of the ML1400 input groups can be wired as sinking or sourcing, and if wired sourcing I would expect to see voltage on the input with nothing connected to it.

ofc, this is another way of saying the same thing as OG: check COM 3. If wired sourcing you will have a 24V wire there.

 

[Taterfarmer]

My apologies, com 0-3 are all tied together. The banks of other inputs don’t have voltage unless they are receiving from a sensor, this is the only bank that I have voltage coming out of the input terminals.

 

[joseph_e2]

We had a failure last year on a 1400 where the inputs in a group were showing voltage when field devices were disconnected. It turned out to be an electronic failure inside the unit which was replaced under warranty.