I have recently had to disable an alarm in an analogue output card channel due to it causing an error in the PLC Software. The Analogue output in question, operates a modulating process valve. This is done by the PLC using a PID linked to temperature control.

As far as I am aware the valve seems to be functioning without any problems (4mA = Closed 20mA = fully open )

however in the PLC I/0 as seen in the control logics project window, I have had disable the internal alarm for this output channel to prevent it from sending an "Analogue error" message via a HMI Screen.

My question

What does this internal alarm actually do. (It must monitor the 4-20 mA signal somehow. (There is no other feebback)?

 

Can someone help me determine how this works

For this Output channel

Is this an internal alarm that sets a bit depending on a pre-set output level (that is pre-adjusted in the I/O properties of the Analogue output card),

or is it an indication that there may be a problem somewhere in the field, with the external circuit (Maybe too much resistance in the 4-20mA Circuit)

Mick

 

You are correct, this is set in the properties of the module. An external fault would show has either hi or lo, or zero, and would flag as a fault or alarm. All this is in the manuals available on line.