I am going to assume the AO is 4-20 mA and I know the hardware is a Flex 1794-OE4.

Has anyone ever been asked to short the analog out [Example terminals A-0 to A-1] of the card under a certain set of safety conditions or any condition?

The terminals would be shorted using a set of contacts from a relay.




Thanks

I have seen "open" it but not short it. Why short it? For what purpose?

Mickey,
I am told they want to do this to force the signal to 0 and in turn force the valve to the closed position.

I saw it once on a vacuum furnace - the over-temperature alarm relay shorted the 4-20mA command coming from the temperature controller. If you have a form C relay then you could use one contact to short across the input of the field device and the other contact to open the 4-20mA loop, that way you don't have to worry about the load on the analog output.

Depending on the input impedance of the the valve and the output impedance of your PLC AO you could throw in a small resistor as long as the overall impedance stayed lower than the max impedance of the AO. In theory the 4-20mA AO should be able to deal with a short but I wouldn't bank on all manufacturers building their product that way. The command shorting relay on the vacuum furnace I mentioned was shorting across a very large rack mount Honeywell controller with a voracious power appetite and a power supply to match.

Alaric,

Here is what they are proposing.

Yes, that's the reason I have done this. But Alaric post number 4 show a good method.
I didn't short the field device though. Just open the circuit.

If that is what the customer is proposing then give them that. Their design. Their responsibility. As I said, I've seen it before. As a precaution however I would configure the output for 0-20mA and program it to output 0 mA when ever CR06330 is off and operate in the 4-mA range whenever CR06330 is on.

Mickey and Alaric,

Thank you for your help.

Bob