4-20mA analog output from DCS

[sat]

Hi,

I need to read three 4-20mA signals from a DCS(ABB) in a remote 6 channel analog input module with RS485 modbus port.

When I connected the three signals individually i can read the raw count values in my AI module,

but if all the three signals connected simultaneously all the three signals stops working in the module.

Is there any alternate way to connect all three signals in a single Analog input module?

Thanks for your help.

 

[Bit_Bucket_07]

Sounds like you need isolation transmitters.

 

[danw]

This is a classic common mode ground loop situation.


The ground reference for the AI circuits in the AI module is at a different potential than the ground reference at the DCS AO. The AI module has a common mode limitation which is not exceeded when a single AO signal is connected but the common mode limitation is exceeded when the all 3 are connected.



In rare cases, floating the DC (-)(removing the connection from (-) to ground) of the power supply powering the AI module (DCS) will solve the problem.


Once and only once, I discovered that the AI needed a ground connection to the AO unit.


The usual approach is to buy 4-20mA isolator modules.