Evaluation of Analog Input cards


Thread Starter

Dan B.


I have been asked to evaluate two different analog input cards, for a possible cost savings.
Let me discribe our current layout:
At our plant, we have a few hundred SCR power
controllers, which we connect to a PLC to do
either power PI control or temperature PI control. These SCRs have voltage, current and power outputs, 0-10 VDC, which we currently wire to our PLCs via a differential isolated analog input cards. The SCRs may be located in different cabinets, and it may also be fed by one of the three different phases of power. These analog outputs are supposed to be isolated, thus I am considering that I don't have to have the PLCs analog inputs also to be isolated.

My concerns are that if I moved towards the non-isolated input cards, if something goes wrong with the SCRs' AO, it will affect all the other process signal and might bring the whole process down. Is there any standards or practices that covers this area?

The second concern is that the 8 ch input card, being a differential input, has a very high noise rejection, and the 16 ch input card, being
single ended, practically has no noise rejection. Our process is very noisy, with all these SCRs. I can't add (software) filters to these inputs, due to the inherit lag intorduced by these filters.

The only plus side to this migration is cost.
The 16 ch input cards are roughly half the cost/ channel of the 8 ch input cards. By multiplying this by 100 * 3, you can see how quickly
the cost adds up.

Thanks for any suggestions, inputs or comment.

Dan B.
Hi Dan,

I have successfully used Action Instruments loop powered analog I/O isolators (model Q500) in our process. I was able to bring the differential outputs from the instrumentation into the inputs of the Action module, then go single ended from the output of the action module to the PLC input card. It has been working for over a year now, no problems so far. "http://www.actionio.com":http://www.actionio.com


Hakan Ozevin

Dear Dan,
You are completely right about your concerns. However this is a performance/price problem.
Of course the isolation of the analog inputs are important and more expensive than non-isolated ones, but there is no standard about it. You, as an engineer, will decide if it worths or not for your process. Remember that you will be asked if the whole process goes down "why?", but if not "was such an investment necessary?" That is what is expected as a decision from an engineer. I have read something from this site from a technician who was complaining about not having enough responsibility as an engineer. That is the point where technicians and engineers distinguish.