analog differential or single ended inputs

[sparky71]

I am wiring four 4-20 mA inputs to a control logix 1759 AI8 module, via IFM module. All four inputs are from 4 wire transmitters, supplying their own power. I have two options,
1. Wire the card as differential analog inputs giving me no spare inputs.
2. Wire the card as single ended inputs, giving me 50% spare capacity.
I am leaning towards option 2, my uncertainty is having the commons all tied together (single ended input). Is this a problem if all the 4-20 transmitters are isolated? I think not but am a little unsure. Thank's for any response. Steve

 

[itsmoked]

Well since 4-20 makes no direct sense to differential are you really converting the 4-20 with a resistor across it?

If yes. I'd go with single ended. Just don't do anything that would prevent you from going to double ended if some hitch comes up.

With your isolated 4-20 I think you're good.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

If the four 4-20 lines are insulated from any potential, including gnd, AND have separate power supplies AND are not used for any other purposes where they may also be connected to gnd or some other potential, then you can use single ended connection to your IFM module.

What you describe seems to be exactly such a system, so just do it.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

Good morning Keith! Yes, of course, a resistor is needed.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Top'O the mornin to you Gunnar!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[sparky71]

Thanks Keith,

The analog current input card has internal 249ohm resisors, so the card does the conversion to 1-5 Volts.

Steve

 

[itsmoked]

Ok thanks.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com