AB analog card 1756-IF16 wiring

dbozic · Jul 1, 2015

Hello,

I dont have any experience in commissioning AB PLC equipment so If anyone could help me with the following questions it would be great.

I have attached a wiring documentation of the analog input module(one in single wiring and one in differential). We have 2-wire(loop powered) and 4-wire(own 24VDC supply) 4-20 mA instruments wired as in attachment.

I have read somewhere that 4-wire instruments should be wired in differential wiring mode. Is that correct?
Also because of the 2 transmitters on the card, I get 8 input card. That is why there is 2 pressure transmitters (2-wire) in differential wiring mode.

Does the attached diagram show the proper wiring?

Thanks!

Bit_Bucket_07 · Jul 1, 2015

dbozic said:
I have read somewhere that 4-wire instruments should be wired in differential wiring mode. Is that correct?

No. That is incorrect. Whether an instrument is loop powered (Two Wire) or provides on-board loop excitation power (Three Wire or Four Wire), has no relation to whether one would choose differential over single ended connection to an analog input module.

If you use differential inputs, then each input is capable of measuring a range of positive or negative voltage on the input. For instance, if you had an instrument whose output is -10VDC to +10VDC, or -20mADC to +20mADC, then you'd need to connect it to a differential input. If, on the other hand, your instrument outputs 4mADC to 20mADC, or 0VDC to 10VDC, then you may measure that signal with a single ended analog input.

crashjr · Jul 1, 2015

Is there not some 4 wire applications that output 4-20ma that will only work correctly on differential input? I think we run into a problem like this with some flex core transducers.

dbozic · Jul 1, 2015

Bit_Bucket_07 said:
No. That is incorrect. Whether an instrument is loop powered (Two Wire) or provides on-board loop excitation power (Three Wire or Four Wire), has no relation to whether one would choose differential over single ended connection to an analog input module.

If you use differential inputs, then each input is capable of measuring a range of positive or negative voltage on the input. For instance, if you had an instrument whose output is -10VDC to +10VDC, or -20mADC to +20mADC, then you'd need to connect it to a differential input. If, on the other hand, your instrument outputs 4mADC to 20mADC, or 0VDC to 10VDC, then you may measure that signal with a single ended analog input.

Thank you for your reply! Much appreciated. So there is not a problem if 2-wire transmitter is connected in differential as I have understood?

crashjr said:
Is there not some 4 wire applications that output 4-20ma that will only work correctly on differential input? I think we run into a problem like this with some flex core transducers.

Yes, I have read this also on some topics so decided to wire it as differential.

Bit_Bucket_07 · Jul 1, 2015

crashjr said:
Is there not some 4 wire applications that output 4-20ma that will only work correctly on differential input? I think we run into a problem like this with some flex core transducers.

On most PLC analog input modules, it is possible to achieve higher A/D numerical resolution with a differential input than with a single ended input. Most applications, however, don't require such enhanced resolution, and using differential inputs unnecessarily is merely a waste of IO slots and money.

At any rate, Two Wire vs. Four Wire is irrelevant to the discussion.

AB analog card 1756-IF16 wiring

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