1756-IF8 active vs passive

[mavrick]

Can you wire a 1756IF8 analog input card for passive and active instruments at the same time or do they have to be separated. Such as wire 1 card for passive and wire 1 card for active.

Page 61 of the manual is confusing me.

Is differential wiring considered active 4 wired instrument?

 

[OkiePC]

If your device produces up to 10vdc or 20mA of current with respect to the analog input channel common terminal, it will work regardless of what the voltage source is. That is the shortest way I know to say it.

Figuring that out in real life, sometimes means interpreting oddly worded manuals from instrument suppliers. With the 1756 analog input modules that can accept differential or single ended current or voltage signals you get another layer of potential confusion.

It is my practice to use my loop calibrator to check whether the device can produce a reading on the meter before I determine whether I need to supply it with voltage. I then use it to drive a signal into the card to ensure the basic wiring is done correctly. In either case (single ended or differential), for a current input, you need to connect the corresponding channel I-RTN terminals to the same common used by the field device.

So yes, you usually can mix active and passive devices on the same card, it's just a matter of making sure the common reference is wired correctly to match the device on each channel.

 

[cardosocea]

4 wire instruments are usually instruments that require a separate power supply (the additional two wires) and can power the loop themselves.



2 wire instruments require the power to come from the loop, so the card must supply them.



Which manual are you looking at? Page 61 of this one:



https://literature.rockwellautomation.com/idc/groups/literature/documents/td/1756-td002_-en-e.pdf


Is for 8IH card.



If you look at page 48 of the linked manual, you'll notice that the 2 wire device (channel 5) has an external power supply, and Channel 0's device doesn't have that power supply. This tells me (as is usual with other Rockwell cards) that the card doesn't supply the loop and as such you can wire it however you want so long as the reference between the two devices is the same (in this instance the RTN terminal).

 

[mavrick]

https://literature.rockwellautomation.com/idc/groups/literature/documents/um/1756-um009_-en-p.pdf
This is the link to the manual. I misspoke its actually page 65

 

[cardosocea]

What's confusing about that image? Notice the 4 wire sensor has a power supply without a reference to the card and the 2 wire sensor has a power supply with a reference to the card.



So, yes. You can wire the card for both passive and active.

 

[mavrick]

My confusion came from the Rockwell support telling me that it could not be done and that the 4 wire sensor is considered a differential wiring. Which it appears the manual doesn't match what the tech support had to say. That's the reason for the question.

 

[Bit_Bucket_07]

My confusion came from the Rockwell support telling me that it could not be done and that the 4 wire sensor is considered a differential wiring.

That's just flat out wrong.

 

[danw]

The use of terminology like "differential wiring" is likely one of inexperience and lack of understanding the analog world.


I suspect that the intention of the comment, "the 4 wire sensor is considered a differential wiring" was to point to the fact that outputs from 4 wire devices frequently reference a different ground than the receiver leading to ground loop problems. One solution to ground loops is to use a differential input, as opposed to a single ended input, which can help if the common mode voltage between the source and receiver is less than whatever common mode the differential input can handle. A solution with higher probability of success is to use a signal isolator.

 

[mavrick]

So should I be able to use an active 4-20ma transmitter (24+ and com for device power, and 2 wires for 4-20ma signal) with the card wired for single ended current? Or does the card need to be wired for differential current. I'm thinking the latter is correct per wire drawings in the manual. If single ended card can work how would I wire the device to the card? Thanks for the help

 

[Bit_Bucket_07]

So should I be able to use an active 4-20ma transmitter (24+ and com for device power, and 2 wires for 4-20ma signal) with the card wired for single ended current?

Yes. You can do that, so long as you don't have a transmitter that provides a non-isolated output, and frankly, those are rare these days, although they do still exist.

 

[mavrick]

Yes. You can do that, so long as you don't have a transmitter that provides a non-isolated output, and frankly, those are rare these days, although they do still exist.

Bit Bucket would I treat the terminations as a looped powered device at that point and terminate wires as such (+ IN-1 ch.4 and minus I-RTN-1 ch.3)

If that is the case I wish Rockwell did better on there schematics and information. Unfortunately I don't do this work everyday. So I have to dive back into the manuals on occasion.

 

[Bit_Bucket_07]

Bit Bucket would I treat the terminations as a looped powered device at that point and terminate wires as such (+ IN-1 ch.4 and minus I-RTN-1 ch.3)

If that is the case I wish Rockwell did better on there schematics and information. Unfortunately I don't do this work everyday. So I have to dive back into the manuals on occasion.

How you terminate depends upon how you've configured the module. If the IF8 module is configured for single-ended inputs, then you can connect 8 devices, one to each amplifier.

If the IF8 module is configured for differential inputs, then two of the eight amplifiers are dedicated to each channel and you may only connect 4 devices to the module.