4-20 I/O Modules

AD@brady · Apr 27, 2021

Hello there PLC Experts,

Quick question for you.

Does a Transmitter with a 4-20ma output need a PLC module with HART capabilities or just any analog module would do?
We currently have a 1756-IF16 Voltage/Current Analog Input Module from AB and it says its input range is 0-20ma.
Maybe all I need is to scale it in the program?
IDK.

Thanks

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geniusintraining · Apr 27, 2021

You need to configure it, see here http://bryceautomation.com/index.php/2017/06/01/controllogix-analog/

AD@brady · Apr 27, 2021

Thank you for the reply.

One thing that I see different on my configuration is that I can only choose 0-20mA and not 4-20mA.

Tom Jenkins · Apr 27, 2021

Note that all Hart includes 4-20 mA but most 4-20 mA signals do not include Hart.

ASF · Apr 27, 2021

AD@brady said:
Thank you for the reply.

One thing that I see different on my configuration is that I can only choose 0-20mA and not 4-20mA.

Scale it 0-20mA and then scale accordingly. For example, if 4-20mA = 0-100%, then 0-20mA = -25-100%.

jonney · Apr 27, 2021

You can use a Voltage converter. Convert to 0-5VDC then output to Analog input card.

ASF · Apr 27, 2021

jonney said:
You can use a Voltage converter. Convert to 0-5VDC then output to Analog input card.

Why? The device is 4-20mA, the card supports 4-20mA, why introduce an additional device?

jonney · Apr 27, 2021

ASF said:
Why? The device is 4-20mA, the card supports 4-20mA, why introduce an additional device?

ASF · Apr 27, 2021

Yes, the module also supports 0-5V, but the transmitter is 4-20mA. Why go to the trouble of converting a 4-20mA signal to 0-5V when you can feed it directly in as 4-20mA without any additional hardware or wiring?

Nova5 · Apr 28, 2021

jonney said:

That doesn't answer WHY should one introduce a middle man when one is not needed. Plus voltage mode has voltage drop issues on long runs. current mode doesn't care nearly as much.

JesperMP · Apr 28, 2021

You can feed a 4-20 mA signal to a 0-20 mA input.
You merely have to take the offset into account in the scaling to engineering values function in the PLC program.

That is what ASF explains in post #5.

BinderNut · Apr 28, 2021

Here is an example of how we have our 1756-IA* cards.
Use the 0-20mA setting and set your desired scaling your engineering units. (This particular one is a temperature probe, and we have it scaled to match the transmitter puck default scaling. But for something like a valve position input we'll typically scale the input 4-20mA = 0-100%).

If your transmitter is Hart-capable and you want to use it, then you will need a Hart card. If all you want to use is the single-value analog, then your -IA16 will be just fine.

4-20 I/O Modules

AD@brady

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geniusintraining

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AD@brady

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Tom Jenkins

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ASF

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jonney

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ASF

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jonney

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ASF

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Nova5

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JesperMP

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BinderNut

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