Analog Input Wiring

coopdegrace · Apr 20, 2016

Hi All,

Probably and easy question for some but I am struggling, so any help appreciated.

I have an AB analog input module number is 1794-IF8IH.

We have a couple of flow meters with 4-20mA HART outputs that we want to read.

Does the analog loop require an external power supply or will it get this from the module? i.e. can I simply connect the +ve and –ve terminal of the flow meter analog out to the analog input channel?

I appreciate any help

Cheers

nonuke · Apr 20, 2016

If your flowmeter is a four-wire current transmitter, you don´t need an external PS.
If your flowmeter is a two-wire current transmitter, you need an external PS.

duckman · Apr 20, 2016

nonuke maybe slightly nuked, as the answer is the opposite of the above.
2 wire the unit will be supplied via the PLC, using just the 2 wires supplying the 24 volt (normally) and reading the current with the same wires.
4 wire will be powered from and external PS, 2 wires for the supply, 2 wires for the current signal back to the PLC.

nonuke · Apr 20, 2016

duckman said:
nonuke maybe slightly nuked, as the answer is the opposite of the above.
2 wire the unit will be supplied via the PLC, using just the 2 wires supplying the 24 volt (normally) and reading the current with the same wires.
4 wire will be powered from and external PS, 2 wires for the supply, 2 wires for the current signal back to the PLC.

So you can understand the 2 wire and 4 wire system. The image is the 1794-IF8IH.

bright676 · Apr 20, 2016

Try this.

nonuke · Apr 20, 2016

bright676 said:
Try this.

Thank you bright676. Same idea but different picture.

This was the question:Does the analog loop require an external power supply or will it get this from the module?

duckman · Apr 20, 2016

Maybe l had some radiation, happy to stand corrected on that module, l new l should have googled that module.

nonuke · Apr 20, 2016

duckman said:
Maybe l had some radiation, happy to stand corrected on that module, l new l should have googled that module.

No problem, it can happen to anyone.

danw · Apr 20, 2016

A Siemens Coriolis flowmeter is a 4 device requiring a separate power supply to make a meaurement.

But it's 4-20mA output can be either 'active', powered by the flowmeter's power supply (4 wire) or 'passive', powered by an external power supply (2 wire), as ordered from the factory (not field selectable).

The model number defines which (of one or more AOs) is which, passive or active.

I've seen other flowmeters that are 4 wire but the AO is passive 2 wire output.

The bottom line is you need to check the flowmeter's documentation to see whether the AO is active or passive.

osmanmom · Apr 20, 2016

OP not mention whether sensor module or transmitter module

mort81 · Apr 20, 2016

danw said:
A Siemens Coriolis flowmeter is a 4 device requiring a separate power supply to make a meaurement.

But it's 4-20mA output can be either 'active', powered by the flowmeter's power supply (4 wire) or 'passive', powered by an external power supply (2 wire), as ordered from the factory (not field selectable).

The model number defines which (of one or more AOs) is which, passive or active.

I've seen other flowmeters that are 4 wire but the AO is passive 2 wire output.

The bottom line is you need to check the flowmeter's documentation to see whether the AO is active or passive.

+1

Endress Hauser flowmeters are the same way, can be source or sink. Need to check the manuals.

coopdegrace · May 11, 2016

Firstly thanks all for your help, much appreciated.

From the 1794-IF8IH manual it tells me the Transmitter can be wired directly to terminals A0 and A1, and power wired to terminal 34(+) and 35(-)

So I do not need to provide power to the loop as it is supplied indirectly via terminals 34 and 35

Have I understood this correctly?

Cheers

Mickey · May 11, 2016

Have I understood this correctly?

No, terminals 34 and 35 power the module. It does not provide power for a loop powered transmitter.

Analog Input Wiring

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