Lose Analog Loop without PLC Power?

crimson75 · May 9, 2016

If I connect a 4-20mA output from say a magnetic flowmeter (4-wire device) to a PLC AI module (CompactLogix) and to a digital display on the door in a loop and the PLC loses power will the digital display on the door read whatever value corresponds to 0.0mA?

i.e.: Does the loop get broken when the PLC AI module is no longer powered?

Thanks all.

Firejo · May 9, 2016

That or the CompactLogix analog inputs are going to ground when the it's not powered up.

crimson75 · May 9, 2016

Meaning we would lose the display on the front panel door if the PLC lost power?

shooter · May 9, 2016

if the loop is powered from separate power(this is recommendable), so the instrument will have power, the display will work too.
please check the maximum resistance of the unit. some display take 10 Volts.

John Soltesz · May 9, 2016

crimson75 said:
If I connect a 4-20mA output from say a magnetic flowmeter (4-wire device) to a PLC AI module (CompactLogix) and to a digital display on the door in a loop and the PLC loses power will the digital display on the door read whatever value corresponds to 0.0mA?

i.e.: Does the loop get broken when the PLC AI module is no longer powered?

Thanks all.

If the flowmeter is powered by a different source (true 4-wire device, 120vac, 24vdc...) and the input card is a 1769-IF4I you should not loose the loop signal to an external meter. The card has a 249 ohm input impedance internally. Check to see if the flowmeter signal is wired to Chx+ and (Chx_iRtn jumpered to Chx-).

shooter · May 11, 2016

249 is only 5 Volts so external meter is possible. when power supply is > 10 Volts. it normally is 24.

Lose Analog Loop without PLC Power?

crimson75

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Firejo

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crimson75

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shooter

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John Soltesz

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shooter

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