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Is the order of transmitter and receiver connection in the current loop important? I have +24VDC power supply unit, PLC with current analog input and the sensor (for test case it's just 2.35kOhm resistor). If I connect it in the typical way (first diagram: PSU -> sensor -> PLC AIN) then everything works well (output = 9.2mA). But when I swap PLC AIN and the sensor (second diagram) then the output is invalid (-0.4mA)

schematic diagram of test loop

Why is that happen? Theoretically current AIN of the PLC is just another resistor (about 250Ohm) in series in the circuit so I thought both connections should work.

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    \$\begingroup\$ is the PSU fully floating, or does it share an unintended unnoticed GND connection with either of the other elements in the loop? \$\endgroup\$
    – Neil_UK
    Commented May 25, 2022 at 11:19
  • \$\begingroup\$ Is the +24V source the same that is used to power the PLC? If yes, than the 4-20mA input of your PLC is probably GND-Referenced and not floating. \$\endgroup\$
    – kruemi
    Commented May 25, 2022 at 11:35
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    \$\begingroup\$ @Neil_UK no, it's the same PSU that powers PLC, so there might be some unnoticed GND connection. If I use another, separated PSU then the measure is accurate. \$\endgroup\$
    – Maciej
    Commented May 25, 2022 at 12:41
  • \$\begingroup\$ @kruemi please see my answer to Neil_UK \$\endgroup\$
    – Maciej
    Commented May 25, 2022 at 12:42
  • \$\begingroup\$ You might have your answer there, then. I would have expected a PLC current loop input to be isolated, for just this reason, but maybe not, , maybe they expect you to use isolated power, or maybe the PLC also has a loop exciting power output that they expect you to use. \$\endgroup\$
    – Neil_UK
    Commented May 25, 2022 at 12:50

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As you are using the same power supply for the PLC and the sensor there is a possibility that the negative input of the ADC for the 4-20mA interface has a connection to GND.

It might be written in the documentation of the PLC or the analogue Module you're using (but it's not always).

I know from experience that the 4-20mA input of a Siemens Logo!8 AM2 Module works even if the - of the analog in is not connected (despite no reference to that fact in the documentation).

If your analog input is not galvanically isolated (look at the datasheet/decumentation) it's probably GND referenced in some way.

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