Okay, let's break down how to wire a 4-20mA PLC output to your telemetry/data recorder. Understanding the nature of the PLC output is key.

Understanding the 4-20mA Current Loop The PLC output is a current source. This is the most crucial point. The PLC actively controls the amount of current (between 4 and 20 milliamps) flowing through the loop. It doesn't provide a voltage in the traditional sense of powering something. It's a loop circuit. The current must flow in a complete circle from the PLC output, through the recorder, and back to the PLC output. Think of it like a one-lane road where the PLC is the gatekeeper, controlling the flow of traffic (current). 4mA represents the minimum value (usually zero) and 20mA represents the maximum value. The values in between represent proportional values. For example, 12mA would represent 50%. The recorder is a load (or a sink) in the circuit. It presents a resistance to the current flow. The PLC has to be able to "push" enough voltage to overcome that resistance and force the desired current through the loop.

Wiring with the Recorder Providing Loop Power (12VDC)

This is generally the preferred and easiest method, as it isolates the PLC power supply from the recorder power supply.1. Configure the Recorder for External Loop Power

Make sure the recorder is configured to use its internal 12VDC loop power supply. The recorder's manual will detail how to do this. Often, it involves setting a DIP switch or a jumper. Look for a setting related to "Loop Power," "2-Wire Transmitter," or "Current Input."2.

Wiring



PLC Output (+): Connect the positive (+) terminal of the PLC's 4-20mA output channel to the positive (+) terminal of the recorder's 4-20mA input. Recorder Output (-): Connect the negative (-) terminal of the recorder's 4-20mA input to the negative (-) terminal of the PLC's 4-20mA output channel.

In other words: PLC (+) to RECORDER (+), RECORDER (-) to PLC (-)

This creates the closed loop circuit. The recorder is now powered and senses the 4-20mA signal.

Important Considerations



Polarity

Incorrect polarity is the most common wiring mistake. Double-check your wiring! Reversing the wires will prevent the loop from working, and could potentially damage the recorder or PLC.

Recorder Input Impedance

The recorder has an input impedance (resistance). The PLC output needs to be able to drive the required current through this impedance. Recorder specifications will provide the impedance details. The PLC documentation will detail the maximum loop impedance. If the total loop impedance is too high, the current loop will not work.

PLC Load Resistance

The PLC output also has a maximum resistance it can drive. The recorder input resistance must be lower than this maximum. If it isn't, then the 4-20mA signal will not work.

Troubleshooting1. Check Wiring

Double, triple-check your wiring for polarity and secure connections.2.

Verify Recorder Configuration

Ensure the recorder is definitely configured to use its internal loop power. If it's expecting an external loop power, it won't work.3.

PLC Output Test

Use a multimeter to measure the PLC's output current. Disconnect the wire from the PLC output going to the recorder. Connect your multimeter in series between the PLC output (+) and the recorder (+). Set your multimeter to measure DC current (mA). The meter becomes part of the loop. Command the PLC to output a specific current (e.g., 12mA).* Verify that the meter reads approximately the commanded current. If not, the PLC output might be faulty. Also, check the PLC configuration to make sure the output is enabled and properly scaled.4.

Recorder Input Test

If the PLC is outputting the correct current, the problem is likely with the recorder. Check the recorder's configuration and input settings. Consult the recorder's manual for troubleshooting guidance.5.

Ground Loops

In some cases, ground loops can cause problems. If you suspect a ground loop, try isolating the signal ground.

Diagram Summary

The key to understanding the diagram is to think about the current flow. It must travel from the PLC output, through the recorder, and back to the PLC output to complete the circuit. Double check all of your wiring.

In Summary

Using the recorder's internal loop power is the easiest solution, but ensure the recorder is properly configured to do so. Double-check your wiring, especially polarity. And finally, measure and test your circuits to ensure they are working properly.Good luck! If you have further questions, please ask! Flag for review