PLC not recognizing 4-20ma signal

[bernie_carlton]

If the module input is set for current I would expect the channel voltage to drop to whatever is needed to drive the current through the input resistance. The card specifications don't seem to show the input resistance when used in current mode. They only show the voltage input resistance (1 M Ohm).

 

[ndzied1]

Just to cover all the bases here, it sounds like this is a system that has been working for a while before this incident. Is that true?

Assming so, then you have to put on your Sherlock Holms hat and find out what changed. If you haven't been around the machine and were just called in to troubleshoot, then you need to talk to the regular operators and maintenance people and get some history. They can't tell you part XYZ is bad but getting information about how the machine regularly operates and especially how it was operating up to the time you got called in is going to help you find the issue.

Has anyone done any welding on the machine lately? We had a customer do that and fry a controller. Did the plant take a lightning strike? That has also been known to make random electronics go flakey.

Did someone change the transmitter or ananlog input card?

Have any features been added to the machine lately? We have had people add things to machines and tap off an existing power supply not realizing that the supplies are not unlimited so they start getting errors when the voltage drops.

Can you verify tha the PLC program is runnig through lights or performing some manual functions on the machine that can only work if the PLC works?

For sure, if it was working before in production, you know something changed. It could be something that seems unrelated but your job is to find it. Of course, the hardest things to find are components that are dying slowly so you can't discount those as well but you should also branch out a little from the immediate objects at hand to make sure you are not missing some ancillary cause.

 

[Bit_Bucket_07]

With the wires disconnected at the PLC, I read 24VDC. When connected to the PLC, reading across the terminals, the voltage drops to like 2 to 5 VDC.

That's normal. You'll read the loop excitation power supply voltage on an open circuit. You're reading the voltage drop across the analog input channel's resistor when the current loop is connected to the input.

Sounds like you may have a ground loop problem with your instrument. Try reading the signal from the instrument while it is disconnected from the analog input. Then attach the negative wire to ground and see if your meter reading changes. If it does, then replace the damaged instrument.

 

[Bit_Bucket_07]

The other thing I noticed, is when I do connect the wiring to the PLC, even though the channel light continues to blink and does not go solid, I do hear a relay click that is controlled by a different channel.

There are no relays in an analog input module.

 

[danw]

> I put a good known signal onto the PLC channel and it did recognize it.

If the known good source is battery powered, like a calibrator, then the fault is likely be a ground loop, because a battery powered device is not grounded. A ground loop can be caused by wiring or by a faulty instrument.

If it was a signal from a device powered by the same power supply as the 'faulty' signal, then there is no issue with the power supply, it can successfully power the 'known good' device.

>The shielded conductors go underground to the equipment about 150 feet away. I shorted out the wiring at 1 end and read about 1.5 ohms resistance

What is the resistance from each wire to ground?

Is any of the wiring outdoors? Check the remote instrument and any junction box for water leakage, which will cause a ground loop. Dry the water up, and check again for operation. Repair the seals.

Underground wiring in conduit tends to collect water. Any nick in a cable's insulation will allow water to conduct to a ground point. I once lay a stretch of cable on the ground which solved a similar problem, proving the underground cable/wiring was faulty.

> I did notice the 24VDC did drop to like 2 to 5 VDC when it was connected.

As others have noted, the voltage across the analog input should drop. The question is, what is the voltage across the power supply itself?

 

[Bit_Bucket_07]

> I put a good known signal onto the PLC channel and it did recognize it.

If the known good source is battery powered, like a calibrator, then the fault is likely be a ground loop, because a battery powered device is not grounded. A ground loop can be caused by wiring or by a faulty instrument.

If it was a signal from a device powered by the same power supply as the 'faulty' signal, then there is no issue with the power supply, it can successfully power the 'known good' device.

>The shielded conductors go underground to the equipment about 150 feet away. I shorted out the wiring at 1 end and read about 1.5 ohms resistance

What is the resistance from each wire to ground?

Is any of the wiring outdoors? Check the remote instrument and any junction box for water leakage, which will cause a ground loop. Dry the water up, and check again for operation. Repair the seals.

Underground wiring in conduit tends to collect water. Any nick in a cable's insulation will allow water to conduct to a ground point. I once lay a stretch of cable on the ground which solved a similar problem, proving the underground cable/wiring was faulty.

> I did notice the 24VDC did drop to like 2 to 5 VDC when it was connected.

As others have noted, the voltage across the analog input should drop. The question is, what is the voltage across the power supply itself?

If you are dealing with a two-wire transmitter, and assuming a (rare) perfectly calibrated loop excitation PS voltage, then If the current is 4 mA, the voltage drop at the AI will be 1 VDC, and the voltage drop across the transmitter should be 23 VDC. The PS should always read 24VDC.

 

[mjmike]

follow-up: The problem was the equipment. There was some issue with a simple circuit board in it. Once the small board was replaced, the PLC accepted the signal. Not sure what the issue with the board was, but something went bad on it.