PLC not recognizing 4-20ma signal

[mjmike]

Hello, first post here and have a puzzling issue. I have recently been troubleshooting a PLC for a client that gets a 4-20mA (24VDC) signal from a remote piece of equipment. It appears the PLC stopped recognizing the signal as the PLC channel light just blinks; started about 2 weeks ago. However, when I put my meter in series, I do read a 4-20ma signal. I can see it change based on what we tell the remote equipment to do. I put a good known signal onto the PLC channel and it did recognize it.

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 so I do not believe the wiring is an issue. I even took a piece of speaker wire and put it directly from the equipment to the PLC and no change.

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. So it seems like the PLC is kind of sensing the signal.

It almost seems light the equipment is putting out maybe an intermittent signal or maybe not a good signal somehow but what gets me is I can read a solid 4-20ma signal to the PLC. Anybody ever experience this before or what other tests to perform as I will be back onsite tomorrow?

 

[Steve Bailey]

People might be better able to offer specific suggestions if you give the PLC model and the part number of the analog module.

 

[PLC Pie Guy]

Can you go online with the PLC and see the raw channel data coming in to the card?
Also, what model of PLC and card are you dealing with?

 

[mjmike]

It is an Allen Bradley 1746-NI8 analog input module. I don't think I can go online.

 

[James Mcquade]

mjmike,

The card requires the I/o to be defined and programmed. The card also requires initialization when powering up or going to run mode.

My first thought is that the data file for the card I/o has gotten corrupted and needs to be redone. The second thing is that the card does not get initialized upon power up and going to run mode via the plc code.

We had the data get corrupted on a plc and finally hard coded the data into the registers with a series of move commands.

regards,
james

 

[mjmike]

Well, I would agree with that except for the fact, I can remove the signal loop from the plc that is causing the issue and connect a known working 4-20 loop from another device and the plc starts to work.

 

[KingPresident]

If you can move the problem, you've found the problem. Look again at your original signal loop.

 

[mjmike]

That is what we will be doing, but if I am getting a 4-20ma signal what would cause the PLC to not recognize it? I know the blinking can mean various things, open, out of range, configuration error, but it isn't open, and we are surely in range as the blinking never stops throughout the entire range.

 

[Racer5039]

Sounds like a short pulling the supply down. A clue is the relay sound/dropping out maybe? I agree with PK, you found the problem "area".

 

[mendonsy]

Start with the simple stuff first! Connect the 4-20ma signal that works to the PLC input and measure the voltage across the input terminals. It will typically be something like 1vdc for 4ma up to 5vdc for 20ma. Then try the same test with the one that doesn't work. The results should lead you to the problem.

 

[KingPresident]

Compare your ohm reading on the faulty loop to the known good loop. I've had minute resistance ruin my day...

 

[mjmike]

Thanks for the info. Where do you take the resistance, with the wires disconnected and shorted at 1 end? I did notice the 24VDC did drop to like 2 to 5 VDC when it was connected. Why does the voltage drop when it is connected?

 

[Racer5039]

That is telling you there is a problem with the equipment or the line going to it. Open the connections at the pc of equipment (at that end) and see it it still pulls the 24Vdc down then. If yes then the problem is in the wiring going to it.

 

[mjmike]

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.

 

[plcDenny]

I had a similar issue a couple months back with a 1762 IF4. While disconnected from the plc, the voltage reading was correct and it would drop when connected to the plc. The problem was that I didn't set the DIP switches on the input card to read voltage. It was set to current

Not sure if this helps