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?
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?