PLC Input Question

[pct_jimmy]

I think I have an idea about what is going on but I wanted to ask and see what some of you guys think.

An OEM has installed a Micrologix 1100 with 1768 series IO. One of the contacts on a level sensor is normally open and goes to a 24VDC input card. The input however will not switch but when a 8 pin, 24VDC, relay is connected the coil on it energizes fine. The 24VDC source wire is a #14 and goes to a few devices on a conduit run close to 100', passes through the contact on the sensor and back the 100' on a #16 conductor.

I'm thinking it could be a distance issue and there may be enough voltage drop to affect the PLC input but not the 8 pin relay. I haven't been on site yet (I'm leaving out early in the morning) and wanted to see if anyone has ran into this before. If anyone has any advice I am all ears.

Thanks,

 

[bernie_carlton]

Or perhaps the input is wired as a sourcing type input. We would need a precise part numer for the card and a complete wiring diagram of the card showwing power connections and the point where this sensor should wire.

Or maybe the card is bad.

 

[Mickey]

First,your ML1100 does not support 1768 series I/O. Those are CompactLogix I/O.

The ML1100 supports 1762 series I/O.

Can you tell us the catalog number of the input card?

100' should not be an issue with that size wire.

 

[pct_jimmy]

I should also point out that yesterday is the first time he powered up and was testing IO. This is a brand new system and whatever series module it is its an IB32. He has tested other inputs on the same module successfully. I should also point out that while both the 1768 and 1769 series IO modules are typically used with Compact logix they are compatible with certain Micrologix processors and I may be telling you the wrong CPU series. I only glimpsed his enclosure last week and I'm going off of memory. I'm just looking for other ideas and I've got a four hour drive to think about it.

 

[T Gibbs]

The relay is more apt to not work because of a voltage drop issue. It takes more power to pull in a relay than it does to activate an input point.

 

[BryanG]

Is the level sensor 2 or 3 wire? If 2 then the high input impedance of the PLC input card wouldn't allow enough current to flow for the sensor to operate. The relay however with its lower impedance will.

 

[T Gibbs]

Is the level sensor 2 or 3 wire? If 2 then the high input impedance of the PLC input card wouldn't allow enough current to flow for the sensor to operate. The relay however with its lower impedance will.

That's right. I didn't think about it possibly being a 2 wire level sensor. I just assumed that it was powered separately.

 

[pct_jimmy]

I did consider that possibility but upon my arrival the installer confirmed one of the other thoughts I had but swept to the back of my head, the young man terminating his connections forgot to connect the dccom onto the module.
Thanks for all the suggestions.

 

[thaddiusjude]

Do you have 2 separate power supplies? Are you using the correct 0V common?

 

[nhatsen]

After 7 months, I suppose the OP solved the problem.

Happy New Year

 

[thaddiusjude]

After 7 months, I suppose the OP solved the problem.

Happy New Year

Noticed that after I responded lol. My fault.