1746-NR4 Powering off the PLC


Thread Starter


Hello All,

New member here with a new problem.

I have an 1747-L532 with a 13 slot rack a P4 PSU 1 I/p module 1 O/P module and a 1746-NR4.

The PLC would intermittently die. I mean no lights on the PSU the CPU or the modules. 120V OK. No load on the PSU 24VDC.

Actions so far. Changed the CPU, both digital I/O Modules the Rack.
All to no avail. Re-did all the grounding. Still the same. I didn't have a spare NR4 so I decided to remove it to see and it did not fail. (It was getting very frequent, like 4 times a day). I put it back in without the Sensors connected and it failed. I ordered another used one and as soon as I put it in it failed. I tried another slot closer to the CPU and it failed immediately.

The PLC ran for 3 days without fail while I awaited the new one.

When I reset the power it stays dead and eventually comes back on.
When the CPU powers back up the ladder has gone. no fault messages.
I can power off manually and the ladder is always there.

I'm pulling my hair out here. I hope someone has some ideas.

Many Thanks,

Bob Peterson

This has the feel of a bad power supply. SLCs are somewhat infamous for strange and intermittent symptoms like this caused either by racks or power supplies that have gone flaky.
Check the simplest things first. It looks to me as if the power supply module has an intermittent problem. You should replace this with a new spare. Another possibility, is that, given the amount of I/O modules, the PS might be short of power. There are 3 models for this: 1746-P1, 1746-P2 and 1746-P4, the P4 being the highest current available for a SLC system. Follow the installation instructions closely to set the input voltage jumper correctly. It won't affect to use a higher current PS than the one in use, but not the opposite.
I forgot to say, I started with a P1 and an A10 and now have a P4 and an A13. Seems weird to have the same fault but not impossible. Both are used.

Also runs fine without the NR4...

I understand exactly what you are saying but 3 modules + the CPU on a 13 slot rack is not a lot. I had a P1 PSU which I replaced with a P4.

Everything is great until I plug in the newer NT4 then it dies. My original NT4 can take up to a few hours to die.
I can add another 4 digital modules without issue on loading.