is she a goner? (SLC-500)

We have an older machine that recently was moved. It uses an SLC-500 (1747-L40C if memory serves). It's connected to a DTAM Micro display for some very simple (unused) editing purposes.

Anyway during the re-install one of the techs hooked up one of the photo eyes a 120VAC hot to the unused normally closed lead due to poor wire labeling. The photo eye was rated for 10-30VDC.

After closing the breaker, It blew out the voltage regulator on the DTAM Micro and took out a completely different eye on the machine. But once I replaced the burned out eye and disconnected the DTAM Micro, everything was all cool.

Then a week later, we started to lose an output. It was half heartedly flickering on and off. I had extra outputs and I know those little relays won't last forever, so I grabbed the laptop to re-assign the point and be on with it.

Two hours later, I gave up. I couldn't get my PCMK pcmcia card to talk to the PLC. Trying 2 other 500's in other identical machines, I was able to connect fine.

I'm guessing when the voltage regulator blew on the DTAM Micro it also zapped something on the SLC's board... I can't find it though. None of the IC's looked or smelled burned and I checked all the fuses and resistors. Nothing looks or smells burned up. My only guess is that it's in one of the IC's

Should I just give up on her? I hate to see an otherwise good PLC go to waste (apart from the bad output, it was doing everything it was supposed to). The only other option I can see is de-soldering and replacing the bad relay output and praying that I never need to monitor the ladder in run-time. That really shouldn't be an issue since the machine has run for damn near ten years without any tinkering in the logic.

So, hello trashcan, or is re-soldering in the burned out output relay worth it?

If you did damage on the backplane, you won't fix it by desoldering the relay output. Best bet is to just replace it if it is critical. If you have time to tinker, tinker away.

I'm not familiar with your hardware. This is based on general electronics, and my experience servicing to the component level.

When 120VAC is connected to the 24 DC line, everything on that buss is destroyed. That's the one guarantee I can give. That's why both photoeyes were blown, along with the DTAM. Because the DTAM's regulator went shorted, 120 was present in the entire device. 120 was also on the communication cable to the PLC.

1) If the relay output module has multiple relays and only one is failing, then the backplane is probably okay. If you loose a backplane line, I expect the entire card to go dead. It also would be dead on day 1, not intermittent 1 week later. Just the contacts on the defective relay got arced. Replacing that relay should be fine.
Was the relay part of the 24DC circuit? If not, then this is an unrelated failure.

2) Is the DTAM on the same port as the programming port? Trace the pins back from the port. The first IC you come to is the bad one. It should be an optoisolator. If so, that's the only bad chip.
Optoisolators provide 4000 volts of isolation. All I/O should have them on a PLC. It is also the norm on the comm ports.
I'm guessing the DTAM was not optoisolated on it's comm port, and that's how it blew the PLC port. If it does have opto's, the the ground line had enough of a spike on it to blow the port.

Thanks for the detailed response. Unfortunately the SLC-500 is an 'all in one' fixed I/O controller. The machine is not super critical to our operation and may be retired soon, so I hate to buy a whole new PLC over this... I have an extra unit that I was saving for another project running it right now.

I'm thinking I might just swap a couple of the relays on the circuit board and see if that gets her back up. Everything else functioned as it was supposed to other than the com port. And in this one case, I'm not too worried about the com port because if absolutely necessary, I could always take the memory module out of it, slap it in another controller, make the changes and replace it.

Anyway, thanks again. You gave me some awesome info.

MH,
I suggest that if you do have to replace it you use a ML1000 (or ML1200). You'll need to mount the ML1000 upside down because the input/output wiring is on the opposite sides from the fixed SLC, but the ML1K wont care. The ML1K is going to be around $400, a lot less money than a fixed IO SLC.

That's a good tip... I hadn't even thought of using a micrologix in it's place. As long as the first point can be used as a high speed counter for the encoder, I'd be set. Thanks Alaric.