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Old August 17th, 2019, 12:20 PM   #1
AutomationTechBrian
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SLC500 (pic)- Output sticks?

I got a call from a guy who says the output on this PLC sticks closed after the output light goes out. It's a couple of hours drive, so I've been thinking about what he told me the imagine the possibilities.

I'm somewhat skeptical that an output sticks closed after the output light goes out, but I've seen stranger things. The output is wired to a contactor coil. He says he noticed it's more likely to happen when the machine is warm, than cold. He also said he just recently replaced the contactors, and that the previous contactor looked like it was "original". He threw the previous contactor away... I guess it was pretty beat up.

I remember a conversation I had in the past about the use of pull-up and pull-down resistors, but I'm not strong enough with my electronics to recall the details... just that they exist. I'm also thinking about ways to verify the issue. The wire has 110AC. A question for the AB experts... will my RS Logix 500 work on this PLC? If so, will my connector kit I got from TW Controls (Tim) have the right cable? I've never encountered a straight "500" before.
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Old August 17th, 2019, 01:04 PM   #2
Ken Roach
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The old 1747-L30 controllers are the original "Fixed I/O SLC-500" controller. They're most similar to the SLC-5/01 or 5/02; they have only a Data Highway 485 port so you need a 1747-UIC or equivalent.

Any edition of RSLogix 500 (except the MicroLogix-only "lite" edition) will connect to allow upload/download and monitoring. The fixed SLCs did not support online editing.

The heyday of these controllers was the late 1980's, so it's very likely that both the controller and that contactor were around 30 years old.

You can see the "RELAY" label on the outputs, so the outputs are genuinely dry relay contacts, not AC solid state triacs.

A common mode of failure for relays that run inductive loads like a contactor coil is that the back-EMF generated when they open the contacts generates a small spark. Over the years that erodes away the contact surface or eventually "welds" or "sticks" the contact closed.

"Snubber" circuits are used to limit the back-EMF spike, but they don't eliminate it entirely (and they can fail or get disconnected too).

One approach is to look for a spare unused output and edit the program to use it instead of the failed one. OUT8 and OUT9 appear to be unused.

But the best recommendation is to get a MicroLogix 1400 and migrate the program.
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Old August 17th, 2019, 01:26 PM   #3
Ron Beaufort
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a little more detail on what Ken has said ...

Quote:
they have only a Data Highway 485 port so you need a 1747-UIC or equivalent.
that's correct - but here's a "gotcha" ready to bite the unwary ...

WARNING! ... the communications port on that unit will look a lot like an Ethernet connection ... do NOT under any circumstances connect your computer's Ethernet port to this socket ... some of the pins in there have 24 volts DC on them - and you can easily destroy your computer's Ethernet connection ...
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Old August 17th, 2019, 01:30 PM   #4
Ronnie Sullivan
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If its 30 years old, just change the output in the program to a spare one.
They'll call you in 30 years when it goes again.
I won't be available then.
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Old August 17th, 2019, 11:23 PM   #5
AutomationTechBrian
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I had a feeling Ken was going to chime in on this post! Thanks, Ken! I sent a text over to the guy today and let him know the situation. I said as long as the PLC didn't have a password, I could re-assign the output to an open terminal. He said that the problem seems to have gone away, but he'll keep that in mind if it returns.

I get a little confused on the DH connections with Allen Bradley. I have this cable that I got from Tim (picture below). It seems that I've been using this a lot lately... probably 3 times in the last 6 months. I get confused with the connection that needs the spendy converter... DH+? ...Right? That one the customer is going to have to buy, should I need it. I almost wish someone would need it. It's the last piece of the SLC communication puzzle for me (except Controlnet, which I've almost never seen).
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