Allen Bradley PLC-3 analog values - some change, some frozen

This hasn't become my problem yet, but it may soon...

Ancient PLC-3 (yeah, I know) has been humming along for decades. It now has some analog values that are moving and some have frozen in place. For example a watts transducer is reading 1056 kw when it should read 0.

That's all I know at this point. I've never been online with a PLC-3. I'm wondering if maybe the block transfers for certain modules have stopped executing for some reason.

Posting here on the off chance someone has seen this before.

This issue was a non-issue. It looks like the logic was written to freeze analogs at last known good value when they go out of range.

Thanks for the follow, I doubt there are many 3's still in service, but you never know.
I know of two in NC, but those are the only one's I've seen this Century.

Ken Moore said:

Thanks for the follow, I doubt there are many 3's still in service, but you never know.
I know of two in NC, but those are the only one's I've seen this Century.

Click to expand...

My plant had dozens of them till about 5 years ago.

kbpatton said:

My plant had dozens of them till about 5 years ago.

Click to expand...

Out of curiosity, what did you do with them when you took them out of service?

I have spent many hours programming on a PLC3 as well as PLC2's
i would recommend that you upgrade it soon a failure could shut down the plant
as for the analog module if memory serves they have a failure / open input mode that you set in the module on a failure drive the input reading either full scale or 0 they
it could be up scale or down scale depending on your application
with a power monitor I would expect it be set for up scale
i would check out the power monitor I have have to replace a few of them over the years in the US i would just replace them with a current input.
A power monitor checks both the line voltage and load current an delivers a single for that. but in the US the line voltage is constant within a narrow range and it doesn't change much so a simple load sensor will do the the job at a much better cost