16-MB Keep relays turn themselves off after one scan of ladder

[Shoprag]

This is a strange problem. The machine woke up one morning with all of the keep relays turned off. When I try to set them, they turn themselves off after one scan of the ladder. I can put the addresses in the trace screen and watch them turn on and back off again. This is true for the normal keep relays as well as the system keep relays. This is a very simple ladder. There is nothing in it that writes to the keep relays. This machine uses K14-K19 for the system keeps. No K900's used.

Fanuc sent a tech in who changed out the master board and some I/O modules. They gave up and said to call the machine builder. This is an old machine that has been running fine for years..

I can alter the timers and data registers. The Counters, Keeps and PMC setting screen is a no go.

I pulled the ladder and imported it into a new program (PMC-RB3) using Ladder III. Exporting this to a memory card and reloading made no difference.
I tried a memory clear and SRAM reload. Still no go.
I deleted the ladder and was able to change the keep relays! When I reloaded the ladder the keep relays turned themselves off again.

I'm thinking an SRAM card or PMC control module. Any ideas?

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[zavateandu]

this is a very strange behaviour indeed,i never met it in practice and i am very interested to see the conclusion
if the keeprelays are not set in the PMC indeed is strange
if you clear the SRAM and start the machine with no parameters inside and go to make one random keep relay to 1 it is go back to 0?
also try to create an equation for a random keeprelay using a phyisical input like edit key or one button that can stay on so you can see if ,even if you turn the key on and make the keeprelay in the PMC ,it will go down again?
i will be here to assist you as i am very interested in this topic

[Shoprag]

I added a rung in the ladder that used a push button to turn on coil K17.1. The trace showed the keep relay turning on, but it turned back off on the next scan. If I kept the button pushed in, the keep relay would not change again. Releasing the button and pressing it again resulted in the same behavior. The ladder showed the button activation, but the coil would not stay energized. It was acting like it was inside of a sub routine that wasn't being called. I put this in level one.. I may end up swapping out the keep relays for data registers to get the machine running. It will be a couple of weeks before I can get back to the machine.

I didn't try clearing the memory and setting a keep relay. Deleting the ladder allowed me to set and retain the keep relays. Reloading it wiped out any settings I made.

[zavateandu]

can i have the ladder?

[Shoprag]

It's a very simple ladder.

I'm jumping on a motorcycle and taking a ride for the next two weeks. I'll let you know what I find when I get back.
Let me know if you see anything that I missed!

[Shoprag]

Resolution - Replacing the PMC module in the control solved the problem. Part # A20B-2901-0960
This was a new one for me.

[zavateandu]

yeah,nice to hear ,good point