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?