slc 5/04 cpu fault and not output card power

Our company has a machine that's running with a SLC 5/04 processor. Sometimes, in the middle of operation, the machine will stop, and the cpu fault light will be steady on, while the battery light blinks. Also, the input cards will still have power, but all the output cards go dead. Sometimes cycling power to the machine will reset whatever problem is occuring, and other times it won't.

We replaced the cpu with a spare, and the same thing happens. We also tried reseating cards, and powering up with the cpu disconnected, then with output cards disconnected. When this fault occurs, communication with the plc can not be established.

I thought there might be something wrong with the backplane, but I really am lost. Anyone have any ideas? Thanks.

quick guess is that you have a ground short on one of your outputs this may be pulling power from your output cards next time it happens try to remove the output wiring to isolate which one may be doing it