Weird PLC...

Hi!

I have two Mitsubishi A1S plcs operating two identical machines, and the program driving the plcs is almost identical (only the I/O numbers change and a few minor details related to them). My problem appeared recently, when I discovered that one of the data registers contained exclusively "ones", printing 4294967296 on the reports. This should NOT happen, as the registers are supposed to be resetted in the program. The only thing I could do to get the register to reset is to actually reset the whole plc by pressing the reset button (I imagine cutting off the power would have worked too...). After doing so, the program worked correctly.

This problem affected only one of the two plcs, and I have no idea what could have produced it. The plcs are a little old and are operated in a quite rugged environment...

I am not quite a plc expert and I would really appreciate if one of you guys had an idea of the potential cause of the problem...

Thanks!

Noise... That's my best guess. If you've got a communication cable connected to the PLC (serial, ethernet, devicenet, etc) then start by checking that it is shielded, the shield is grounded on only one end to an actual ground, and that any noise generating device (high voltage AC lines, etc) aren't anywhere near the comm cable.