I had an old Series 90-20 mysteriously go out of run mode last night. I have never worked with the GE before but could not find a run/program switch of any kind. Luckily we had a spare programmed and ready to go. We installed it, powered it on and it went right to run mode and we were good to go. How would it have been taken out of run mode? How do you even put these things into to run mode? Thanks for your help.

JS

To start a 90-20 running if it has stopped, you need either a hand-held programmer or Logicmaster programming software. It can be configured to try to start running at power-up, but the default configuration is to do the same thing it was doing when power went down. If it was running, it will power-up running. If it was stopped, it will power-up stopped.

That's an old unit. GE Fanuc stopped selling it more than a decade ago. You may want to consider replacing it with something more readily available.

Yes I know that this is a very old unit. We will be replacing shortly with an AB. I was just trying to figure out what might have caused this from happening. The 90-20 should power up in run mode or at least it has been for the past 16 years. Not sure why it would of suddenly stopped unless something went bad inside. Since I don't have the programmer or software I guess we'll never know. Oh well, thanks for your time.

JS

There are probably any number of potential causes, power instability being the most likely. That's not to say that your plant power is unstable. It could be that some component in the PLC's power supply is on the verge of failure. After 16 years of operation, dust may have built up on circuit boards leading to hot spots. There is probably a clue to what caused the shutdown written into the fault table. If the battery is still connected (and still good), someone with the software might be able to find out for you.