GE 90-30 ALW_ON is not ON

The Alw_On bit on my 90-30 plc is not on anymore. What causes this bit to turn off? How do I fix this problem? Thanks!

If you're online and Equal with the PLC and the PLC is running, the ALW_ON bit should be true. Have you verified it as being off by looking in the reference view table? Its address is %S0007.

I have verified the bit in the reference view. %S0007 is a 0 when online with the PLC. When I verify the program it says everything is equal with no mismatches. I don't have a lot of experience with the 90-30. Is there anything else I can check or verify that would keep that bit from being a 1?

I don't know if this is even possible in the 90-30 but could something have overwritten this bit?

Is the PLC in Run mode?

I just checked with Logicmaster and a CPU363 and there was no way I could turn off the %S0007 bit. When I stopped the PLC, the bit remained on. I tried to override it and Logicmaster wouldn't allow overrides to %S memory. I tried to program a rung with a -(R)- coil addressed to address %S0007 with the same result. I tried using a MOV_BIT instruction with no luck.

I found that it is possible to turn on or off %SA, %SB and %SC memory bits, so it may have been possible to control %S memory in earlier firmware versions or with some software other than Logicmaster. I haven't tried with VersaPro or Machine Edition.

I suggest going online with your CPU and trying to toggle the bit on. If that doesn't work, try clearing the PLC memory and see if the bit comes back on. If it does, try downloading your program again. If it doesn't come back on after clearing the memory, try reflashing the CPU if its the type that has flash memory. Otherwise you may have to replace the CPU.