1756-L55 Loss of communication

Hi Everyone,

I have a AB 1756-L55 processor that I have went online with and uploaded the program and have went online a couple other times with. I had went to go download some changes that I have made and when I plugged in the rs-232 null modem cable it had auto configured but when I went to go online the processor faulted out before I could. Also the rs-232 light on the processor lit up red. I figured the cable must have went bad on me and got a new one. When I used the new cable It auto configured and let me upload. After the upload about 5 minutes later it said I "lost communication to RS-Linx" and when I checked the processor it had faulted out again. I cycled power on the processor, got green lights and as soon as I plugged in the RS-232 the processor faulted out again. Has anyone had this happen and is there anything else I can check without having to buy a new processor? Thank in advance.

If doing this with a laptop, unplug it from it's AC power and try just using it's battery
power. I have heard of ground problems if the laptop and the PLC have different power sources.

I agree that this sounds a lot like there is current running between the controller ground and the computer ground via the RS-232 cable shell (or Common pin).

The 1756-L55 controllers had their share of memory parity check problems (both real and self-inflicted) but since there seems to be a strong correlation between having the serial cable connected and getting a fault (I'll bet it's stored as a Type 1 Code 60), check the voltage between your PC RS-232 shell and the controller's RS-232 shell (and likewise between the grounds).