Friday afternoon the managed switch in the MCC died. Six PowerFlex 525s on the accumulation line, all on EtherNet/IP off a CompactLogix 5380. Five of them dropped out and stopped, the way they should. Number four kept going at its last speed for a good minute. Operator hit stop on the PanelView and nothing happened. He hit the e-stop and it coasted down.

Switch is swapped, everything talks again, line's running. I went online and compared the logic first, and drive four is the same AOI as the other five, same rungs, and the HMI stop button writes the same tag on all six.

So is the HMI at fault here, or the program? Same code on six drives and one of them behaves different. Is there something in the drive itself that decides what it does when the comms go, rather than anything in my code?