PLC5-40 suspected fault

[gromit 4]

While monitoring a PLC5/40 with RSLogix5 an incident occurred that caused the entire Demin water treatment process to trip offline. The only indication was the appearance of a popup error window that stated "Loss of Communication" After the process was assessed via the CiTect HMI, the ladder logic was monitored via RSLogix5,on the same PC, it was discovered that the process/program was running and communication established without any manual intervention. The operator spent the next hour starting up equipment and aligning valves and controllers. Once I could get back on the PC, I looked at the processor status, but saw no current faults. One thing that I noticed is the maximum scan time was 505 ms, which exceeded the 500 ms watchdog threshold. The current scan time ranged from 25 to 30 ms. When I looked in the data table of a previously backed up program, the maximum scan time indicated 505 ms as well. I thought that this could have cause a processor fault, until I noticed that the data table of a backup program indicated the same max scan time. Anyway what is the safest way to reset the maximum scan time to allow it to be captured during a subsequent event? If indeed the watchdog threshold was exceeded causing the processor to fault, would the processor recover from it and continue to process without manual intervention? What could be some other probable causes of the event described? Could monitoring the ladder logic via RSLogix5 from the same PC that is operating CiTect HMI software caue the processor to fault or scan time to increase dramatically, say from 30 ms to over 500 ms? What is the best way to safeguard this from happening in the future?

[Conor 4]

Hi Gromit, I just searched for this on the tech support website and it says that Last Scan S:8 and Max Scan S:9 can be reset by putting a zero into these bits via RSLogix5 software. The article number on tech support is 61150. I had checked the Enhanced user manual, 1785-UM012D-EN-P, but couldn't find anything in that about resetting. Conor Edited 9 Oct 2013 by Conor

[Mickey 71]

A little more on the above.

[gromit 4]

thanks