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We are still facing a problem regarding communication loss in PLC-5/40L with 1771-ASB Adaptors (8 chassis and with single slot addressing we have racks 2 to 17 in Remote IOs) as described below:

NATURE OF PROBLEM

1 - The systems goes to faulted state frequently

2 - The major error code we receive is S:12=83 “Watchdog timer error”. And the bits S11 gets status 0000-0001-0000-0000 (this indicates only watchdog timer error)

3 - All the Remote IO communication module loss communication as soon as this fault occurs.

4 - This fault is removed as soon as we recycle the power.

TROUBLESHOOTING ACTIVITIES DONE:
1 - The scan time of PLC-5 was increased from 500ms to maximum (due to this, frequency of fault has decreased but it’s still there).

2 - The fault routine was developed in which as soon as this fault occurs the system goes to this fault routine and find the status of error. If error code matches the fault code 83.

The system removes the fault. Due to this, now as soon as this fault comes, the PLC doesn’t go in fault state but the Remote IOs goes in non-responding state again. This is recovered at once only by recycling power.

3 - All the Remote IO cables were checked and if there was some error in cables then the system must not get OK at once after power up.

4 - The configuration of Remote IO adaptors is changed from manual startup to Auto Startup after fault.

But still the problem is same. Is there any remedy for it?

Assuming it's over DH+, have you checked over the run length of the cable and the terminating resistors. Anything like node addresses? Just a first few simple things to check.

While the system is operating, check the number of retries in the IO statusfile to see if there are abnormal retries on one chassis. Watchdog covers application program as well as housekeeping so I/O scan is part of that.  I have seen issues when the PLC is the middle of the IO chain. Separate the racks onto 2 different channels? May want to set the baud rate back to 115K or 57.6. Could be a bad adapter module?Cheers from: Jeremy Pollard, CETThe Caring Canuckian!www[.]tsuonline.comControl Design  www[.]controldesign.com Manufacturing Automation  www[.]automationmag.com3 Red Pine Court, RR# 2Shanty Bay, Ontario  L0L 2L0705.739.7155  Cell # 705.725.3579