Thanks for your replies!
I looked through the code, and there is no JMPs of any sort of looping in the logic, it's just plain ladder logic with subroutines listed in the file 2 (each subroutine has return and end statements at their ends), contacts, timers, counters, math (only one division with constant, that is 2), scaling and comparison.
Rung 72 in file 2 is the program end. I was partially involved (programed the Quick panels) at the beginning of the project in 1996 (retrofit was done two years ago), and I remember vaguely something very unusual with Quick panels' addressing: the alarms tags were T4:xx.DN, and they worked only after we had added T4:99.DN (Quick panel's way of AB addressing) tag as "alarm enabler".
I've cleared the fault and switch the PLC to run mode, without reloading the program. Next time I go there, I would change the watchdog timer (as far as I can remeber from other PLCs, watchdog timers are usualy less than 500ms).
Regards,
zif