Example of a Utility Function Block – to enable bumpless Switch over

Consider the on-delay timer HA_MOD_TON (V2)/ HaModTon (V3).

HaModTon utility function block with internal synchronization

Both PLCs require the same function block called in the program. Under normal operating conditions the elapsed time ET and output, Q of the timer is synchronized internally for both PLCs. ET and Q data are available and can be attached to local or global variables in the program as per application requirements. If PLC A shuts down due to a fault, the primary status switches over to PLC B.

In the event of a switch over, the moment the PLC B becomes the primary, the timer on this PLC will keep running. Till the time of PLC A failure the timer on the PLC B was synchronized, the actual process remains unaffected by the switch over.