Example of a Utility Function Block - Switch Over

Consider the on-delay timer HA_CS31_TON (refer to HA_CS31_TON - HA Turn-On Delay Timer).

HA_CS31_TON

Both CPUs 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 CPUs. ET and Q data are available and can be attached to local or global variables in the program as per application requirements. If CPU A shuts down due to a fault, the primary status switches over to CPU B.

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

All possible use cases for switch over are described under Functionality.