A retentive timer works like an ON-delay timer with one difference.
That is, when its rung condition changes from true to false, the timer simply stops timing, but its Accumulate value is not reset to zero.
Retentive timer
When its rung condition goes from false to true again, the retentive timer’s Accumulate value counts up from where it stopped the last time.
To reset a retentive timer’s Accumulate value to zero, a reset instruction with the same timer address must be used.
A reset instruction is a controlled instruction, which means it must be located next to the right rail in a rung.
When its rung is true, the reset instruction becomes enabled and resets the timer according to the address that is assigned to the reset instruction.