Some PLCs also have retentive timers. A retentive timer (RTO), such as that of Allen- Bradley (Figure 9.15), lets the timer start and stop without resetting the accumulated value. The retentive timer operates the same as a TON timer except that the accumulator is not reset when the timer enable goes to 0 and continues to increment whenever the enable bit goes to 1. When the accumulator bit reaches the preset value, the timer timing
bit goes to 0 and the done bit to 1 and the accumulator bit remains in that state until a reset instruction is received.