PMMI Programmable Logic Controllers (PLC) 1 Practice Test

Count Down decrements the accumulated value by one every time the rung containing it changes from off (false) to on (true). This edge-triggered behavior means the count only updates when the input transitions to true, not just because it’s currently true. The accumulated value starts from a preset value and can continue down toward zero; when it reaches zero, the counter’s done/empty status typically activates, signaling the target count has been reached. In contrast, counting up would increase on the same rising edge, and counting on a falling edge would respond to the input leaving its on state, not its arrival.