What does the Count Down (CTD) instruction do?

What does the Count Down (CTD) instruction do?

• A. Increases the Accumulated value by one each time the rung containing the instruction goes from false to true.
• B. Increases the Accumulated value by one each time the rung containing the instruction goes from true to false.
• C. Decreases the Accumulated value by one each time the rung containing the instruction goes from false to true.
• D. Decreases the Accumulated value by one each time the rung containing the instruction goes from true to false.

Answer: (C)

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.

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.