Which of the following describes when the CTU's Done bit is set?

• A. When the rung goes from true to false.
• B. When the preset value is cleared.
• C. When the ACC value is greater than or equal to the preset value.
• D. When the rung is false.

Answer: (C)

In a CTU, the Done bit signals that the counter has reached its preset value. The counter increments its accumulator (ACC) on each true CU input, and it typically saturates at the preset value, so once ACC reaches PRE it won’t go higher. That makes ACC >= PRE true exactly when the counter has reached the preset, which is when the Done bit is set. After that point, ACC stays at PRE and DN stays on as long as the rung conditions keep the counter in that state.

So the moment ACC is at least the preset value is the moment DN goes high, which is why that option best describes when the Done bit is set. The other statements don’t describe the CTU’s completion signal: the Done bit isn’t tied to the rung turning false, to clearing the preset, or to the rung being false in general.