Which bit is set when the CTU instruction is activated?

When the CTU (Count Up) instruction is activated in programmable controller systems, the DN (Done) bit is set to indicate that the counter has reached its preset value. This bit serves as a flag that signals the completion of the counting operation, providing feedback to the controlling program about the state of the counter.

Setting the DN bit upon activation of the CTU instruction allows the program to make decisions based on whether the counting operation has been completed. This is particularly useful in control applications where certain actions are to be triggered after a specific event or count is reached, thus providing a clear and effective way of managing and responding to process conditions.

In this context, the other bits mentioned serve different purposes. The RESET bit would be used to reset the counter, the PRESET bit is typically involved in defining the maximum count that the counter should achieve, and the CTD (Count Down) bit pertains to counting down operations, which are not related to the CTU instruction directly. Therefore, the activation of the CTU instruction specifically aligns with setting the DN bit to represent the successful completion of a counting task.