When is the 'done' status bit set in a count up counter?

The 'done' status bit in a count up counter is set when the accumulated value equals the preset value. This represents the condition where the counter has successfully counted up to the defined target, indicating that the counting operation is complete. At this point, the counter has fulfilled its intended task of reaching a specific count, and the 'done' bit serves as a signal that the counter has achieved its goal.

This functionality is crucial for understanding how PLCs manage counting operations within control processes. It allows other parts of the program to react or trigger subsequent actions once the count is complete. When the accumulated value is less than the preset value or the counter is reset, the 'done' bit would not be set as the counting process is still ongoing or has been interrupted. Additionally, a count up counter may have a maximum limit, but reaching that limit does not automatically signify the counting operation's completion in the context of a preset target; it signifies a different condition that may require separate handling.