When is the DN bit set for the count up instruction tag?

The DN bit, or Done bit, serves a crucial role in programmable controller systems, particularly in relation to counting instructions. When utilizing a count up instruction, the DN bit is set when the accumulated value reaches or exceeds the preset value. This reflects the completion of the counting task since the target set by the preset has been met.

Setting the DN bit in this manner allows the programmer to implement subsequent logic based on the counting operation's status. For example, once the DN bit is true, other parts of the program can respond accordingly, which is vital for effective control system design.

In the context of counting, it's important to recognize that the DN bit indicates a key transition point—when the operation has successfully fulfilled its condition, as specified by the preset. This characteristic streamlines program efficiency, ensuring that operations dependent on the count achieve the intended outcomes seamlessly.