Tag Archives: plc scan time calculation

What is the execution order of the PLC program?

PLC program execution order

(1) The ladder diagram should be programmed with the left bus as the starting point and the right bus as the end point, from left to right, written line by line.

The beginning of each line is the starting condition, consisting of normally open, normally closed contacts or a combination thereof. The rightmost coil is the output. After one line is written, one line is written from top to bottom. Do not write contacts on the right side of the coil


(2) The contacts should be drawn on the horizontal line and cannot be drawn on the vertical branch line.

As shown in the figure, the contact X005 is drawn on the vertical line, which makes it difficult to correctly identify the relationship with other contacts, and the circuit should be rearranged.

(3) When several series circuits are connected in parallel, the branch with the most contacts should be placed at the top of the ladder diagram.

For the case where several parallel circuits are connected in series, the parallel circuit with the most contacts should be placed at the far left of the ladder diagram. This arrangement can make the program simple and clear, and the instruction statement is also less.