FAQ04305 of Programmable Relays FAQ

With the following ladder program, the interlock circuit will operate normally if the reverse (or forward) switch turns ON during forward (or reverse) operation. Both forward and reverse operation, however, will be applied simultaneously if the forward and reverse switch turn ON at the same time. If that is a possibility, then write a program like the one shown in Program Example 2.

Program Example 1

ZEN Program Execution

The ZEN executes the entire ladder program from the first to the last line at one time. Each rung is executed in order from left to right starting from the left bus bar. If I0 (forward switch) and I1 (reverse switch) turn ON simultaneously in the first input rung of the circuit above, NC contacts of Q0 and Q1 in the second input rung are ON status, so Q0 (forward) and Q1 (reverse) in the first output rung turn ON simultaneously.

Programmable Controller Program Execution

The ZEN and the Programmable Controller execute programs quite differently.

The Programmable Controller executes a circuit a starting on the left from the bus bar and then moves to the right. It then repeats this with circuit b. If I0 (forward switch) and I1 (reverse switch) simultaneously turn ON, Q0 (forward) turns ON in circuit a and then circuit b executes, so Q1 (reverse) will not turn ON.

Program Example 2

Only Q0 (forward) turns ON if forward and reverse operation are applied at the same time.

In this program, I0 (forward switch) has priority if I0 (forward switch) and I1 (reverse switch) turn ON at the same time.