4 Step Ladder Instructions 4.3 How does a Step Ladder Instruction Work? STL instruction is used for designing the syntax of a sequential function chart (SFC), making the program designing similar to drawing a flow chart and allowing a more explicit and readable program. From the figure in the left hand side below, we can see very clearly the sequence to be designed, and we can convert the sequence into the step ladder diagram in the right hand side. RET instruction has to be written at the end of every step sequence, representing the end of a sequence. There can be more than one step sequence in a program. Therefore, we have to write in RET at the end of every step sequence.
There is no limitation on the times of using RET which is used together with S0 ~ S9. If there is no RET instruction at the end of a step sequence, errors will be detected by WPL editor. M1002 initial pulse M1002 S0 S S21 S S22 S S23 S S0 RET 1. Actions of Step Ladder: A step ladder is composed of many steps and every step controls an action in the sequence. The step ladder has to: a) Drive the output coil b) Designate the transition condition c) Designate which step will take over the control from the current step DVP-PLC Application Manual 4-3 Alternative convergence. More than 2 steps transfer to the same step by transition condition. Simultaneous divergence.
The same step transfers to more than 2 steps by the same transition condition. Simultaneous convergence. More than 2 steps transfer to the same step by a single transition condition. S0 S21 S22 S23 02 SET S0 SET S21 1 SET S22 SET S23