4 Step Ladder Instructions 8. Other Points to Note: The instruction used for transferring the step (SET S □ or OUT S □ ) can only be executed after all the relevant outputs and actions in the current status are completed. See the figure below. The executed results by the PLC are the same, but if there are many conditions or actions in S10, it is recommended that you modify the diagram in the left hand side into the diagram in the right hand side. SET S20 is only executed after all relevant outputs and actions are completed, which is a more explicit sequence. Y0 S10 S S10 S S20 Y1 S20 S Y2 S Y2 S0 S20 X1 S Make sure to add RET instruction after STL at the end of RET the step ladder diagram.
S0 S20 S X1 RET 4.4 Things to Note for Designing a Step Ladder Program 1. The first step in the SFC is called the “initial step", S0 ~ S9. Use the initial step as the start of a sequence and end a complete sequence with RET instruction. 2. If STL instruction is not in use, step S can be a general-purpose auxiliary relay. 3. When STL instruction is in use, the No. of step S cannot be repeated. 4. Types of sequences: Single sequence: There is only one sequence without alternative divergence, alternative convergence, simultaneous divergence and simultaneous convergence in a program. Complicated single sequence:
There is only one sequence with alternative divergence, alternative convergence, simultaneous divergence and simultaneous convergence in a program. Multiple sequences: There are more than one sequence in a program, maximum 10 sequences, S0 ~ S9. 5. Separation of sequence: Multiple sequences are allowed to be written into the step ladder diagram. a) See the diagram in the right hand side. There are two sequences OUT S1 S0 OUT S0 and S1. The program writes in S0 ~ S30 first and S1 ~ S43 next. S21 S41 b) You can designate a step in the sequence to jump to any step in OUT another sequence.
S42 c) When the condition below S21 is true, the sequence will jump to S30 S43 step S42 in sequence S1, which is called “separating the step”. DVP-PLC Application Manual 4-9 0 Y0 Y1 SET S20 SET S20 SET S20