4 Step Ladder Instructions 4.1 Step Ladder Instructions [STL], [RET] Explanations: STL Sn constructs a step. When STL instruction appears in the program, the program will enter a step ladder diagram status controlled by steps. The initial status has to start from S0 ~ S9. RET instruction indicates the end of a step ladder diagram starting from S0 ~ S9 and the bus returns to a normal ladder diagram instruction. SFC uses the step ladder diagram composed of STL/RET to complete the action of a circuit. The No. of S cannot be repeated. Explanations: RET indicates the end of a step. There has to be a RET instruction in the end of a series of steps.
One PLC program can be written in maximum 10 steps (S0 ~ S9) and every step should end with a RET. Program Example: Ladder diagram: SFC: M1002 S0 X0 S20 Y0 X1 S30 Y1 X2 X3 S40 Y2 S0 DVP-PLC Application Manual 4-1 Mnemonic Function Program steps Controllers ES EX SS SA SX SC EH2 SV EH3 SV2 STL Step Transition Ladder Start 1 Controllers ES EX SS SA SX SC EH2 SV EH3 SV2 S0 ~ S1023 Operand Mnemonic Function Program steps Controllers RET Step Transition Ladder Return 1 ES EX SS SA SX SC EH2 SV EH3 SV2 N/A Operand M1002 ZRST S0 S127 SET S0 S0 X0 S SET S20 S20 S Y0 X1 SET S30 S30 S Y1 X2 SET S40 S40 S Y2 X3 S0 RET END ZRST S0 S127 END SET S0 SET S20 SET S30 SET S40