Delta Electronics Programmable Logic Controller DVP-PLC - Manual (Page 15)

1 Basic Principles of PLC Ladder Diagram
DVP-PLC Application Manual 1-11
7. STL instruction: Used for designing the syntax of the sequential function chart (SFC).
STL instruction allows the program designer a clearer and readable picture of the sequence of the program as
when they draw a sequence chart. From the figure below, we can see clearly the sequence to be planned. When the
step S moves to the next step, the original S will be “Off". Such a sequence can then be converted into a PLC ladder
diagram and called “step ladder diagram”.
M1002
S0
SET S0
S0
SET S21
SET S22
S
S21
S
RET
S22
S
M1002
8. RET instruction: Placed after the completed step ladder diagram.
RET also has be placed after STL instruction. See the example below.
RET
S20
S
RET
S20
S
X1
X1
See step ladder instructions [STL], [RET] in Ch. 4 for the structure of the ladder diagram.