1. PLC Concepts
1-9
Branch
instruction Branch
Symbol Description
MPS ┬ Start of branches. Stores current result of
program evaluation. Max. 8 MPS-MPP pairs can
be applied
MRD ├ Reads the stored current result from previous
MPS
MPP └ End of branches. Pops (reads then resets) the
stored result in previous MPS
Note: When compiling ladder diagram with WPLSoft, MPS, MRD and MPP could be automatically
added to the compiled results in instruction format. However, sometimes the branch instructions
are ignored by WPLSoft if not necessary. Users programming in instruction format can enter branch
instructions as required.
Connection points of MPS, MRD and MPP:
MPS
MRD
MPP MPP
MPS
Note: Ladder diagram editor in ISPSoft does not support MPS, MRD and MPP instructions. To
achieve the same results as branch instructions, users have to connect all branches to the left
hand bus bar.
WPLSoft
ISPSoft
1.5.11 STL (Step Ladder Programming)
STL programming uses step points, e.g. S0 S21, S22, which allow users to program in a clearer
and understandable way as drawing a flow chart. The program will proceed to next step only if the
previous step is completed, therefore it forms a sequential control process similar to SFC
(Sequential Function Chart) mode. The STL sequence can be converted into a PLC ladder diagram
which is called “step ladder diagram” as below.