
1. GENERAL
B–61863E/15
STEP SEQUENCE FUNCTION
968
The ladder method is most often used for programming the sequence
control governed by a programmable controller. This method, shown in
Fig.1.1(a) , was derived from relay-panel control circuits. Since it has
been in use for years, many sequence control engineers are already
familiar with it. This method is also used in PMC sequence
programming.
Fig. 1.1 (a) Ladder method
The greater the number of functions implemented by the PMC for a CNC
system, the larger and the more complicated the sequence program
becomes. A large-scale system requires a larger program and a greater
number of processes, making it hard for the ladder method to control the
overall process. This is because the ladder method does not describe the
order of control. While the ladder method is suitable for describing partial
control, it is hard to apply it to the description of the flow of control
overall.
To overcome this problem, structured programming has been introduced
into sequence control. A PMC that supports the subprogram function
enables the use of modular programs. As shown in Fig.1.1(b), a
large-scale program is divided into subprograms for each function,
simplifying the unit of processing. Since the programmer determines
how to divide the main program into subprograms and the control flow
used to call the subprograms, however, the programs are not necessarily
easy-to-understand by other programmers.
CALL
CALL
Subprogram
Subprogram
Fig. 1.1 (b) Module method
1.1
STEP SEQUENCE
METHOD