Specifications

General programming notes
8.4.4
Program components
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8.4.4 Program components
 Modules
A program consists of the following levels of modules:
• Modules that are executed at start up (OB100)
• Modules that are called from OB1 and executed in synchronization with scanning
• Modules that are executed in interrupt processes
The basic parts of a program are started by OB1, OB40, and OB100 as shown below.
The illustration above shows only basic the parts of a program. An actual program may be
more complicated including some other interrupt processes. For details on OB, FC and FB,
see the System Software for S7-300/S7-400 (System and Standard Functions).
OB100
restart process
OB1
cycle process
(at each scanning)
User program
OB40
interrupt process
FC
alarms
User program
FC
cycle process
FC
start up process
The system and user data blocks
(program) check parameters and
other important data at the time of
start up, and stops the PLC if any
abnormality is found.
The following processes are done
during normal scanning:
・
Sending and receiving control status
signals
・
Sending and receiving auxiliary and
G functions
・
M decode (M00-99)
・
M, S, and F code
・
Communication between machine
control panel and NCK
・
Logging of user errors and operating
messages
Interrupt process for a process alarm
User program