PLC concepts
5.1 Execution of the user program
S7-1200 Programmable controller
92 System Manual, V4.2, 09/2016, A5E02486680-AK
Organization blocks (OBs)
OBs control the execution of the user program. Specific events in the CPU trigger the
execution of an organization block. OBs cannot call each other or be called from an FC or
FB. Only an event such as a diagnostic interrupt or a time interval can start the execution of
an OB. The CPU handles OBs according to their respective priority classes, with higher
priority OBs executing before lower priority OBs. The lowest priority class is 1 (for the main
program cycle), and the highest priority class is 26.
Program cycle OBs execute cyclically while the CPU is in RUN mode. The main block of the
program is a program cycle OB. This is where you place the instructions that control your
program and where you call additional user blocks. You can have multiple program cycle
OBs, which the CPU executes in numerical order. Main (OB 1) is the default.
The program cycle event happens once during each program cycle (or scan). During the
program cycle, the CPU writes the outputs, reads the inputs and executes program cycle
OBs. The program cycle event is required and is always enabled. You might have no
program cycle OBs, or you might have multiple OBs selected for the program cycle event.
After the program cycle event occurs, the CPU executes the lowest numbered program cycle
OB (usually "Main" OB 1). The CPU executes the other program cycle OBs sequentially (in
numerical order) within the program cycle. Program execution is cyclical such that the
program cycle event occurs at the following times:
● When the last startup OB finishes execution
● When the last program cycle OB finishes execution
Table 5- 2 Start information for a program cycle OB
True for initial call of the OB
True if retentive data are available
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