Technical data

Table Of Contents
Organization Blocks
System Software for S7-300/400 System and Standard Functions - Volume 1/2
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A5E00709327-01
1.2 Program Cycle Organization Block (OB1)
Description
The operating system of the S7 CPU executes OB1 periodically. When OB1 has
been executed, the operating system starts it again. Cyclic execution of OB1 is
started after the startup has been completed. You can call other function blocks
(FBs, SFBs) or functions (FCs, SFCs) in OB1.
Understanding the Operation of OB1
OB1 has the lowest priority of all of the OBs whose run-times are monitored, in
other words, all of the other OBs except OB90 can interrupt the execution of OB1.
The following events cause the operating system to call OB1:
The startup is completed.
The execution of OB1 (the previous cycle) has finished.
When OB1 has been executed, the operating system sends global data. Before
restarting OB1, the operating system writes the process-image output table to the
output modules, updates the process-image input table and receives any global
data for the CPU.
S7 monitors the maximum scan time, ensuring a maximum response time. The
value for the maximum scan time is preset to 150 ms. You can set a new value or
you can restart the time monitoring anywhere within your program with SFC43
"RE_TRIGR." If your program exceeds the maximum cycle time for OB1, the
operating system calls OB80 (time error OB); if OB80 is not programmed, the CPU
changes to the STOP mode.
Apart from monitoring the maximum scan time, it is also possible to guarantee a
minimum scan time. The operating system will delay the start of a new cycle
(writing of the process image output table to the output modules) until the minimum
scan time has been reached.
Refer to the manuals /70/ and /101/ for the ranges of the parameters "maximum"
and "minimum" scan time. You change parameter settings using STEP 7.