Technical data

Basics of Designing a Program Structure
4.2 Blocks in the User Program
Programming with STEP 7
106 Manual, 05/2010, A5E02789666-01
4.2.4.5 Startup Organization Blocks (OB100 / OB101 / OB102)
Startup Types
There are three distinct types of startup:
• Hot restart (not in S7-300 and S7-400H)
• Restart (warm restart)
• Cold restart
The following table shows which OB the operating system calls in each startup type.
Startup Type Related OB
Hot restart OB101
Restart (warm restart) OB100
Cold restart OB102
Start Events for Startup OBs
The CPU executes a startup after the following events:
• After power up
• After you switch the mode selector from STOP to RUN/RUN-P
• After a request from a communication function
• After synchronizing in multicomputing mode
• In an H system after link-up (only on the standby CPU)
Depending on the start event, the CPU used, and its set parameters the relevant startup OB
(OB100, OB101, or OB102) is called.
Startup Program
You can specify the conditions for starting up your CPU (initialization values for RUN, startup
values for I/O modules) by writing your program for the startup in the organization blocks OB100 for
restart (warm restart), OB101 for hot restart, or OB102 for cold restart.
There are no restrictions to the length of the startup program and no time limit since the cycle
monitoring is not active. Time-driven or interrupt-driven execution is not possible in the startup
program. During the startup, all digital outputs have the signal state 0.
Startup Type After Manual Restart
On S7-300 CPUs only a manual restart (warm restart) or cold restart (CPU 318-2 only) is possible.
On some S7-400 CPUs, you can restart manually using the mode selector and the startup type
switch (CRST/WRST) if this is permitted by the parameter assignment you made with STEP 7. A
manual restart (warm restart) is possible without specifically assigning parameters.