Technical data

Programming with STEP 7
Manual, 05/2010, A5E02789666-01 557
27 Appendix
27.1 Operating Modes
27.1.1 Operating Modes and Mode Transitions
Operating Modes
Operating modes describe the behavior of the CPU at a particular point in time. Knowing the
operating modes of CPUs is useful when programming the startup, testing the controller, and for
troubleshooting.
The S7-300 and S7-400 CPUs can adopt the following operating modes:
• STOP
• STARTUP
• RUN
• HOLD
In STOP mode, the CPU checks whether all the configured modules or modules set by the default
addressing actually exist and sets the I/Os to a predefined initial status. The user program is not
executed in STOP mode.
In STARTUP mode, a distinction is made between the startup types "warm restart," "cold restart,"
and "hot restart:"
• In a warm restart, program processing starts at the beginning of the program with initial settings
for the system data and user address areas (the non-retentive timers, counters, and bit
memory are reset).
• In a cold restart, the process-image input table is read in and the STEP 7 user program is
processed starting at the first command in OB1 (also applies to warm restart).
- Any data blocks created by SFC in the work memory are deleted; the remaining data
blocks have the preset value from the load memory.
- The process image and all timers, counters, and bit memory are reset, regardless of
whether they were assigned as retentive or not.
• In a hot restart, the program is resumed at the point at which it was interrupted (timers,
counters, and bit memory are not reset). A hot restart is only possible on S7-400 CPUs.
In RUN mode, the CPU executes the user program, updates the inputs and outputs, services
interrupts, and process error messages.
In HOLD mode, processing of the user program is halted and you can test the user program step
by step. The HOLD mode is only possible when you are testing using the programming device.
In all these modes, the CPU can communicate via the multipoint interface (MPI).