Siemens S5-135U/155U appendix RUN setting, the CPU 928B processes the user program when

CPUs, Memory Cards, Memory Submodules, Interface Submodules

Mode Switch	The mode switch has two settings:
RUN	In the RUN setting, the CPU 928B processes the user program when the
	green RUN LED is lit.
STOP	The CPU 928B goes to the stop state when you switch from RUN to STOP.
	The red STOP LED then lights up.

Momentary-	You can initiate the Overall Reset, Reset and Restart functions with the
Contact Mode	momentary-contact mode switch:
Switch
OVERALL RESET	Momentary-contact switch down
	With an overall reset, all RAM areas are erased and initialized (both on the
	CPU and on a RAM submodule).
Reset	Momentary-contact switch up
	During a reset, all flags, timers, counters and the process image will be
	erased. OB 20 will be invoked. Processing of the user program will start from
	the beginning again.
Restart	Momentary-contact switch at midpoint
	With a restart, processing of the user program will continue from the point of
	interruption. The statuses of flags, timers, counters and the process image are
	retained during stoppage of the CPU.

5-36	System Manual
	C79000-G8576-C199-06