Siemens S5-135U/155U System Manual page 166

CPUs, Memory Cards, Memory Submodules, Interface Submodules
Restart
You can also carry out a manual restart of the CPU 948 with the mode
switch. The CPU 948 Programming Guide will indicate when a manual
restart is permissible.
Step
For maintenance purposes or in the event of a fault, this startup without user
program in single-processor operation can serve to establish whether the CPU
is operating without errors.
5-26
Action
1 Set the mode switch from
STOP to RUN
Result
– Red STOP LED goes off
– RUN LED lights up
– Red BASP LED goes off
System Manual
C79000-G8576-C199-06