Siemens SIMATIC S7-300 Hardware And Installation Manual page 146

Commissioning
start-up of CPU 31x-2 DP/31xC-2 DP as DP master
On start-up, CPU 31x-2 DP/31xC-2 DP compares the preset configuration of your
DP master systems to the actual configuration.
If preset configuration = actual configuration the CPU switches to RUN mode.
If the preset configuration  WR WKH DFWXDO FRQILJXUDWLRQ WKH FRQILJXUDWLRQ RI
parameter start-up if preset configuration  DFWXDO FRQILJXUDWLRQ determines
the start-up behavior of the CPU.
start-up with preset configuration 
actual configuration = yes (Default
setting)
CPU 31x-2 DP/31xC-2 DP switches to RUN
mode.
(BUSF LED flashes if any of the DP slaves
cannot be addressed)
9-28
start-up with preset configuration 
actual configuration = no
CPU 31x-2 DP/31xC-2 DP remains in STOP
mode. On expiration of the Monitoring time
for parameter transfer to modules the
BUSF LED starts flashing.
The flashing BUSF LED indicates that at
least one DP slave cannot be accessed. In
this case, check whether all DP slaves are
switched on or correspond with your
configuration, or read out the diagnostic
buffer with STEP 7.
S7-300 Programmable Controller Hardware and Installation
A5E00105492-02