Siemens SINUMERIK 840Di Function Manual page 957

2.7.1 Startup and synchronization of NCK PLC
Loading the basic program
The basic program must be loaded with the S7 tool when the PLC is in the Stop state. This
ensures that all blocks in the basic program will be initiated correctly the next time they are
called. An undefined state may otherwise develop on the PLC (e.g., all PLC LEDs flashing).
Startup
NCK and PLC synchronization is performed during startup. The system and user data blocks
are checked for integrity and the most important basic program parameters are verified for
plausibility. In the event of an error, the basic program outputs an error identifier to the
diagnostic buffer and switches the PLC to STOP.
A warm restart is not provided, i.e., following system initialization, the operating system runs
organization block OB 100 and always commences cyclic execution at the start of OB 1.
Synchronization
The PLC is synchronized with the MMC and NCK during powerup.
Sign of life
After a proper initial start and the first complete OB1 cycle (initial setting cycle) the PLC and
NCK continuously exchange sign of life signals. If the sign of life signal from the NCK fails to
arrive, the PLC/NCK interface is neutralized and the signal "NCK CPU ready" in DB 10 is set
to zero.
2.7.2 Cyclic operation (OB1)
General
The NCK PLC interface is processed completely in cyclic mode. From a chronological
viewpoint, the basic program runs ahead of the user program. In order to minimize the
execution time of the basic program, only the control/status signals are transmitted cyclically;
transfer of the auxiliary functions and G functions only takes place on request.
Power Line Basic PLC Program (P3)
Function Manual, 08/2005 Edition, 6FC5397-0BP10-0BA0
2.7 Structure and functions of the basic program
Detailed description
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