Siemens SINUMERIK 840D sl Configuration Manual page 47

Cyclic operation (OB1)
From a chronological viewpoint, the PLC basic program runs ahead of the PLC user
program. The complete processing of the NCK-PLC interface is carried out in cyclic mode. A
cyclic monitoring function is activated between PLC and NCK once boot-up and the first OB1
cycle have been completed. A PLC failure produces the "2000 Sign-of-life monitoring PLC"
alarm in SINUMERIK Operate.
Start-up behavior of the PLC
The PLC always starts up in RESTART mode, i.e. the PLC operating system runs OB100
after initialization and starts cyclic operation at the beginning of OB1. No return is made to
the interruption point (for example, in the event of a power failure).
There are both retentive and non-retentive areas for the markers, timers and counters. The
areas are contiguous and are divided by a parameterizable limit, where the area with the
higher-value address range is defined as the non-retentive area. Data blocks are always
retentive.
RESTART start type (OB100)
If the retentive area has no battery backup (backup battery is empty) start-up is prevented.
The following operations are performed during a cold restart:
● UStack, BStack and non-retentive flags, timers and counters are deleted
● The process output image (POI) is deleted
● Process and diagnostics alarms are canceled
● The system status list is updated
● Parameterization objects of modules (from SDB100 onwards) are evaluated or default
parameters are output to all modules in single-processor mode
● Cold restart (OB100) is executed
● The process input image (PII) is imported
● The command output inhibit (BASP) is canceled
Further information
The block descriptions and other information about the PLC basic program are contained in
the P3 section of the SINUMERIK 840D sl Basic Functions Manual. Basic PLC program.
SINUMERIK STEP 7 Toolbox V14 SP1
Configuration Manual, 05/2017
3.1 General information about the PLC program
Programming the PLC
47