Siemens SINUMERIK 840D sl Configuration Manual page 57

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 V15.1
Configuration Manual, 12/2018, A5E46279635B AA
4.1 General information about the PLC program
Programming the PLC
57