Siemens SINUMERIK 840D sl Commissioning Manual page 86

PLC commissioning
5.4 Creating a PLC program
Figure 5-3
PLC status
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).
Start-up behavior of the PLC
There are both retentive and non-retentive areas for the markers, timers and counters. The
areas are continuous 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 startup type (OB100)
If the retentive area is not buffered (the buffer battery is empty) start-up is prevented. The
following operations are performed during a restart:
● Delete the UStack, BStack and non-retentive flags, timers and counters
● Delete the process output image (POI)
● Cancel the process and diagnostics alarms
86
PLC program structure
CNC Commissioning: NC, PLC, Drive
Commissioning Manual, 12/2019, A5E48312804B AA