Siemens SINUMERIK 840D sl Commissioning Manual page 307

Figure 15-4
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. There is no re-entry at
the point of interruption (e.g. on power failure).
IBN CNC: NCK, PLC, drive
Commissioning Manual, 07/2007, 6FC5397-2AP10-3BA0
PLC program structure
Fundamentals
15.4 PLC program
307