System Bits are the Operating System interface to the user writing the application. System Bits are reserved by the Operating System for particular functions. Some System Bits, for example, are connected to the PLC's keypad keys.
There are 256 SBs (Address SB 0 - SB 255).
Only certain SBs may be written into by the programmer:
SB 80: Activate Linearization
SB 200 -SB 215: M90 Network Operand
Click on the System Bits folder in the Program Navigation pane to display the complete list of System Bits. Scroll down to view the complete list