The Modicon M340, Modicon M580, Premium, Atrium, Quantum, and Momentum PLCs use %Si system bits which indicate the state of the PLC, or they can be used to control how it operates.

These bits can be tested in the user program to detect any functional development requiring a set processing procedure.

Some of these bits must be reset to their initial or normal state by the program. However, the system bits that are reset to their initial or normal state by the system must not be reset by the program or by the terminal.