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First Scan Relay Flags

First Scan Relay Flags (F001)​

The first scan flags, F001, provide initialization statuses during the first scan of the PLC after a program start or reset.
These flags are commonly used to initialize variables, settings, or other logic components in a predictable way at the start of program execution.

First Scan Relay Flags
Flag BitNameDescriptionUse Case
F001.0Always ONThis flag is always ON (1)Used for triggering logic that should always execute
F001.1Always OFFThis flag is always OFF (0)Used for disabling specific conditions or logic paths
F001.2ON at First Scan OnlyThis flag is ON (1) only during the first scan after the program starts or resetsUseful for initializing variables or settings at program start
F001.3OFF at First Scan OnlyThis flag is OFF (0) only during the first scan after the program starts or resetsUsed to prevent execution during initialization
F001.4Toggle at Each ScanToggles between ON (1) and OFF (0) at each scanUsed for timing logic or alternating states

F001.0 - Always ON​

Description​

The Always ON flag is ON (1) regardless of the program execution or conditions.

Use Case​

  • Used as a base condition to ensure specific logic or actions are always executed.
  • Can serve as a default true condition in ladder logic or other control processes.

F1.0 ON​

F001.1 - Always OFF​

Description​

The Always OFF flag is OFF (0) regardless of the program execution or conditions.

Use Case​

  • Useful for logic paths or operations that should never trigger.
  • Can disable unused parts of logic for testing or development.

F1.1 OFF​

F001.2 - ON at First Scan Only​

Description​

The ON at First Scan Only flag is ON (1) only during the first scan after a program starts or resets.
After the first scan, this flag remains OFF (0).

Use Case​

  • Initializes variables, counters, timers, or settings during startup.
  • Used to reset program states or trigger setup routines.

F1.2 ON​

F1.2 OFF​

F001.3 - OFF at First Scan Only​

Description​

The OFF at First Scan Only flag is OFF (0) only during the first scan after a program starts or resets.
After the first scan, this flag remains ON (1).

Use Case​

  • Used to delay certain operations or logic until after initialization is complete.
  • Helps prevent unintended execution during the first program cycle.

F1.3 OFF​

F1.3 ON​

F001.4 - Toggle at Each Scan​

Description​

The Toggle at Each Scan flag alternates between ON (1) and OFF (0) with each scan cycle.

Use Case​

  • Ideal for creating alternating states or timing events.
  • Can be used for blinking indicators or flip-flop logic.

F1.4 OFF​

F1.4 ON​