Scan Bit New!: Beckhoff First

| Brand | First Scan Mechanism | Beckhoff Equivalent | |--------|----------------------|----------------------| | Siemens | OB100 | INIT section or FB_FirstScan | | Rockwell | S:FS (First Scan) bit | bInit variable manually set | | Codesys | bInit in PLC_PRG | Same (TwinCAT is based on Codesys) | | Beckhoff Native | INIT , FB_Init , or F_TRIG | Choose based on scope |

bFirstScanSys := TwinCAT_SystemInfoVarList._FirstScan;

In PLC (Programmable Logic Controller) programming, initialization is a critical phase of machine control. When a control system transitions from to Run mode, or when the controller power-cycles, variables must be set to safe default states, communication links must be verified, and system handshakes must be established.

// Reset the flag after first cycle IF bFirstScanDone THEN bFirstScan := FALSE; END_IF beckhoff first scan bit

Method 2: System Task Array Linkage (TwinCAT 2 & 3 Compatibility)

In Beckhoff’s TwinCAT 3 environment, there isn’t a single hard-coded bit in the global memory by default, but the system provides a specialized mechanism to create one that is far more powerful than a simple boolean. What is the First Scan Bit?

VAR fbGetTaskIndex : FB_GetCurTaskIndex; nCycleCount : UDINT; END_VAR fbGetTaskIndex(); nCycleCount := _TaskInfo[fbGetTaskIndex.index].CycleCount; IF nCycleCount = 1 THEN // This is the first scan END_IF Use code with caution. | Brand | First Scan Mechanism | Beckhoff

In TwinCAT 3, the First Scan Bit is represented by the system variable FirstScan . Here's an example of how to use it in a simple PLC program:

The "Beckhoff first scan bit" is not a single feature but a concept implemented through various patterns. For simple projects, a F_TRIG on a TRUE variable is sufficient. For robust, reusable code, use FB_Init in function blocks. For system-wide initialization that must run before cyclic logic, use the INIT section.

In Beckhoff TwinCAT 3 , efficiently capturing the is essential for initializing variables, resetting state machines, and executing one-time startup routines . Unlike many legacy PLC platforms that offer a simple, hardcoded system flag (like S₀ or SM0.1) for the very first execution cycle, TwinCAT handles this dynamically through system structures. What is the First Scan Bit

PROGRAM MAIN VAR bInit : BOOL := TRUE; // Our first scan flag bFirstScanDone : BOOL; fbFirstScan : F_TRIG; fbEcMaster : FB_EcCoEADsRead; // EtherCAT utility nState : INT; END_VAR

(* Clear alarms *) bAlarmReset := TRUE;