First Scan Bit: Beckhoff

| 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 |

Key difference: Beckhoff's system does not have a mandatory, automatic FirstScan system bit. You must explicitly create it. This gives you more control but requires discipline.

Beckhoff provides additional system flags for finer control: beckhoff first scan bit

Use these in combination:

IF FirstScan THEN
    IF bInit_Cold THEN
        // Full factory reset
    ELSIF bInit_Warm THEN
        // Restore retain variables, but reinit comms
    END_IF
END_IF

During the first scan, you typically want to: | Brand | First Scan Mechanism | Beckhoff

Without a proper first scan routine, your machine might start with actuators in unpredictable positions or unfinished states from a previous run.

PROGRAM MAIN
VAR
    bFirstScan    : BOOL := TRUE;   // Initialize as TRUE
    bFirstScanDone: BOOL := FALSE;
END_VAR
// First scan detection
IF bFirstScan AND NOT bFirstScanDone THEN
bFirstScanDone := TRUE;
// First scan logic here
END_IF Beckhoff provides additional system flags for finer control:
// Reset the flag after first cycle
IF bFirstScanDone THEN
bFirstScan := FALSE;
END_IF

⚠️ Caveat: This method can fail if the PLC is stopped/started without power cycle. Always prefer the system library method.