Similarly, the condition could be placed in the transition condition of a step or as a guard clause within Instruction List (IL) . This cross-language support ensures that no matter your team's coding preference, you have access to this critical system flag.
The most common approach is to create a boolean bFirstScan and a variable to track the PLC state. structuredtext
When you instantiate a function block, TwinCAT automatically calls its FB_Init method once.
: The logic trapped inside this IF statement evaluates only once when the controller boots up or transitions from CONFIG to RUN mode. Why is the First Scan Bit Important? beckhoff first scan bit
// Drive reset pulse IF bResetDrives THEN // Pulse drive reset signal bResetDrives := FALSE; END_IF
: The INIT section runs before the first cyclic scan. This is actually earlier than a typical "first scan bit." If you need to guarantee initialization occurs before any other logic, INIT is superior.
// 3. Request axes to home (if needed) fbAxis1.bHomeRequest := TRUE; Similarly, the condition could be placed in the
In PLC programming, ensuring that a system starts in a known state is critical for safety and process reliability. The (sometimes called First Cycle bit) is a special bit that is TRUE only during the very first scan of a PLC program after powering on or transitioning from STOP to RUN mode.
PROGRAM MAIN VAR bInit : BOOL := TRUE; // Our first scan flag bFirstScanDone : BOOL; fbFirstScan : F_TRIG; fbEcMaster : FB_EcCoEADsRead; // EtherCAT utility nState : INT; END_VAR
END_IF
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VAR bFirstScan : BOOL := TRUE; // Initializes to TRUE on PLC startup bInitializationDone : BOOL; // Your application variables nState : INT; fTargetVelocity : LREAL; END_VAR // --- PLC Execution Code --- IF bFirstScan THEN // 1. Initialize State Machines nState := 10; // 2. Set Default Configurations fTargetVelocity := 1500.0; // 3. Clear Historical Fault Arrays or Buffers // (Your code here) // 4. Reset the first scan bit so this block never runs again bFirstScan := FALSE; END_VAR // Normal, cyclic PLC logic continues here Use code with caution. How It Works during Online Changes