PROGRAM MAIN VAR bInit : BOOL := TRUE; // Our first scan flag bFirstScanDone : BOOL; fbFirstScan : F_TRIG; fbEcMaster : FB_EcCoEADsRead; // EtherCAT utility nState : INT; END_VAR
The most straightforward way is to declare a global variable that resets itself after the first cycle.
If you perform a or Download , the variables reset to their initial values, and the first scan logic will execute again. 2. Keep Logic Lightweight
If you are using programming, TwinCAT provides implicit variables called SFC Flags . beckhoff first scan bit
Beckhoff TwinCAT supports persistent and remanent variables that retain their values through a power cycle. However, this stored data can sometimes become corrupted or invalid. On the first scan, you can implement logic to read this persistent data, perform a checksum or validity check, and if it fails, either reset the variables to safe defaults or set a system warning.
If you prefer a portable method that works across almost any IEC 61131-3 platform, you can create your own logic using a global variable.
The first scan bit is a boolean flag that is exactly TRUE for the very first execution cycle of a PLC task and switches to FALSE for all subsequent cycles. PROGRAM MAIN VAR bInit : BOOL := TRUE;
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If you are using Retentive Variables (variables that keep their value after power-loss), you may need bFirstScan to reset them to a default safe state upon the first startup, but not subsequent power cycles.
Note: While functional, this method hardcodes the task index ( [1] ), making it less adaptable if your PLC project structure changes down the road. Method 3: The Custom Software-Based Flag Keep Logic Lightweight If you are using programming,
If you are using Function Blocks, TwinCAT 3 supports the FB_init method. This is a specialized sub-method that runs when the block is instantiated (during PLC startup or after a download), making it the cleanest way to handle block-specific initializations. Why use a First Scan Bit?
: Ensures equipment begins in a safe, known state.
Initialize critical error flags to TRUE (forcing a safe state) and non-critical outputs to safe FALSE values. 4. Common Pitfalls and Troubleshooting 1. "My First Scan Logic Runs Too Often"
The most common, portable, and standard-compliant way to create a first scan bit in TwinCAT Structured Text (ST) is by declaring a local or global initialization variable. Implementation