Without a proper first scan routine, your machine might start with actuators in unpredictable positions or unfinished states from a previous run.
VAR fbGetCurTaskIdx : GETCURTASKINDEX; // Function block to find the current task ID bFirstScan : BOOL; END_VAR fbGetCurTaskIdx(); // Call the FB // Access the system's internal task info array bFirstScan := _TaskInfo[fbGetCurTaskIdx.index].FirstCycle; IF bFirstScan THEN // Insert initialization logic here END_IF Use code with caution. Copied to clipboard beckhoff first scan bit
The "First Scan" bit in Beckhoff TwinCAT PLC systems is a boolean status flag indicating the PLC program's initial execution cycle after startup, download, or reset. It enables safe, deterministic initialization of variables, hardware states, and communication interfaces before normal cyclic operation proceeds. Without a proper first scan routine, your machine
: It is built into the runtime and is highly reliable for initializing state machines or variables. Implementation Example You must fetch the specific task index first
TwinCAT provides an array called _TaskInfo that holds real-time diagnostic data for every task running on the processor. You must fetch the specific task index first to avoid hardcoding errors.
Network interfaces, serial buffers, or fieldbus master configurations may contain stale data after a restart.
Many developers prefer a manual method for simplicity or for use in older versions of TwinCAT where system structures might differ. This relies on the initialization value of a boolean variable.