To ensure deterministic startup behavior and avoid page faults or memory violations, adhere to these architectural rules: 1. Avoid Complex Pointer Manipulations
The First Scan Bit is a system-defined boolean variable that is TRUE only during the very first PLC scan after a TwinCAT runtime startup or program download. On the second scan and all subsequent scans, the bit is FALSE . Why Use the First Scan Bit? beckhoff first scan bit
: In your Global Variable List (GVL) or program, declare a BOOL with an initial value of TRUE . VAR_GLOBAL bFirstScan : BOOL := TRUE; END_VAR Use code with caution. Copied to clipboard To ensure deterministic startup behavior and avoid page
Resetting historical error flags or step indexes in Sequential Function Charts (SFC) to ensure the machine starts from a known, safe state. Why Use the First Scan Bit
A common "best practice" for portability across different PLC brands is to create your own flag in a Global Variable List (GVL) . :
TwinCAT provides system variables via its standard libraries that can detect system startup conditions. Implementation
It is more robust than manual "first scan" flags (like using a boolean that you set to false at the end of the code), as the PLC runtime handles its state directly. Usage Example