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.
Forcing sequential state machines (e.g., SFC or CASE statements) to start safely at State 0 .
VAR RETAIN bInitialized : BOOL; END_VAR VAR bFirstScanSys : BOOL; END_VAR beckhoff first scan bit
END_IF
// Check if the flag is still false IF NOT bFirstCycleDone THEN // Place your one-time initialization logic here // ... Similarly, the condition could be placed in the
Telling other machines or HMI that the PLC has just powered on. 2. Implementing the First Scan Bit in TwinCAT 3 (ST)
Without a initialization bit, your PLC logic simply resumes from its last state or starts with default values that might not be appropriate for a running machine. Common use cases include: VAR RETAIN bInitialized : BOOL; END_VAR VAR bFirstScanSys
The most common approach is to create a boolean bFirstScan and a variable to track the PLC state. structuredtext
But this is still a workaround. The most professional method in Tc3_Standard is to use the StandardLib (from Beckhoff Automation) which includes FB_FirstScan directly.