Beckhoff First Scan Bit _verified_ May 2026

In Beckhoff TwinCAT systems, there is no single global "S:FS" bit like those found in Rockwell (Allen-Bradley) controllers . Instead, users typically leverage the PlcTaskSystemInfo

Part 4: Best Practice – The FB_Init Method (Object-Oriented)

For advanced TwinCAT 3 users working with Function Blocks, the most elegant and robust "first scan" is not a bit at all—it's the object constructor: FB_Init. beckhoff first scan bit

In Beckhoff TwinCAT, first scan detection is achieved via the PlcTaskSystemInfo.FirstCycle In Beckhoff TwinCAT systems, there is no single

• The _FirstScan from TwinCAT_SystemInfoVarList is reset on each login – useful for debugging.
• For persistent one-time init across power cycles, store a flag in VAR RETAIN.

Declare a global or local variable: bInitialized : BOOL := FALSE; Logic: Declare a global or local variable: bInitialized :

Logic: Place code at the very end of your main program that sets this bit to FALSE. Because the variable is initialized to TRUE, it remains so for the entire first scan before being permanently toggled off. Comparison and Review PlcTaskSystemInfo.FirstCycle Manual Custom Bit Reliability Native to TwinCAT; handles task-specific restarts. Highly reliable if implemented at the program's end. Complexity Requires calling GETCURTASKINDEX. Extremely simple to declare and use. Best Use Case

// EXIT section runs when program stops EXIT myOutput := FALSE;

The most robust method involves using the built-in PlcTaskSystemInfo structure. This provides real-time data about the current task. Variable: _TaskInfo[index].FirstCycle