Beckhoff First Scan Bit Here

Here are the general steps to use the First Scan Bit:

IF _TaskInfo[GETCURTASKINDEX()].FirstCycle THEN // Your initialization logic here END_IF; Use code with caution. Copied to clipboard Custom Variable Method

To understand the importance of the First Scan bit, one must first appreciate the architecture of a PLC. A PLC operates on a scan cycle: reading inputs, executing logic, and writing outputs. Under normal operation, this cycle repeats endlessly. However, the very first cycle after a power-up or a program reset presents a unique problem. At this specific moment, input data may not have settled, variables may be holding default values rather than retained ones, and physical actuators might be in unknown positions. If the control logic were to execute standard commands immediately, it could lead to unintended consequences, such as commanding a cylinder to extend before verifying it is retracted, or resetting a recipe to default values instead of loading the last saved state. beckhoff first scan bit

FB_init is a specialized method that executes implicitly the PLC task starts its cyclic execution. It runs during the initialization code phase. How to use FB_init :

| Start type | First scan bit behavior | |------------|--------------------------| | Cold start | TRUE once | | Warm start | TRUE once | | Stop → Run | FALSE (not set) | Here are the general steps to use the

VAR bFirstScan : BOOL := TRUE; // Initialized to TRUE END_VAR

The First Scan Bit is an internal Boolean variable that acts as a one-shot trigger. It acts as the signal that the PLC program has just loaded into memory and is beginning its first cycle of execution. Key Characteristics: It is TRUE on the very first cycle. Under normal operation, this cycle repeats endlessly

Forcing sequential state machines (e.g., SFC or CASE statements) to start safely at State 0 .

The first scan bit is a small but mighty element in the Beckhoff TwinCAT programmer's toolkit. It provides the essential, dependable signal needed to perform one-time setup, ensuring that every control program starts from a clean, reliable, and safe state.