Beckhoff First Scan Bit [better] Guide

One of the most critical uses is to reset latched alarms or safety circuits at startup. This prevents a machine from automatically restarting a potentially dangerous process after a power cycle.

The most robust and recommended method is to use the firstCycle variable from the TcSystem library. This variable is part of the SYSTEMTASKINFOTYPE structure, which is accessible via the SystemTaskInfoArr array. The index in this array (typically 1) corresponds to the PLC task ID.

The core of this tracking relies on the system structure , which contains a dedicated Boolean flag named FirstCycle . This variable reads TRUE during the very first cycle of the task and switches permanently to FALSE for every subsequent cycle. Method 1: The Standard Global Task Info Hook beckhoff first scan bit

Populating operational variables with safe baseline data before the operator inputs custom values.

Proper utilization of the first scan bit is a hallmark of clean, structured Beckhoff programming. Common use cases include: One of the most critical uses is to

The array _TaskInfo is automatically exposed by the TwinCAT runtime engine. Fetching the index programmatically ensures that if you reuse this code across different tasks (e.g., a fast 1 ms motion task vs. a slow 50 ms housekeeping task), it will always monitor the correct execution context.

The primary utility of the First Scan bit lies in initialization. It serves as the logical "clean slate" mechanism. For instance, in complex motion control applications involving Beckhoff’s NC (Numerical Control) or robotics, the First Scan routine is used to verify the actual position of axes against their commanded positions. It allows the programmer to suppress motion commands until the system has verified that communication with servo drives is healthy. Furthermore, it is instrumental in state machine logic. By forcing the state machine into a specific "Init" or "Home" state on the first scan, the engineer ensures the machine follows a strict, safe sequence of startup events, regardless of the state the machine was in when it was last powered off. This variable is part of the SYSTEMTASKINFOTYPE structure,

// 3. Request axes to home (if needed) fbAxis1.bHomeRequest := TRUE;

// Main logic uses bIsFirstScan... // Final line of code: bIsFirstScan := FALSE; Use code with caution. Copied to clipboard 3. SFC Initialization Flag