Beckhoff First Scan Bit

The "First Scan" bit in Beckhoff TwinCAT PLC systems is a boolean status flag indicating the PLC program's initial execution cycle after startup, download, or reset. It enables safe, deterministic initialization of variables, hardware states, and communication interfaces before normal cyclic operation proceeds.

Implementation Method 1: The Local Initialization Attribute (Recommended)

: Provides the exact number of cycles the task has completed since boot—a fantastic diagnostic tool for tracking task execution frequency.

You can implement this programmatically in Structured Text (ST) by declaring the system information variables and the GETCURTASKINDEX function block. beckhoff first scan bit

The array index _TaskInfo[1] points to your primary PLC task. The property .CycleCount increments automatically on every cycle. On the very first pass, it evaluates to 1 , rendering bFirstScan true. On cycle two and all subsequent cycles, bFirstScan becomes false automatically. Method 2: The Classic IEC 61131-3 "Inverted Flag" Approach

In older TwinCAT 2 systems, sometimes developers used specialized _Init programs. However, in modern TwinCAT 3, using a bFirstScan boolean in the main task is the preferred approach for readability and consistency. 5. Common Pitfalls and Debugging

This is particularly useful for initializing complex data structures, allocating memory with __NEW , or passing parameters to function blocks before they are used in the program. The "First Scan" bit in Beckhoff TwinCAT PLC

PROGRAM MAIN VAR fbGetCurTaskIndex : GETCURTASKINDEX; // Function block to get task index nTaskIndex : UINT; // Variable to store the index bIsFirstScan : BOOL; // Flag indicating the first scan // Example application variables nSystemInitCount : INT; // Tracks how many times init is run END_VAR // 1. Call the function block to get the index of the currently executing task fbGetCurTaskIndex(); nTaskIndex := fbGetCurTaskIndex.index; // 2. Read the system info for this specific task bIsFirstScan := _TaskInfo[nTaskIndex].FirstCycle; // 3. Execute First Scan Logic IF bIsFirstScan THEN // Place your startup and initialization code here nSystemInitCount := nSystemInitCount + 1; END_IF Use code with caution. 2. How the Code Works

While the core concept is universal, the naming and implementation of the first scan differ across platforms.

: Retrieves the Distributed Clock (DC) timestamp, which is absolutely vital for high-precision motion control and EtherCAT synchronization. You can implement this programmatically in Structured Text

In PLC (Programmable Logic Controller) programming, initialization is a critical phase of machine control. When a control system transitions from to Run mode, or when the controller power-cycles, variables must be set to safe default states, communication links must be verified, and system handshakes must be established.

If you perform a or Download , the variables reset to their initial values, and the first scan logic will execute again. 2. Keep Logic Lightweight

Understanding the Beckhoff First Scan Bit: Implementation, Use Cases, and Best Practices

The First Scan Bit, also known as the "First Cycle Bit" or "Initialization Bit," is a special bit in Beckhoff PLCs that indicates when the PLC is executing its first scan cycle. In other words, it signals that the PLC is starting up and executing its program for the first time.