If you are initializing NC axes (Motion Control), do not use the first scan bit to issue movement commands or reset drives directly. Instead, use the first scan bit to initialize the state variables of a sequential state machine (e.g., CASE statement), and let the state machine handle the drive enablement ( MC_Power ) and homing sequences sequentially over multiple cycles. 3. Scope Your Bits Correctly
Understanding the Beckhoff First Scan Bit: Implementation, Use Cases, and Best Practices
When a TwinCAT configuration activates and the PLC task starts, the system runtime allocates memory and sets variables to their default values. The very first execution cycle—Cycle 0—is where initialization logic must execute.
// In your main PLC program (e.g., MAIN) IF firstScanFlag THEN // Execute one-time actions // ... firstScanFlag := FALSE; // Reset for next cycles END_IF beckhoff first scan bit
Beckhoff's TwinCAT 3 environment offers multiple methods to implement a first scan, ranging from a simple, self-created flag to using the more official, built-in system structures.
TwinCAT 3 allows you to use attributes to ensure initialization happens properly, especially when dealing with complex function blocks or specialized task handling.
Next time you write a new PLC program, ask yourself: “If this were a cold start right now, would my system behave safely?” If the answer isn’t an immediate “yes,” you need FirstScan . If you are initializing NC axes (Motion Control),
Understanding and Implementing the Beckhoff First Scan Bit in TwinCAT
Pre-loading recipe parameters, positioning targets, or setting default calibration values before the main control loops execute.
VAR bFirstScan : BOOL := TRUE; // Initializes to TRUE on PLC startup bInitializationDone : BOOL; // Your application variables nState : INT; fTargetVelocity : LREAL; END_VAR // --- PLC Execution Code --- IF bFirstScan THEN // 1. Initialize State Machines nState := 10; // 2. Set Default Configurations fTargetVelocity := 1500.0; // 3. Clear Historical Fault Arrays or Buffers // (Your code here) // 4. Reset the first scan bit so this block never runs again bFirstScan := FALSE; END_VAR // Normal, cyclic PLC logic continues here Use code with caution. How It Works during Online Changes Scope Your Bits Correctly Understanding the Beckhoff First
boolean, which signals the initial task execution, or by creating a custom global variable initialized to TRUE. Alternative methods include utilizing
Some hardware modules (e.g., high-speed counters, PWM generators) need a setup block executed exactly once.
This ensures that regardless of any retained data from previous runs, the machine will always begin its operation logic from a defined starting point.
The first scan bit is a fundamental concept for writing robust and predictable PLC programs in Beckhoff TwinCAT. Whether you choose the system variable SystemTaskInfoArr[1].firstCycle for standard cold-start initialization or a custom RETAIN flag for more control, understanding this feature will help you build more reliable automation systems.
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