Beckhoff First Scan Bit [hot] 🚀

IF bFirstScan THEN bFirstScan := FALSE; // One-time init code END_IF

If multiple POUs across different application layers need to know it is the first scan, declare a global variable (GVL) named GVL_System.bFirstScan . Reset it to FALSE at the very end of your primary, highest-priority cyclic task. Summary Checklist Implementation Strategy Simple POU Reset

Some hardware modules (e.g., high-speed counters, PWM generators) need a setup block executed exactly once.

If you are using Object-Oriented Programming (OOP) with Function Blocks, you should generally use the method for hardware checks or setup, rather than a "First Scan" bit inside the body logic. This runs before the first cyclic call and is cleaner for object initialization.

The most robust, precise, and professional way to find out if the runtime is on its very first loop execution is by tapping into TwinCAT's built-in task system structures. beckhoff first scan bit

Resetting Modbus, OPC UA, or TCP/IP communication buffers to ensure stale data isn't transmitted during boot-up.

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

You can use the attribute 'call_after_init' pragma to mark a method to be executed immediately after the FB_init call but still before the first cyclic call of the FB. This gives you another layer of control over the object's lifecycle.

Because multiple tasks can run concurrently in a Beckhoff controller, you must programmatically fetch the specific task index of the executing code using the GETCURTASKINDEX function block. Implementation in Structured Text (ST) IF bFirstScan THEN bFirstScan := FALSE; // One-time

In Beckhoff TwinCAT, the First Scan Bit is accessed via the _TaskInfo system array using the FirstCycle property. This bit is uniquely TRUE during the first execution cycle of a task, allowing for precise system initialization. First Scan Bit - OpenPLC Forum

Are you handling during this first scan?

FirstScan gives you a deterministic moment to reset, preset, or home everything before normal operation begins.

When the TwinCAT PLC runtime transitions to , memory is allocated and bFirstScan is initialized to TRUE . If you are using Object-Oriented Programming (OOP) with

TwinCAT 3 allows you to use attributes to ensure initialization happens properly, especially when dealing with complex function blocks or specialized task handling.

along the rung when bInitDone is FALSE . Set the Coil bInitDone at the end of that same rung.

In TwinCAT 2, developers frequently monitored the SystemInfoCtrl or utilized standard system flags. However, for TwinCAT 3, is highly preferred due to its independence from specific system task architectures and superior portability across different hardware targets (ARM vs. x86/x64).

: This bit is TRUE during the very first execution cycle of the PLC task and automatically switches to FALSE for all subsequent cycles. Implementation Example (ST)

Note: It is worth highlighting that restarting or stopping/starting the PLC code via the engineering UI may not always toggle this bit. It strictly relies on the full initialization of the TwinCAT runtime system.

Purging old data arrays, pointers, or FIFO queues from previous run cycles.