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If you are working on older legacy controllers (such as the Beckhoff CX1010 or CX1020 series) or maintaining an legacy system, you may find the global system array variable explicit:
Typical uses and patterns
: The FirstCycle bit typically triggers when the Runtime starts . Simple PLC "Stop" and "Start" commands in the IDE might not always reset this bit unless the entire TwinCAT system is restarted or a new configuration is activated .
The Beckhoff First Scan bit is a small, easily overlooked tool that separates professional, robust PLC code from fragile, “works-most-of-the-time” logic. By taking explicit control of the first cycle, you eliminate startup surprises, protect hardware, and ensure your TwinCAT application starts every time in a predictable, safe state. beckhoff first scan bit
In Beckhoff’s TwinCAT environment, the First Scan Bit is a fundamental diagnostic tool used to initialize logic, reset variables, or trigger specific startup sequences the moment the PLC transitions from Config/Stop
// Also override certain retained values if needed IF NOT bRetainValid THEN // Force defaults END_IF This public link is valid for 7 days
But here’s the catch: unlike traditional PLCs (e.g., Siemens with OB100 or Rockwell with FirstScan ), Beckhoff’s approach is more flexible—but also more confusing for newcomers. This article will dissect every method to detect and utilize the first scan cycle in TwinCAT, from standard PLC libraries to advanced object-oriented techniques.
// -- Wait for EtherCAT sync -- nState := fbEcMaster.GetState(); IF nState <> 8 THEN RETURN; // Don't run logic until bus is operational END_IF Can’t copy the link right now
By leveraging the robust and standardized method involving the GETCURTASKINDEX function block and the _TaskInfo system array, developers can create highly maintainable and transparent initialization logic. Whether you are using Structured Text, Ladder Diagram, or any other IEC 61131-3 language, the ability to trap the very first scan cycle is universally available.
This relies on the fact that standard, non-retained PLC variables always initialize to FALSE (or 0 ) upon a cold reset. Code Implementation (Structured Text)