Beckhoff First Scan Bit Jun 2026
To illustrate the utility of the first scan bit, here are a few common industrial automation use cases:
By checking if the current task cycle count equals 1 , you can identify the first scan globally.
By pairing a boolean variable defaulted to TRUE with an immediate reset to FALSE inside the code, you create an automatic, single-cycle trigger. Structured Text (ST) Implementation
If you need to initialize something, but it requires that I/O modules are fully active, the first scan might be too early. beckhoff first scan bit
Instead of an absolute global bit, TwinCAT provides an array of structures named _TaskInfo . This array tracks the real-time performance and diagnostics of every active PLC task running inside the TwinCAT execution environment. Within the PlcTaskSystemInfo structure , Beckhoff natively updates a boolean property called FirstCycle . During the first-ever execution cycle of that specific task, this variable is set to TRUE ; on every subsequent cycle, it automatically remains FALSE .
When working with the first scan bit, keep these points in mind to avoid common pitfalls:
If you are maintaining legacy TwinCAT 2 systems or want to utilize structural system flags, you can monitor the state of the PLC task control flags. TwinCAT provides standard libraries containing system structures that reflect the state of the execution engine. To illustrate the utility of the first scan
Large projects with multiple tasks or complex initialization. Quick, single-task projects or basic logic. Summary of Benefits
For a machine to operate safely, it must always start from a well-defined "home" state. The first scan bit is the ideal place to force a state machine to begin at its initial state, such as ST_IDLE or ST_STOPPED . This prevents the machine from trying to resume a partially completed operation after a power cycle, which could be hazardous.
If you prefer a portable method that works across almost any IEC 61131-3 platform, you can create your own logic using a global variable. Instead of an absolute global bit, TwinCAT provides
: Simplest to implement and easy to read for engineers coming from other platforms.
The First Scan Bit is an internal Boolean variable that acts as a one-shot trigger. It acts as the signal that the PLC program has just loaded into memory and is beginning its first cycle of execution. Key Characteristics: It is TRUE on the very first cycle.
