Beckhoff First Scan Bit ✓

The FirstCycle bit is a member of the PlcTaskSystemInfo data type. This structure is part of the TwinCAT 3 PLC library and provides diagnostic data directly from the real-time kernel. Accessing it requires the GETCURTASKINDEX function block to identify which task is currently executing, as TwinCAT can run multiple tasks with different cycle times. 2. Alternative "Manual" Method

#Beckhoff #TwinCAT #PLCProgramming #Automation #ControlsEngineering #IEC61131

There are two primary ways to access or create a "First Scan" signal in Beckhoff TwinCAT: beckhoff first scan bit

| Variable | Data type | Description | | :--- | :--- | :--- | | active | BOOL | TRUE if the task is active. | | taskName | STRING(16) | The name of the task. | | | BOOL | TRUE only on the first scan cycle after system startup; FALSE thereafter. | | cycleTimeExceeded | BOOL | TRUE if the task's execution time exceeds the configured cycle time. | | cycleTime | UDINT | The configured cycle time of the task (in 100 ns increments). | | lastExecTime | UDINT | The execution time of the task during the previous cycle (in 100 ns increments). | | priority | BYTE | The priority of the task. | | cycleCount | UDINT | The number of completed cycles since the task started. |

This is the standard, robust method in TwinCAT 3. You check the FirstCycle property within the PlcTaskSystemInfo array. The FirstCycle bit is a member of the

: Triggering a one-time read from a CSV file or database.

But this is still a workaround. The most professional method in Tc3_Standard is to use the StandardLib (from Beckhoff Automation) which includes FB_FirstScan directly. | | | BOOL | TRUE only on

A common pitfall: If you mark variables as RETAIN , they a warm start. But on a first scan (especially after download), you may want to override retain values.

The most common, efficient, and standard-compliant way to create a first scan bit in TwinCAT Structured Text (ST) is by using a local or global BOOL variable that defaults to FALSE (or TRUE , depending on your logic preference).