Beckhoff First Scan Bit !!hot!! Instant

IF FirstScan THEN IF bInit_Cold THEN // Full factory reset ELSIF bInit_Warm THEN // Restore retain variables, but reinit comms END_IF END_IF

// Check if the flag is still false IF NOT bFirstCycleDone THEN // Place your one-time initialization logic here // ...

: Do not use the first scan bit to overwrite PERSISTENT or RETAIN variables unless you intentionally want to ignore their saved values upon every reboot.

The system automatically resets this bit to FALSE after the first cycle completes. beckhoff first scan bit

PROGRAM MAIN VAR bFirstScan : BOOL; rst : BOOL; END_VAR

If you perform an "Online Change" (modify code without full download), the first scan bit does trigger. Your initialization code will not run. To force reinitialization, use Reset or Reset Cold from the TwinCAT runtime.

In PLC (Programmable Logic Controller) programming, initialization is a critical phase of the execution lifecycle. When a controller switches from Config Mode or Stop Mode into Run Mode, variables must be set to safe initial states, communication links must be established, and system histories must often be cleared. IF FirstScan THEN IF bInit_Cold THEN // Full

The cleanest and most modern way to retrieve the first scan status in is by utilizing the function block GETCURTASKINDEX paired with the global variable array _TaskInfo . This dynamically captures the cycle state of whichever task is currently calling the code. Structured Text (ST) Implementation

FB_init is a specialized method you can add to any Function Block. It executes automatically before the PLC task starts its cyclic execution. Right-click your Function Block →right arrow →right arrow Method . Name the method exactly FB_init . It automatically generates the required signature:

| TwinCAT Version | Variable Name | Scope | |----------------|---------------|-------| | TwinCAT 2 | bInit | Global (in Standard.lib ) | | TwinCAT 3 | FirstScan | Per-POU (automatic) | PROGRAM MAIN VAR bFirstScan : BOOL; rst :

The behavior of the first scan bit can vary depending on your specific Beckhoff hardware platform:

This is particularly useful for initializing complex data structures, allocating memory with __NEW , or passing parameters to function blocks before they are used in the program.

The Beckhoff First Scan bit is your "clean slate" button. Whether you use a simple boolean flag or the system's cycle counter, implementing this ensures that your machine starts up with the correct parameters every time, preventing "ghost" data from causing erratic behavior during commissioning.