Beckhoff First Scan Bit Jun 2026

The most common, portable, and standard-compliant way to create a first scan bit in TwinCAT Structured Text (ST) is by declaring a local or global initialization variable. Implementation

(* Your regular cyclic code runs here *) nCounter := nCounter + 1;

Remember: A well-implemented first scan routine separates unreliable prototypes from industrial-grade automation. Take the time to initialize deliberately—your machine's safe operation depends on it.

Clear old error flags that might have occurred during the previous shutdown. Establish Communication: beckhoff first scan bit

If you store data in VAR_RETAIN or VAR_PERSISTENT , these variables survive a PLC restart. If your first scan bit blindly overwrites these variables with hardcoded default values on every boot, you defeat the entire purpose of persistent memory. Ensure your initialization logic leaves persistent recipes intact unless explicitly forced by a "Factory Reset" flag.

The standard method to detect the first cycle in TwinCAT is to use two key tools: the GETCURTASKINDEX function block and the global _TaskInfo array.

// Get the index of the current PLC task nTaskIdx := GETCURTASKINDEXEX(); The most common, portable, and standard-compliant way to

If you are developing your application using Ladder Logic or Function Block Diagram (FBD) within TwinCAT, you can replicate Method 1 visually using a sealed coil or an interlocking contact. Visual Logic Sequence

As noted in industry forums, the first scan bit in Beckhoff is tied to the Runtime Started state, rather than just the "RUN" mode switch. 3. Best Practices for First Scan Logic

To use the First Scan Bit in a Beckhoff PLC, you need to access the PLC's system variables. The First Scan Bit is typically represented by a specific system variable, such as FirstScan or InitDone . Clear old error flags that might have occurred

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

Do not call asynchronous function blocks (like file I/O operations, ADS read/write blocks, or socket communications) inside the first scan IF statement. These blocks require multiple PLC cycles to return a bBusy = FALSE status. They will stall or fail if executed only once.

PROGRAM MAIN VAR bFirstCycleDone : BOOL := FALSE; END_VAR