Beckhoff First Scan Bit -

// -- Reset the first scan trigger for next init -- bInit := FALSE;

The "First Scan" bit in Beckhoff TwinCAT PLC systems is a boolean status flag indicating the PLC program's initial execution cycle after startup, download, or reset. It enables safe, deterministic initialization of variables, hardware states, and communication interfaces before normal cyclic operation proceeds.

// Your logic here... IF bFirstScan THEN // Initialization code END_IF // Last line of the program bFirstScan := FALSE; Use code with caution. Copied to clipboard

In the world of industrial automation, specifically within the Beckhoff TwinCAT environment, the "first scan bit" is a fundamental concept used to initialize logic, reset variables, or trigger one-time events when a PLC program transitions from Stop to Run mode. beckhoff first scan bit

A common misconception is that the first scan bit is only relevant upon a full system power-up. In TwinCAT, however, using the SystemTaskInfoArr[1].firstCycle variable will be TRUE when the controller physically starts up (a "cold start"). It will not be set to TRUE when you simply restart the PLC program from STOP to RUN.

// Main cyclic code myOutput := TRUE; // Normal logic

By utilizing the standard initialization behavior of variables in TwinCAT, you can create a self-resetting flag. // -- Reset the first scan trigger for

A common pattern is to use a RETAIN variable with the first scan bit to determine if the controller is starting up after a power loss versus a fresh program download:

Use these in combination:

Here’s a technical feature article exploring the in Beckhoff TwinCAT systems. IF bFirstScan THEN // Initialization code END_IF //

mapped to a retentive or standard flag named bInitDone .

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

The first scan bit is a versatile tool used in nearly every professional PLC project.

When working with the first scan bit, keep these points in mind to avoid common pitfalls:

Have a tricky Beckhoff initialization issue? Share your experience in the comments or contact your local Beckhoff automation partner.