Beckhoff First Scan Bit

Here is a simple example in Structured Text (ST) demonstrating the usage:

The principle is to declare a global BOOL variable and ensure it retains its value across cycles:

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).

For engineers working with Beckhoff TwinCAT 3 (and TwinCAT 2), the concept of the (often implemented via the bInit or bFirst variable) is the cornerstone of safe and robust machine initialization. beckhoff first scan bit

Unlike some other PLC platforms (like Allen-Bradley or Siemens) that have a pre-defined system bit (e.g., First_Scan ), Beckhoff TwinCAT requires you to create this bit yourself. Fortunately, it is very simple to do. Method A: The Classic TwinCAT Approach (Code-Based)

During the first scan, you typically want to:

If you perform a or a Reset Original , the memory is cleared, variables re-initialize to their default values, and bFirstScan will correctly fire again on the next startup. Method 2: Utilizing TwinCAT System Variables Here is a simple example in Structured Text

Wiping the slate clean on startup so old errors don't prevent a start.

This is the most common application. When a PLC starts, the initial values of many variables must be set to a known, safe state. While you can assign initial values in variable declarations, a first scan routine can perform more complex logic that might depend on other system conditions that are not yet ready at compile time.

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. Fortunately, it is very simple to do

In all subsequent cycles, the condition evaluates to false, skipping the initialization block.

IF TwinCAT_SystemInfoVarList._FirstScan THEN // One-time actions END_IF

: Triggering initial requests for external fieldbus devices like EtherNet/IP Beckhoff Information System Function Block Diagram