Beckhoff First Scan Bit

A first scan bit is a boolean flag that remains TRUE for exactly one execution cycle of the PLC task. After the first logic solve is complete, the bit drops to FALSE and stays there until the PLC is restarted.

Beckhoff's TwinCAT 3 environment does not have a dedicated pre-defined "first scan" system bit like Allen-Bradley's S:FS . Instead, developers typically implement this functionality manually using an initial value or by referencing specific PLC task variables. beckhoff first scan bit

| TwinCAT Version | Variable Name | Scope | |----------------|---------------|-------| | TwinCAT 2 | bInit | Global (in Standard.lib ) | | TwinCAT 3 | FirstScan | Per-POU (automatic) | A first scan bit is a boolean flag

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

The First Scan Bit is essential for several reasons:

Caveat : This method is cycle-dependent. If your cycle time is 10ms, set PT to at least 1ms — but ensure it's longer than one cycle but shorter than two.