|best| — Beckhoff First Scan Bit

Without a mechanism to detect the first scan, your program would execute initialization logic (like setting default values, clearing counters, or resetting state machines) every single time the code runs. Key scenarios requiring the First Scan Bit include:

Many EtherCAT terminals require a one-time configuration command after start-up to function correctly. The first scan bit is the perfect place to send these commands.

FirstScan gives you a deterministic moment to reset, preset, or home everything before normal operation begins.

If you are maintaining legacy TwinCAT 2 systems or want to utilize structural system flags, you can monitor the state of the PLC task control flags. TwinCAT provides standard libraries containing system structures that reflect the state of the execution engine. beckhoff first scan bit

| TwinCAT Version | Variable Name | Scope | |----------------|---------------|-------| | TwinCAT 2 | bInit | Global (in Standard.lib ) | | TwinCAT 3 | FirstScan | Per-POU (automatic) |

bit may only trigger when the TwinCAT runtime is fully restarted, rather than just stopping and starting the PLC code execution via the developer interface. Initialization Importance

Initialize critical error flags to TRUE (forcing a safe state) and non-critical outputs to safe FALSE values. 4. Common Pitfalls and Troubleshooting 1. "My First Scan Logic Runs Too Often" Without a mechanism to detect the first scan,

METHOD FB_init : BOOL VAR_INPUT bInitRetains : BOOL; bInCopyCode : BOOL; END_VAR

bInitTriggered is FALSE . The code enters the first IF statement. bFirstScan becomes TRUE . At the end of the block, bInitTriggered is flipped to TRUE .

In Beckhoff TwinCAT (2 and 3), there is no single "magic" global bit like the S:FS in Allen-Bradley . Instead, you can access the "First Scan" status through built-in system variables or by creating a custom initialization flag. 1. Using Built-in System Info ( FirstCycle ) FirstScan gives you a deterministic moment to reset,

⚠️ Always use the first scan bit to put your machine into a Safe State . Never assume the hardware is in the same position it was when the power went out.

The initialization code executes, and bInitialized is toggled to TRUE .