Beckhoff First Scan Bit Hot! Guide

Demystifying the Beckhoff "First Scan Bit": 3 Ways to Handle PLC Initialization in TwinCAT 3

Its primary purpose is to provide a controlled "clean slate" for the machine's logic. It's the ideal trigger for executing initialization routines that should only run once at startup, ensuring the system begins in a predictable and safe state, rather than resuming from an unknown or unsafe condition.

: The timer’s output starts FALSE . On the first cycle, IN is TRUE , but the timer hasn't elapsed, so Q remains FALSE . Thus bFirstScan = TRUE . On the second cycle, Q becomes TRUE , IN becomes FALSE , and bFirstScan becomes FALSE permanently. beckhoff first scan bit

If you are working on older legacy controllers (such as the Beckhoff CX1010 or CX1020 series) or maintaining an legacy system, you may find the global system array variable explicit:

PROGRAM MAIN VAR bInit : BOOL := TRUE; // Our first scan flag bFirstScanDone : BOOL; fbFirstScan : F_TRIG; fbEcMaster : FB_EcCoEADsRead; // EtherCAT utility nState : INT; END_VAR Demystifying the Beckhoff "First Scan Bit": 3 Ways

END_IF

In TwinCAT 2, it is typically FirstCycle or FirstScan . In TwinCAT 3, it is available via Tc2_System function block or directly as FirstScan in some contexts. On the first cycle, IN is TRUE ,

: Starting handshake sequences with external devices or HMIs.

While Structured Text is powerful, TwinCAT supports all five IEC 61131-3 programming languages, allowing you to work in the environment you are most comfortable with. Here is a pseudo-code example of how one might check the same condition in . You would call the GETCURTASKINDEX function block and then use a contact for _TaskInfo[Index].firstCycle .