Beckhoff First Scan Bit -

In Beckhoff TwinCAT, the equivalent of a "first scan bit" is the firstCycle variable found within the system task information. This bit is automatically set to TRUE only during the very first execution cycle of a PLC task, making it ideal for one-time initialization logic. How to Access the First Scan Bit

You can access this feature through the implicit _TaskInfo array, which contains data for every task running in your PLC project. Syntax (Structured Text):

IF _TaskInfo[GETCURTASKINDEXEX()].firstCycle THEN // Your initialization code here (e.g., setting default parameters) END_IF Use code with caution. Copied to clipboard Key Characteristics

Automatic Reset: The system automatically resets this bit to FALSE after the first cycle completes.

Task-Specific: Because it is part of the task info structure, it correctly identifies the first scan for the specific task it is called within.

Reliability: It is more robust than manual "first scan" flags (like using a boolean that you set to false at the end of the code), as the PLC runtime handles its state directly. Usage Example

Commonly used to load recipes, initialize communication drivers, or reset state machines to their starting position upon a PLC cold or warm start. If you'd like, I can show you: beckhoff first scan bit

How to manually create a first-scan bit if you're using an older version of TwinCAT.

How to use it to initialize persistent variables from a file.

The difference between a cold start and a warm start in relation to this bit. Beckhoff CX1010 first scan | PLCtalk - Interactive Q & A

Here’s a concise guide to the First Scan Bit in Beckhoff TwinCAT (IEC 61131-3).

Why Do You Need It?

During the first scan, you typically want to:

Set default values for global variables (since retain/persistent variables may hold old data).
Home axes or reset servo drives to a known state.
Clear alarm buffers that existed before the controller went offline.
Initialize communication buffers (EtherCAT, OPC UA, etc.).
Prevent unintended motion by setting all outputs to a safe state before logic runs.

Without a proper first scan routine, your machine might start with actuators in unpredictable positions or unfinished states from a previous run. In Beckhoff TwinCAT, the equivalent of a "first

Where It Appears

TwinCAT PLC runtime exposes First Scan information via the system variable(s) and libraries.
Typical names/places:
- SysTime or SysState related system areas.
- Tc2_SystemLibrary or ADS/GVL-provided flags in some templates.
- Many engineers implement their own FIRST_SCAN boolean set on startup (see Implementation Patterns).

Note: exact symbol names can vary by TwinCAT version and project conventions.

The Standard Solution: The Initialization Method

In TwinCAT, the PROGRAM or FUNCTION_BLOCK structure has a specific order of execution. The VAR section declares variables, but it doesn't execute logic. To run logic on the first scan, you declare a boolean flag and check its state.

The Code:

PROGRAM MAIN
VAR
    bFirstScan : BOOL := TRUE; (* Initialize as TRUE *)
    nCounter   : INT;
END_VAR
(* Logic Section )
IF bFirstScan THEN
( --- This runs ONLY on the first scan --- )
nCounter := 0;          ( Reset variables )
( Perform other startup routines *)
bFirstScan := FALSE;    (* Set to false so this never runs again *)

END_IF
(* Your regular cyclic code runs here *)
nCounter := nCounter + 1;
 Without a proper first scan routine, your machine

Example with Retain (power-cycle safe)

VAR RETAIN
    bInitialized : BOOL;
END_VAR
VAR
    bFirstScanSys : BOOL;
END_VAR
bFirstScanSys := TwinCAT_SystemInfoVarList._FirstScan;
IF bFirstScanSys AND NOT bInitialized THEN
// Run once in device lifetime
bInitialized := TRUE;
END_IF

Pitfall 3: Race Conditions with EtherCAT

If you set outputs on the first scan before the EtherCAT bus is fully operational (state OP), your writes may be ignored or cause errors. Always wait for EtherCAT Master State = OP before critical I/O initialization.

IF fbFirstScan() AND (nEtherCATState = 8) THEN  // 8 = OP
    EnableOutputs();
END_IF

The Definition

The First Scan Bit is a Boolean flag that is TRUE only for a single PLC cycle immediately after the runtime system starts executing the application. On the second cycle, it becomes FALSE and remains FALSE until the next controller reboot or program download.