Beckhoff First Scan Bit -

Understanding the Beckhoff First Scan Bit: A Comprehensive Guide

In the world of industrial automation, programmable logic controllers (PLCs) play a crucial role in controlling and monitoring various processes. Beckhoff, a renowned German-based company, has been at the forefront of PLC technology, providing innovative solutions for a wide range of industries. One of the key features of Beckhoff PLCs is the "First Scan Bit," a concept that has significant implications for PLC programming and operation. In this article, we will delve into the world of Beckhoff PLCs and explore the concept of the First Scan Bit in detail.

What is a Beckhoff PLC?

Before we dive into the First Scan Bit, let's take a brief look at Beckhoff PLCs. Beckhoff Automation GmbH & Co. KG is a leading global supplier of automation technology, including PLCs, human-machine interfaces (HMIs), and motion control systems. Their PLCs, also known as TwinCAT (Twin Computer) systems, are widely used in various industries, such as automotive, aerospace, food and beverage, and more.

Beckhoff PLCs are known for their flexibility, scalability, and high performance. They offer a range of PLC platforms, from compact, entry-level devices to high-end, rack-based systems. One of the key features of Beckhoff PLCs is their ability to execute PLC code in a Windows-based environment, allowing for seamless integration with other Windows applications.

What is the First Scan Bit?

The First Scan Bit, also known as the "First Cycle Bit" or "Initialization Bit," is a special bit in Beckhoff PLCs that indicates when the PLC is executing its first scan cycle. In other words, it signals that the PLC is starting up and executing its program for the first time.

The First Scan Bit is a digital output that is automatically set by the PLC during its startup sequence. When the PLC is powered on or reset, it executes a series of internal checks and initializations before starting to execute the user program. During this first scan cycle, the First Scan Bit is set to TRUE (or 1).

Why is the First Scan Bit important?

The First Scan Bit is essential for several reasons:

Initialization: The First Scan Bit provides a clear indication that the PLC is starting up and executing its program for the first time. This allows the user program to perform any necessary initialization tasks, such as setting default values, initializing variables, or executing specific startup routines.
Program execution: By detecting the First Scan Bit, the user program can determine when to execute specific code segments, such as initialization routines or startup sequences.
Safety: In safety-critical applications, the First Scan Bit can be used to ensure that safety-related functions are executed during startup, ensuring the safe operation of the machine or process.

How to use the First Scan Bit in Beckhoff PLCs

To use the First Scan Bit in a Beckhoff PLC, you need to access the PLC's system variables. The First Scan Bit is typically represented by a specific system variable, such as FirstScan or InitDone.

Here are the general steps to use the First Scan Bit:

Access the system variables: Open the TwinCAT PLC programming software (e.g., TwinCAT 3) and navigate to the system variables section.
Identify the First Scan Bit: Locate the system variable that represents the First Scan Bit (e.g., FirstScan).
Use the First Scan Bit in your program: In your user program, you can use the First Scan Bit to execute specific code segments during the first scan cycle.

Example: Using the First Scan Bit in TwinCAT 3

In TwinCAT 3, the First Scan Bit is represented by the system variable FirstScan. Here's an example of how to use it in a simple PLC program:

PROGRAM Example
VAR
    FirstScan    : BOOL;
END_VAR
BEGIN
    IF FirstScan THEN
        // Execute initialization code here
        // e.g., set default values, initialize variables
        FirstScan := FALSE;
    END_IF
// Rest of the user program...
END_PROGRAM

In this example, the FirstScan system variable is used to execute an initialization code segment during the first scan cycle. Once the initialization is complete, the FirstScan bit is reset to FALSE.

Conclusion

The Beckhoff First Scan Bit is a powerful feature that allows PLC programmers to execute specific code segments during the first scan cycle of a PLC. By understanding the concept of the First Scan Bit, developers can create more efficient, safe, and reliable PLC programs. Whether you're a seasoned PLC programmer or just starting out, the First Scan Bit is an essential concept to grasp when working with Beckhoff PLCs.

In this article, we've provided a comprehensive overview of the Beckhoff First Scan Bit, including its definition, importance, and usage. By following the guidelines outlined here, you'll be able to harness the power of the First Scan Bit in your own PLC projects.

In Beckhoff TwinCAT, the First Scan Bit is a system flag used to execute logic exactly once when the PLC transitions from Config/Stop to Run mode. It is essential for initializing variables, resetting timers, or triggering one-time communication handshakes. How to Access the First Scan Bit

TwinCAT provides a built-in system variable for this purpose within the PlcAppSystemInfo structure. You do not need to create a global variable manually; you can access it via the TwinCAT_SystemInfoVarList. Variable Name: _AppInfo.bFirstCycle Data Type: BOOL

Behavior: This bit is TRUE during the very first execution cycle of the PLC task and automatically switches to FALSE for all subsequent cycles. Implementation Example (ST)

You can use this bit in Structured Text to set default values at startup:

IF _AppInfo.bFirstCycle THEN // Initializing setpoints fTargetTemperature := 22.5; bSystemReady := FALSE; // Resetting operational counters nCycleCounter := 0; END_IF Use code with caution. Copied to clipboard Key Use Cases

Variable Initialization: Setting non-persistent variables to a known starting state.

Pulse Triggers: Triggering a TP (Timer Pulse) or R_TRIG that needs to fire immediately upon startup.

Communication Reset: Sending a "Reset" or "Init" command to external devices (like drives or Vision systems) over EtherCAT.

Loading Recipes: Reading a default parameter set from a file or database during the first task execution. Important Considerations beckhoff first scan bit

Multiple Tasks: If your project has multiple PLC tasks, _AppInfo.bFirstCycle is local to the context of the task it is called in.

Persistent Variables: Do not use the first scan bit to overwrite PERSISTENT or RETAIN variables unless you intentionally want to ignore their saved values upon every reboot.

Manual Implementation: If you prefer not to use the system global, you can create a local "Init" flag:

IF NOT bInitDone THEN // Do startup logic here bInitDone := TRUE; END_IF Use code with caution. Copied to clipboard

In the world of industrial automation, specifically within the Beckhoff TwinCAT environment, the "first scan bit" is a fundamental concept used to initialize logic, reset variables, or trigger one-time events when a PLC program transitions from Stop to Run mode.

Understanding how to implement and utilize this bit effectively ensures that your machines start up in a safe, predictable state every time the controller is powered on or the code is restarted. What is a 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.

Unlike some traditional PLCs (like Allen-Bradley’s S:FS bit) that have a predefined system variable, Beckhoff’s TwinCAT allows for several ways to achieve this functionality depending on your version and preference. Methods to Implement First Scan in TwinCAT 1. Using the TwinCAT System Info (The Pro Way)

The most robust method involves using the built-in PlcTaskSystemInfo structure. This provides real-time data about the current task. Variable: _TaskInfo[index].FirstCycle

How it works: This system variable is automatically managed by the TwinCAT runtime.

Benefit: No manual coding is required to reset the bit; it is inherently tied to the task execution. 2. Manual Logic (The Classic Way)

If you prefer a portable method that works across almost any IEC 61131-3 platform, you can create your own logic using a global variable.

Step 1: Declare a global boolean, e.g., bFirstScan : BOOL := TRUE;.

Step 2: At the very end of your MAIN routine, add: bFirstScan := FALSE;.

How it works: On startup, the variable initializes to TRUE. The logic runs once, and then the assignment at the bottom kills the bit for all subsequent cycles. 3. Using the 'Init' Attribute

TwinCAT 3 supports the attribute 'init_on_on_online_change' or specific FB_init methods. While more advanced, these are used to run initialization code specifically when a function block is instantiated or the PLC starts. Common Use Cases

🚀 Initialization of SetpointsEnsures that PID gains, speed limits, or timers start with default "safe" values rather than zeros.

🔄 Resetting State MachinesForces your Sequential Function Chart (SFC) or CASE statements to jump to the 'IDLE' or 'INIT' state regardless of where they were during the last shutdown.

📡 Communication HandshakesTriggers a "Hello" or synchronization pulse to external devices, such as HMIs or SQL databases, to signal that the PLC is back online. Best Practices and Pitfalls

Execution Order Matters: If you use a manual first scan bit, ensure it is set to FALSE at the very end of your program. If you do it at the top, the rest of your logic won't see the TRUE state.

Distributed Tasks: If your TwinCAT project has multiple tasks (e.g., a fast 1ms task and a slow 10ms task), remember that each task has its own "first cycle."

Avoid Logic Overload: Don't cram too much heavy processing into the first scan. If you have massive data to load, consider a dedicated "Initialization" state that spans multiple cycles.

⚠️ Key Reminder: 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.

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. Understanding the Beckhoff First Scan Bit: A Comprehensive

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:

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 complete, ready-to-use post explaining the Beckhoff First Scan Bit in TwinCAT PLC.

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;

Method 1: `Tc2_Standard` (TwinCAT 2 Legacy)

In legacy TwinCAT 2 and early TwinCAT 3 projects using the Tc2_Standard library, the standard way to get a first scan bit is:

PROGRAM MAIN
VAR
    fbFirstScan : TON;
    bFirstScan : BOOL;
END_VAR
// Implementation
fbFirstScan(IN := NOT fbFirstScan.Q, PT := T#1MS);
bFirstScan := NOT fbFirstScan.Q;

How it works: 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.

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.

Part 4: Best Practice – The `FB_Init` Method (Object-Oriented)

For advanced TwinCAT 3 users working with Function Blocks, the most elegant and robust "first scan" is not a bit at all—it's the object constructor: FB_Init.

When you instantiate a function block, TwinCAT automatically calls its FB_Init method once.

FUNCTION_BLOCK FB_DriveController
VAR_INPUT
    bEnable : BOOL;
END_VAR
VAR_OUTPUT
    bReady : BOOL;
END_VAR
VAR
    fSpeed : REAL;
END_VAR
// This runs once when the FB is created (first scan of the FB)
METHOD FB_Init : BOOL
VAR_INPUT
bInitRetains : BOOL;  // TRUE if retain variables are restored
bInCopyCode : BOOL;   // TRUE if FB is copied
END_VAR
fSpeed := 0.0;        // Initialize internal variable
bReady := FALSE;
END_METHOD

Advantages:

No extra Boolean flags scattered in your code.
Encapsulation: Each object initializes itself.
Support for retain variables: The bInitRetains parameter tells you if previous retain values were loaded.

When to use this: In large OOP-based TwinCAT projects with many reusable function blocks.

Method 2: The `Tc3_Standard` Library (Modern)

With TwinCAT 3, Beckhoff introduced the Tc3_Standard library, which includes a dedicated function block: F_TRIG combined with a system flag is no longer needed. Instead, use:

PROGRAM MAIN
VAR
    bFirstScan AT %Q* : BOOL;  // Not directly. Better:
    fbFirstScan : F_TRIG;
    bInit : BOOL := TRUE;
END_VAR
// Implementation
fbFirstScan(CLK := bInit);
bFirstScan := fbFirstScan.Q;
bInit := FALSE;

Better yet: The official Tc3_Standard provides a clean helper:

PROGRAM MAIN
VAR
    bFirstScan : BOOL;
    rst : BOOL;
END_VAR
// In your implementation section
bFirstScan := NOT rst;
rst := TRUE;

But this is still a workaround. The most professional method in Tc3_Standard is to use the StandardLib (from Beckhoff Automation) which includes FB_FirstScan directly.

Best Practices Summary

Use a single, well-documented FirstScan mechanism project-wide.
Guard all critical initialization with the flag and clear it promptly.
Coordinate initialization across tasks and modules via explicit readiness flags.
Validate EtherCAT/fieldbus states before manipulating hardware.
Design initialization to be fast, deterministic, and safe; defer noncritical work.

Conclusion: Choose the Right Tool for the Job

The "Beckhoff first scan bit" is not a single feature but a concept implemented through various patterns. For simple projects, a F_TRIG on a TRUE variable is sufficient. For robust, reusable code, use FB_Init in function blocks. For system-wide initialization that must run before cyclic logic, use the INIT section.

Final recommendation for most engineers:

For small to medium machines: Use the F_TRIG method at the top of your MAIN program.
For large, object-oriented projects: Implement FB_Init in every function block.
For safety-critical initialization: Combine INIT with a check of the EtherCAT state machine.

Remember: A well-implemented first scan routine separates unreliable prototypes from industrial-grade automation. Take the time to initialize deliberately—your machine's safe operation depends on it.

Further Reading:

Beckhoff Infosys: "TwinCAT PLC Control – Initialization of Variables"
TwinCAT 3 Tc3_Standard Library Documentation
EtherCAT State Machine (ESM) – FB_EcCoEADsRead

Have a specific first scan challenge? Visit the Beckhoff Community forums or consult your local Beckhoff support engineer.

In Beckhoff TwinCAT, there is no single global system bit like the Allen-Bradley S:FS. Instead, you use task-specific system information or a manual initialization variable. 1. Built-in FirstCycle Bit

The most robust way is to use the PlcTaskSystemInfo structure, which contains a FirstCycle boolean. This bit is TRUE only during the very first execution of that specific task after the TwinCAT runtime starts. Implementation Example (Structured Text):

VAR fbGetCurTaskIdx : GETCURTASKINDEX; // Function block to find the current task ID bFirstScan : BOOL; END_VAR fbGetCurTaskIdx(); // Call the FB // Access the system's internal task info array bFirstScan := _TaskInfo[fbGetCurTaskIdx.index].FirstCycle; IF bFirstScan THEN // Insert initialization logic here END_IF Use code with caution. Copied to clipboard

Behavior: This bit usually only triggers when the TwinCAT Runtime is started or restarted. Simply stopping and starting the PLC code with the "Start/Stop" commands in the IDE may not reset this bit. 2. Custom Initialization Variable

If you need a bit that resets every time you "Stop" and "Start" the PLC program (warm restart), the standard practice is to create a non-persistent variable that defaults to TRUE and is set to FALSE at the end of the scan. Implementation Example:

Declare a global or local variable: bInitialized : BOOL := FALSE; Logic:

IF NOT bInitialized THEN // Initialization code here bInitialized := TRUE; END_IF Use code with caution. Copied to clipboard

Behavior: Because TwinCAT resets non-persistent variables to their initial state upon a program start, bInitialized will be FALSE on the first scan, allowing your logic to run once. 3. Legacy Hardware (CX1010)

On older hardware like the CX1010, some technical references mention a specific status bit (Bit 4) in certain control parameters that is set for the first scan after power-up. Key Comparison PlcTaskSystemInfo.FirstCycle Custom bInitialized Variable Best for System-level startup/initialization Logic-level resets/warm restarts Reset Trigger TwinCAT Runtime Restart PLC Program Start (Start/Stop) Setup Requires GETCURTASKINDEX Simple declaration

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

Part 3: The Native TwinCAT Method – Using `PRG.INIT` and `PRG.EXIT`

Beckhoff TwinCAT offers built-in program-local initialization flags that many users overlook: INIT and EXIT sections within a program.

PROGRAM MAIN VAR myOutput : BOOL; END_VAR // INIT section runs once when program is loaded/started INIT myOutput := FALSE; // Set safe state // Home axes, clear arrays, etc. // Main cyclic code myOutput := TRUE; // Normal logic

// EXIT section runs when program stops EXIT myOutput := FALSE;

Key nuance: The INIT section runs before the first cyclic scan. This is actually earlier than a typical "first scan bit." If you need to guarantee initialization occurs before any other logic, INIT is superior.

But warning: If you call the program multiple times (e.g., as an action or method), the INIT behavior may change. Use it carefully within the main cyclic task.

2. Set Outputs to Safe State

Physical outputs (via PA_ or AT%Q*) often keep their previous value through a restart unless explicitly cleared. Initialization : The First Scan Bit provides a

IF FirstScan THEN
    // Force all digital outputs OFF
    myDigitalOutput := FALSE;
// Set analog output to 0V/0mA
myAnalogOutput := 0;
// Disable drives
driveEnable := FALSE;

END_IF

Beckhoff First Scan Bit -

The Standard Solution: The Initialization Method

Method 1: Tc2_Standard (TwinCAT 2 Legacy)

Part 4: Best Practice – The FB_Init Method (Object-Oriented)

Method 2: The Tc3_Standard Library (Modern)