First Scan Bit: Beckhoff

If you are coming from the Allen-Bradley world, you are likely very comfortable with the S:FS (First Scan) bit. It’s a staple for initializing logic, resetting counters, or clearing buffers on startup.

When you transition to Beckhoff TwinCAT, you won’t find a system bit named exactly "First Scan" in the global variable list. This often leads to the question: How do I run logic exactly once when the PLC starts?

Here is the standard way to achieve this in TwinCAT 3 (IEC 61131-3).

If you are using Object-Oriented Programming (OOP) with Function Blocks, you should generally use the FB_Init method for hardware checks or setup, rather than a "First Scan" bit inside the body logic. This runs before the first cyclic call and is cleaner for object initialization.

However, for standard ladder logic or structured text programs, the boolean flag method above is the industry standard for Beckhoff systems.

Have you found other uses for a "First Scan" bit in your machines? Let me know in the comments!

#Beckhoff #TwinCAT #PLCProgramming #Automation #ControlsEngineering #IEC61131

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. beckhoff first scan bit

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:

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:

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.

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. Method 1: Manual Boolean (Most Common) If you are coming from the Allen-Bradley world,

The most straightforward way is to declare a global variable that resets itself after the first cycle.

Declare Variable: In your Global Variable List (GVL) or program, declare a BOOL with an initial value of TRUE. VAR_GLOBAL bFirstScan : BOOL := TRUE; END_VAR Use code with caution. Copied to clipboard

Reset Logic: At the very end of your MAIN program (or the last program executed in your task), set this bit to FALSE.

// Your logic here... IF bFirstScan THEN // Initialization code END_IF // Last line of the program bFirstScan := FALSE; Use code with caution. Copied to clipboard Method 2: Using PLC Task System Variables

For a more "built-in" feel, you can access internal PLC task information. Beckhoff provides a structure that tracks the scan count of a task. Variable: _TaskInfo[Index].CycleCount

Logic: If the CycleCount is 1, it is the first scan. This is useful if you need to know the first scan of a specific task rather than the entire PLC. Method 3: Initialization in POUs (FB_init)

If you are using Function Blocks, TwinCAT 3 supports the FB_init method. This is a specialized sub-method that runs once when the block is instantiated (during PLC startup or after a download), making it the cleanest way to handle block-specific initializations. Why use a First Scan Bit?

Resetting Latches: Clearing OTL (Latches) or variables that must start in a known state.

Loading Recipes: Triggering a one-time read from a CSV file or database.

Communication Init: Establishing initial handshake bits with HMIs or other PLCs.

PROGRAM MAIN
VAR
    fbSystemFirstScan : FB_FirstScan;
    bGlobalFirstScan : BOOL;
END_VAR
// Call this every cycle (it will return TRUE only on first)
bGlobalFirstScan := fbSystemFirstScan();

The FB_FirstScan function block monitors the system’s cycle counter and reliably returns TRUE for exactly one cycle after application start, even if multiple programs call the same FB instance. Have you found other uses for a "First

During the first scan, you typically want to:

Without a proper first scan routine, your machine might start with actuators in unpredictable positions or unfinished states from a previous run.

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;

Beckhoff provides additional system flags for finer control:

Use these in combination:

IF FirstScan THEN
    IF bInit_Cold THEN
        // Full factory reset
    ELSIF bInit_Warm THEN
        // Restore retain variables, but reinit comms
    END_IF
END_IF

Physical outputs (via PA_ or AT%Q*) often keep their previous value through a restart unless explicitly cleared.

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

END_IF

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