Beckhoff First Scan Bit

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

Beckhoff First Scan Bit — Essay

The Beckhoff “first scan” bit is a small but important concept in TwinCAT PLC programming that helps ensure deterministic, safe, and predictable behavior during controller startup. While its implementation is straightforward, understanding its purpose and correct use is essential for robust automation systems.

What the first scan bit is

• The first scan bit is a boolean flag that is true only during the PLC’s initial program cycle after the runtime transitions to RUN mode (or after a cold start). It indicates that the PLC is executing its very first program scan.
• In TwinCAT environments, this bit is typically provided by the runtime system (for example, as SysInit or a compiler/runtime variable) rather than being a user-created flag. Different Beckhoff/TwinCAT versions may expose named system variables or recommended idioms to detect first-scan behavior.

Why it matters

• Initialization: Many variables, hardware outputs, or state machines must be set to known safe values at startup. The first scan bit provides a reliable trigger to perform one-time initialization actions (e.g., zeroing integrators, setting actuators to safe states, loading defaults).
• Preventing transient actions: Without first-scan detection, logic that assumes valid sensor data or active communications might execute incorrectly during startup. The first-scan bit helps avoid unintended actuator movement, alarms, or communication attempts until initialization completes.
• Determinism and safety: Explicit first-scan initialization improves deterministic behavior between restarts and ensures the system moves from an undefined power-up state to a controlled, validated state.
• Commissioning and diagnostics: During commissioning, the first scan can be used to log startup conditions, set diagnostic flags, or run self-tests once per start.

Typical uses and patterns

• One-time initialization: Assigning initial values to persistent variables, resetting timers/counters, or toggling state-machine states to an initial state.
• Safe output setup: Force digital outputs to safe values (e.g., OFF) on first scan until the logic verifies sensors and interlocks.
• Communication setup: Defer network communication or fieldbus message transmissions until after first-scan initialization has completed.
• Conditional transitions: Use the first-scan bit to gate transitions in state machines so that the controller starts in a defined state rather than progressing based on stale or default inputs.
• Feature gating: Enable diagnostic or setup code paths only on first scan to avoid interfering with normal runtime behavior.

Implementation examples (conceptual)

• First-scan guarded initialization:
  • if FIRST_SCAN then
    • VarA := DefaultA;
    • MotorEnable := FALSE;
    • Reset counters/timers;
  • end_if
• One-shot initialization with persistent flag:
  • if FIRST_SCAN and NOT Initialized then
    • Perform initialization;
    • Initialized := TRUE;
  • end_if

Best practices

• Use the runtime-provided first-scan indicator when available rather than relying on timers or custom startup delays.
• Keep first-scan logic concise and deterministic—do not perform long-running operations directly on first scan; instead, set state and let cyclic logic perform staged initialization.
• Combine first-scan checks with hardware interlocks and watchdogs. First-scan should not be the only safety mechanism.
• Document initialization behavior clearly so operators and maintainers understand startup sequencing.
• Test cold starts, warm restarts, and safe-stop/restore scenarios to ensure the first-scan logic covers required cases.

Version and platform considerations

• TwinCAT or Beckhoff runtime versions may expose different system variable names or recommended approaches for detecting first-scan. Consult version-specific documentation or system libraries (e.g., SysLib) for the canonical variable or function block.
• Some installations distinguish between cold start (power loss) and warm start (download or restart). Decide whether initialization should run for both and implement accordingly.

Conclusion The Beckhoff first scan bit is a simple but essential tool for controlled startup in TwinCAT PLC programs. Properly used, it ensures variables and outputs are initialized predictably, prevents unsafe transient actions, and supports reliable commissioning and diagnostics. Adhering to concise, documented first-scan patterns and combining them with broader safety practices produces safer, more maintainable control software.

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:

1. 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.
2. 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.
3. 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. beckhoff first scan bit

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

1. Access the system variables: Open the TwinCAT PLC programming software (e.g., TwinCAT 3) and navigate to the system variables section.
2. Identify the First Scan Bit: Locate the system variable that represents the First Scan Bit (e.g., FirstScan).
3. 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 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. Here’s a concise guide to the First Scan

🔄 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 systems, there is no single global "S:FS" bit like those found in Rockwell (Allen-Bradley) controllers . Instead, users typically leverage the PlcTaskSystemInfo

structure or create a custom initialization variable to manage first-scan logic. Beckhoff Information System Key Ways to Implement a First Scan Bit

There are two primary methods used in TwinCAT to achieve "first scan" functionality: System Variable Method : The most robust way is using the FirstCycle member of the PlcTaskSystemInfo structure. How it works : Every PLC task has a system variable which contains a boolean FirstCycle . This bit is only during the very first cycle of that specific task.

: It is built into the runtime and is highly reliable for initializing state machines or variables. Implementation Example

IF _TaskInfo[GETCURTASKINDEX()].FirstCycle THEN // Your initialization logic here END_IF; Use code with caution. Copied to clipboard Custom Variable Method

: You can manually create a non-retentive boolean variable initialized as How it works : Define a with an initial value of

. At the very end of your main program, set this variable to

. It will then act as a first-scan bit for every subsequent cycle. : Simple to understand and portable across different IEC 61131-3 platforms. Beckhoff Information System User Experience and "Reviews" Behavioral Quirks : Some users have noted that the FirstCycle

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

: In industrial settings, a first-scan bit is considered essential for resetting retentive memory and ensuring equipment starts in a safe, predictable state. Alternative for Advanced Users

: For complex setups, some developers prefer using a dedicated Initialization (INIT) block

or state machine that runs once before the main cyclic logic begins. DigiKey TechForum

For official documentation on these system variables, you can refer to the Beckhoff Information System Are you looking to initialize specific variables or are you migrating logic from another PLC platform RSLogix 5000 First Scan Bit (S:FS) Programming Guide

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 The first scan bit is a boolean flag

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

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

Here’s a technical feature article exploring the First Scan Bit in Beckhoff TwinCAT systems.

Typical Use Cases

🧪 Testing the First Scan Bit

You can easily test it by:

1. Creating a simple counter:

   IF fbFirstScan.bFirstScan THEN
       nFirstScanCounter := nFirstScanCounter + 1;
   END_IF
   

2. Run → Login → Download
   → Counter increments by 1

3. Stop → Start PLC (without download)
   → Counter does not increment (warm start without new program)
   → In many cases, bFirstScan stays FALSE here unless configured to detect warm starts.

Typical Implementation Patterns

1. Built-in FirstScan flag (recommended if provided)

   • Use runtime-provided FIRST_SCAN boolean (if available) evaluated within cyclic tasks.
   • Code runs guarded by IF FIRST_SCAN THEN ... END_IF.

2. Software-driven flag (common)

   • Create a global BOOL FirstScan := TRUE; in initialization.
   • In the first PLC cycle, execute initialization and set FirstScan := FALSE.
   • Use a short delay or event to ensure I/O mapping is ready if needed.

3. Edge-detected approach

   • Use a R_TRIG on a system state transition (e.g., from PREOP to RUN) to detect the first RUN cycle.

4. Supervisor/Task-based initialization

   • Use a dedicated initialization POUs (FUNCTION_BLOCK or PROGRAM) called first in task order with highest priority.
   • After initialization, signal other tasks that startup is complete via a semaphore or boolean Ready flag.

4. One‑Time Hardware Configuration

Some hardware modules (e.g., high-speed counters, PWM generators) need a setup block executed exactly once.

IF FirstScan THEN
    // Configure encoder input
    Encoder_SetMode(ENC_MODE_QUADRATURE);
    Encoder_SetResolution(4096);
END_IF