The Beckhoff First Scan Bit is a simple but powerful tool for safe PLC initialization. Always use the system library version (FB_FirstScan), and remember: first scan ≠ warm start. Use it to enforce a clean startup state, especially after program downloads or power cycles.
Have you encountered any unexpected behavior with first scan in TwinCAT? Let me know in the comments — I’ve debugged many tricky startup issues and can help!
The Beckhoff "First Scan Bit" refers to a signal used in TwinCAT PLC programming to execute initialization logic exactly once when the controller starts or enters run mode. Unlike some other PLC platforms that have a fixed system bit like Allen-Bradley's S:FS, Beckhoff TwinCAT provides this functionality through specific system variables or custom logic. Standard Implementation Methods
There are two primary ways to access or create a "First Scan" signal in Beckhoff TwinCAT:
System Info Variable (PlcTaskSystemInfo):The most reliable built-in method in TwinCAT 3 is using the FirstCycle boolean found within the PlcTaskSystemInfo structure.
How to use it: Access the global array _TaskInfo[index] using the GETCURTASKINDEX function block to retrieve the current task's index. Snippet Example:
IF _TaskInfo[fbGetCurTaskIndex.index].FirstCycle THEN // Your initialization code here END_IF Use code with caution. Copied to clipboard
Manual Bit Creation:A common "homemade" solution involves declaring a boolean variable with a default value of TRUE.
Logic: Place code at the very end of your main program that sets this bit to FALSE. Because the variable is initialized to TRUE, it remains so for the entire first scan before being permanently toggled off. Comparison and Review PlcTaskSystemInfo.FirstCycle Manual Custom Bit Reliability Native to TwinCAT; handles task-specific restarts. Highly reliable if implemented at the program's end. Complexity Requires calling GETCURTASKINDEX. Extremely simple to declare and use. Best Use Case
Large projects with multiple tasks or complex initialization. Quick, single-task projects or basic logic. Summary of Benefits
Initialization: Essential for setting default values for retentive memory variables.
State Reset: Ensures equipment begins in a safe, known state.
Task Specificity: Allows initialization to be tied to the specific timing of individual tasks rather than just global power-up. RSLogix 5000 First Scan Bit (S:FS) Programming Guide
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 beckhoff first scan bit
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.
The Sentinel of Startup: Understanding the Beckhoff First Scan Bit
In the realm of industrial automation, the difference between a smoothly running machine and a catastrophic collision often comes down to timing. While the cyclical nature of Programmable Logic Controllers (PLC) implies a repetitive, predictable existence, the transition from a powered-down state to an operational one is a critical window of uncertainty. It is in this precise moment that the "First Scan" bit proves its worth. Within the Beckhoff automation ecosystem—specifically utilizing TwinCAT software—the First Scan bit acts as the essential sentinel of initialization, ensuring that logic executes correctly before the physical world is engaged.
To understand the importance of the First Scan bit, one must first appreciate the architecture of a PLC. A PLC operates on a scan cycle: reading inputs, executing logic, and writing outputs. Under normal operation, this cycle repeats endlessly. However, the very first cycle after a power-up or a program reset presents a unique problem. At this specific moment, input data may not have settled, variables may be holding default values rather than retained ones, and physical actuators might be in unknown positions. If the control logic were to execute standard commands immediately, it could lead to unintended consequences, such as commanding a cylinder to extend before verifying it is retracted, or resetting a recipe to default values instead of loading the last saved state.
In the Beckhoff TwinCAT environment, this functionality is primarily handled through system-defined variables, specifically within the System task info structures. While various versions of TwinCAT exist, the concept remains consistent: the system generates a Boolean flag that is TRUE for exactly one cycle—the very first cycle after the controller enters the "Run" state. In TwinCAT 3, this is often accessed via the PlcTaskSystemInfo structure or the _TaskInfo interface, providing developers with a programmable trigger that occurs once and only once per startup.
The primary utility of the First Scan bit lies in initialization. It serves as the logical "clean slate" mechanism. For instance, in complex motion control applications involving Beckhoff’s NC (Numerical Control) or robotics, the First Scan routine is used to verify the actual position of axes against their commanded positions. It allows the programmer to suppress motion commands until the system has verified that communication with servo drives is healthy. Furthermore, it is instrumental in state machine logic. By forcing the state machine into a specific "Init" or "Home" state on the first scan, the engineer ensures the machine follows a strict, safe sequence of startup events, regardless of the state the machine was in when it was last powered off.
Beyond simple initialization, the First Scan bit is crucial for the handling of Retain variables. TwinCAT offers robust mechanisms for persistent data, but logic is often required to handle the scenario where no retained data exists (a first boot) versus when it does. The First Scan bit allows the code to differentiate between these scenarios, loading defaults only when necessary or validating the integrity of retained data before use.
It is important to distinguish the First Scan bit from the "Cold Start" or "Warm Start" methods. While the First Scan bit fires in both scenarios, the logic attached to it can be bifurcated. A skilled programmer may use additional system flags to determine if the startup is a warm restart (retaining memory) or a cold restart (memory cleared), allowing the First Scan logic to behave differently depending on the severity of the reset. This granularity offers a level of control that separates robust industrial code from hobbyist experimentation.
In conclusion, the Beckhoff First Scan bit is far more than a simple Boolean flag; it is a foundational element of reliable software engineering in automation. It bridges the gap between the static, powered-down world and the dynamic, moving machine. By providing a deterministic method to execute initialization logic exactly once, it safeguards machinery, protects processes, and ensures that every production cycle begins with a known, safe, and calculated start. In the symphony of industrial control, the First Scan bit is the conductor’s initial tap of the baton—the signal that establishes order before the performance truly begins.
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:
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
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 Mastering the Beckhoff First Scan Bit: A Comprehensive
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
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)
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.
In the world of Beckhoff TwinCAT and industrial automation, the "First Scan Bit" is a fundamental tool for ensuring your PLC starts in a predictable, safe state. If you’ve ever worked with Siemens (where it’s a system bit like FirstScan) or Allen-Bradley (using the S:FS bit), you know how vital this is.
In Beckhoff’s TwinCAT 3 environment, there isn’t a single hard-coded bit in the global memory by default, but the system provides a specialized mechanism to create one that is far more powerful than a simple boolean. What is the First Scan Bit?
The First Scan Bit is a flag that is TRUE for exactly one PLC cycle when the controller moves from "Config" or "Stop" mode into "Run" mode. After that first execution of the logic, the bit turns FALSE and remains so until the PLC is restarted or the code is re-downloaded. Why Do You Need It?
Without a initialization bit, your PLC logic simply resumes from its last state or starts with default values that might not be appropriate for a running machine. Common use cases include:
Initializing Setpoints: Setting default temperatures, speeds, or timers.
Resetting State Machines: Ensuring your sequences (SFC) start at "Step 0."
Clearing Alarms: Wiping the slate clean on startup so old errors don't prevent a start.
Communication Handshakes: Establishing a "heartbeat" or initial connection status with HMIs or third-party devices. How to Implement "First Scan" in TwinCAT 3 There are two primary ways to handle this in Beckhoff. 1. The Manual Method (Most Common) The Beckhoff "First Scan Bit" refers to a
Most TwinCAT developers create a global boolean variable and set it to TRUE by default. At the very end of their main program, they set it to FALSE. Variable Declaration (GVL): VAR_GLOBAL bFirstScan : BOOL := TRUE; END_VAR Use code with caution. Main Logic (MAIN PRG):
IF bFirstScan THEN // Perform Initialization Tasks here iTargetVelocity := 1500; bMachineReady := FALSE; END_IF // All other machine logic goes here... // The very last line of the program: bFirstScan := FALSE; Use code with caution. 2. Using FB_GetCurTaskIndex (The Pro Method)
TwinCAT provides internal system information via the Tc2_System library. You can check if the current cycle is the very first one by looking at the system task info.
VAR fbGetTaskIndex : FB_GetCurTaskIndex; nCycleCount : UDINT; END_VAR fbGetTaskIndex(); nCycleCount := _TaskInfo[fbGetTaskIndex.index].CycleCount; IF nCycleCount = 1 THEN // This is the first scan END_IF Use code with caution.
Note: This method is more robust because it relies on the system's own cycle counter rather than a variable you might accidentally overwrite elsewhere. Best Practices
Placement Matters: If you use the manual variable method, ensure the line bFirstScan := FALSE; is at the very bottom of your MAIN task. If you put it in a sub-function, other parts of your program might miss the "True" state.
Avoid "Retain" Variables: Never make your First Scan bit a RETAIN or PERSISTENT variable. It needs to reset every time the PLC power cycles.
Safety First: Use the first scan to ensure all physical outputs are in a "Safe/Off" state before the logic takes over.
The Beckhoff First Scan bit is your "clean slate" button. Whether you use a simple boolean flag or the system's cycle counter, implementing this ensures that your machine starts up with the correct parameters every time, preventing "ghost" data from causing erratic behavior during commissioning.
Here’s a concise guide to the First Scan Bit in Beckhoff TwinCAT (IEC 61131-3).
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.
In the world of industrial automation, a clean start is everything. When a Beckhoff PLC (Programmable Logic Controller) boots up—whether from a power cycle, a download of a new TwinCAT configuration, or a manual restart—the system enters a critical, fleeting state. Variables are uninitialized, previous runtime values linger in memory, and physical outputs may hold their last state. How do you reliably distinguish this “first scan” from normal cyclic operation?
The answer lies in a small, single-purpose variable: FirstScan (or bInit in older TwinCAT 2 nomenclature). Though it appears only once per startup, its impact on system reliability is profound.
The "First Scan" bit in Beckhoff TwinCAT PLC systems is a boolean status flag indicating the PLC program's initial execution cycle after startup, download, or reset. It enables safe, deterministic initialization of variables, hardware states, and communication interfaces before normal cyclic operation proceeds.
Simple flag:
VAR_GLOBAL
FirstScan : BOOL := TRUE;
END_VAR
IF FirstScan THEN
// initialize variables, outputs, communication
Output1 := FALSE;
ResetAlarms();
FirstScan := FALSE;
END_IF
Edge detection using task cycle counter:
VAR
CycleCount : UDINT := 0;
END_VAR
CycleCount := CycleCount + 1;
IF CycleCount = 1 THEN
// first scan logic
END_IF
Initialization block with handshake:
FB_Init : InitFB;
// InitFB runs once, sets InitDone BOOL when finished
IF InitDone THEN
// normal operation
ELSE
// hold outputs safe
END_IF
PROGRAM MAIN VAR bFirstScan : BOOL := TRUE; // Initialize as TRUE bFirstScanDone: BOOL := FALSE; END_VAR// First scan detection IF bFirstScan AND NOT bFirstScanDone THEN bFirstScanDone := TRUE; // First scan logic here END_IF
// Reset the flag after first cycle IF bFirstScanDone THEN bFirstScan := FALSE; END_IF
⚠️ Caveat: This method can fail if the PLC is stopped/started without power cycle. Always prefer the system library method.
Built-in FirstScan flag (recommended if provided)
Software-driven flag (common)
Edge-detected approach
Supervisor/Task-based initialization