Engineers transitioning from Rockwell Automation or Siemens to TwinCAT often search the system variables for a universal TwinCAT_System_Variables.FirstScan flag. It does not exist globally for one primary reason:
: Triggering a TP (Timer Pulse) or R_TRIG that needs to fire immediately upon startup.
| Do | Don't | |----|-------| | Use FB_FirstScan from Tc2_System | Rely on manual := TRUE flags | | Keep first scan logic short | Put heavy processing inside first scan | | Set outputs to safe defaults | Assume all retentive data is valid | | Combine with Tc2_Standard for consistent behavior | Use first scan for normal logic flow |
For Object-Oriented Programming (OOP) in TwinCAT, encapsulate your startup logic within a dedicated FB_Init method or an explicit M_Initialize() method called during the first cycle. If you are working on a specific architecture, let me know:
for function block-specific initialization. These approaches enable reliable initialization of communication, variables, and logging upon system startup. AllTwinCAT First cycle - AllTwinCAT beckhoff first scan bit
If you have multiple programs ( MAIN , Safety , HMI ), ensure you know which program runs first to set the bFirstScan bit. It is usually best to handle initialization in the main task of your main program.
Instead of an absolute global bit, TwinCAT provides an array of structures named _TaskInfo . This array tracks the real-time performance and diagnostics of every active PLC task running inside the TwinCAT execution environment. Within the PlcTaskSystemInfo structure , Beckhoff natively updates a boolean property called FirstCycle . During the first-ever execution cycle of that specific task, this variable is set to TRUE ; on every subsequent cycle, it automatically remains FALSE .
| Mistake | Consequence | Fix | |---------|-------------|-----| | Using FirstScan inside a function block | The FB’s FirstScan is local to that instance – may trigger multiple times. | Use a global g_FirstScanDone flag in the main PLC cycle. | | Assuming FirstScan runs before I/O update | I/O is typically updated before the first PLC cycle, so outputs may glitch. | Explicitly write safe values in FirstScan even if I/O was read. | | Forgetting FirstScan in interrupt tasks | Fast tasks or alarms may execute before MAIN ’s FirstScan clears. | Add FirstScan logic to every task, or use a global semaphore. | | Relying on FirstScan after online change | Online change (warm restart) also triggers FirstScan . | If you need cold start only, check bInit_Cold in SysLibCallback . |
Are you using standard or Object-Oriented Programming (methods/properties) ? If you are working on a specific architecture,
Method 1: The Native TwinCAT System Info Approach (Recommended)
Mastering the Beckhoff First Scan Bit: A Complete Guide to PLC Initialization
If you would like to expand on any of these implementation steps, let me know:
Because it is part of the task info structure, it correctly identifies the first scan for the specific task it is called within. It is usually best to handle initialization in
It is cleaner to have one IF bFirstScan block at the beginning of your code rather than scattering multiple IF bFirstScan blocks throughout your program, which can make debugging difficult. 4. First Scan Bit vs. PLC_PRG_Init
: The code inside your IF firstCycle THEN block executes repeatedly. Common Cause : You forgot to reset the flag or the condition remains TRUE. Solution : Ensure you reset the flag within the same cycle. Use R_TRIG (rising edge detection) if you're using a custom flag approach, or simply call SystemTaskInfoArr[1].firstCycle directly, which automatically resets after the first cycle.
When working with , one of the most fundamental but often misunderstood system flags is the First Scan Bit . Properly using this bit ensures your machine or process starts in a safe, defined state.
Engineers transitioning from Rockwell Automation or Siemens to TwinCAT often search the system variables for a universal TwinCAT_System_Variables.FirstScan flag. It does not exist globally for one primary reason:
: Triggering a TP (Timer Pulse) or R_TRIG that needs to fire immediately upon startup.
| Do | Don't | |----|-------| | Use FB_FirstScan from Tc2_System | Rely on manual := TRUE flags | | Keep first scan logic short | Put heavy processing inside first scan | | Set outputs to safe defaults | Assume all retentive data is valid | | Combine with Tc2_Standard for consistent behavior | Use first scan for normal logic flow |
For Object-Oriented Programming (OOP) in TwinCAT, encapsulate your startup logic within a dedicated FB_Init method or an explicit M_Initialize() method called during the first cycle. If you are working on a specific architecture, let me know:
for function block-specific initialization. These approaches enable reliable initialization of communication, variables, and logging upon system startup. AllTwinCAT First cycle - AllTwinCAT
If you have multiple programs ( MAIN , Safety , HMI ), ensure you know which program runs first to set the bFirstScan bit. It is usually best to handle initialization in the main task of your main program.
Instead of an absolute global bit, TwinCAT provides an array of structures named _TaskInfo . This array tracks the real-time performance and diagnostics of every active PLC task running inside the TwinCAT execution environment. Within the PlcTaskSystemInfo structure , Beckhoff natively updates a boolean property called FirstCycle . During the first-ever execution cycle of that specific task, this variable is set to TRUE ; on every subsequent cycle, it automatically remains FALSE .
| Mistake | Consequence | Fix | |---------|-------------|-----| | Using FirstScan inside a function block | The FB’s FirstScan is local to that instance – may trigger multiple times. | Use a global g_FirstScanDone flag in the main PLC cycle. | | Assuming FirstScan runs before I/O update | I/O is typically updated before the first PLC cycle, so outputs may glitch. | Explicitly write safe values in FirstScan even if I/O was read. | | Forgetting FirstScan in interrupt tasks | Fast tasks or alarms may execute before MAIN ’s FirstScan clears. | Add FirstScan logic to every task, or use a global semaphore. | | Relying on FirstScan after online change | Online change (warm restart) also triggers FirstScan . | If you need cold start only, check bInit_Cold in SysLibCallback . |
Are you using standard or Object-Oriented Programming (methods/properties) ?
Method 1: The Native TwinCAT System Info Approach (Recommended)
Mastering the Beckhoff First Scan Bit: A Complete Guide to PLC Initialization
If you would like to expand on any of these implementation steps, let me know:
Because it is part of the task info structure, it correctly identifies the first scan for the specific task it is called within.
It is cleaner to have one IF bFirstScan block at the beginning of your code rather than scattering multiple IF bFirstScan blocks throughout your program, which can make debugging difficult. 4. First Scan Bit vs. PLC_PRG_Init
: The code inside your IF firstCycle THEN block executes repeatedly. Common Cause : You forgot to reset the flag or the condition remains TRUE. Solution : Ensure you reset the flag within the same cycle. Use R_TRIG (rising edge detection) if you're using a custom flag approach, or simply call SystemTaskInfoArr[1].firstCycle directly, which automatically resets after the first cycle.
When working with , one of the most fundamental but often misunderstood system flags is the First Scan Bit . Properly using this bit ensures your machine or process starts in a safe, defined state.