Suppose we have a PLC program that requires the initialization of several variables during the startup of the PLC. We can use the First Scan Bit to execute this initialization task only once, during the first scan of the PLC.
By properly implementing the first scan concept in TwinCAT, you ensure that your Beckhoff industrial PCs boot safely, predictably, and seamlessly transition into active machine control.
What or axes are you trying to initialize during the first scan?
Pre-loading custom parameters, setting initial process target temperatures, or defaulting PID controller parameters before the main continuous loops take over. beckhoff first scan bit
If you are working on older legacy controllers (such as the Beckhoff CX1010 or CX1020 series) or maintaining an legacy system, you may find the global system array variable explicit:
VAR_GLOBAL FirstScan : BOOL := TRUE; END_VAR
| Use Case | Purpose | |----------|---------| | | Set outputs to safe defaults | | Clear retentive variables selectively | Override old retentive values on first start | | Home axes | Trigger homing sequence only once | | Reset alarms | Clear boot-time fault flags | | Load configuration | Load parameters from file or EEPROM once | Suppose we have a PLC program that requires
Because TwinCAT executes code in tasks, accessing the correct task index ensures your first scan logic fires reliably regardless of where it is called in your program. 1. The Code Structure
Creating a simple counter:
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? What or axes are you trying to initialize
The Beckhoff First Scan Bit, also known as the "FirstScan" bit, is a special bit in the TwinCAT 3 system that indicates when the PLC (Programmable Logic Controller) is executing its first scan cycle. In other words, it is a flag that is set during the initial scan of the PLC program. This bit is typically used to execute specific code or actions only once, during the first scan of the PLC.
What specific (e.g., setting parameters, resetting axes, clearing arrays) are you trying to perform?
VAR_GLOBAL bIsFirstScan : BOOL := TRUE; // Starts TRUE when the PLC runtime begins END_VAR Use code with caution. Copied to clipboard