If you are using Object-Oriented Programming (OOP) with Function Blocks, you should generally use the 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.
A common misconception is that the first scan bit is only relevant upon a full system power-up. In TwinCAT, however, using the SystemTaskInfoArr[1].firstCycle variable will be TRUE when the controller physically starts up (a "cold start"). It will not be set to TRUE when you simply restart the PLC program from STOP to RUN. beckhoff first scan bit
If you put this logic in the main code, it would run every cycle, preventing the machine from operating. The first scan bit provides a "fire-once" trigger to set these initial conditions without interfering with the ongoing machine operation [2]. 2. How to Implement the First Scan Bit in TwinCAT If you are using Object-Oriented Programming (OOP) with
Note: exact symbol names can vary by TwinCAT version and project conventions. In TwinCAT, however, using the SystemTaskInfoArr[1]
user wants a long article about "beckhoff first scan bit". This is a technical topic related to Beckhoff TwinCAT PLC programming. I need to provide a comprehensive guide covering what it is, how to implement it, and best practices. To do this effectively, I should search for relevant information from various sources, including official documentation, tutorials, and community forums. I will follow the search plan provided in the hint. search results didn't show direct matches for "first scan bit". The user's question is about a specific technical feature in Beckhoff TwinCAT. I need to provide a comprehensive answer. I will structure the article with an introduction, methods to implement, usage examples, and best practices. To gather enough information, I need to perform multiple searches covering different aspects. I will also search in German for broader results. search results have provided some useful information. The German forum thread "Erster Zyklus nach Neustart" mentions a system variable SystemTaskInfoArr[1].FirstCycle . This is likely the key. I need to open that forum thread to get the details. Also, the PLCtalk.net thread "One Shot vs. FirstScan System Bit" might contain useful discussions. I'll open both to gather more information. forum thread mentions SystemTaskInfoArr[1].FirstCycle . I should also search for "TwinCAT first cycle bit" to get more official documentation. have found several sources that explain the SystemTaskInfoArr[1].firstCycle variable. Now I need to gather more details. I will open the results from the search to get the official documentation and examples. have gathered information on the system variable SystemTaskInfoArr[1].firstCycle . I also have some information on alternative methods. I need to structure the article. The article should cover: what the first scan bit is, why it's needed, how to implement it using the system variable, alternative methods (like self-created flags), important caveats, best practices, and advanced use cases. I'll also need to mention the library TcSystem.lib . I will search for more details on "TcSystem.lib" and "first cycle" to ensure comprehensive coverage. I have a good amount of information. I will structure the article with an introduction, key information, implementation steps, and best practices. I will cite the sources. Now, I will write the article. "first scan bit" is a core concept in PLC programming, and in the Beckhoff TwinCAT ecosystem, it serves as a critical tool for managing application start-up. This article provides a comprehensive guide to the Beckhoff first scan bit, explaining what it is, why it's essential, and how to implement it effectively in your projects.
Because the variable is declared with RETAIN , it will preserve its FALSE state across subsequent program starts, until a full power cycle. If you need it to trigger on every STOP->RUN transition, you would declare it without the RETAIN keyword, which forces a TRUE initialization on each warm start.
IF _TaskInfo[GETCURTASKINDEX()].FirstCycle THEN // Your initialization logic here END_IF; Use code with caution. Copied to clipboard Custom Variable Method