Hello Everyone! As the title indicates, I'm seeking guidance on how to implement a "First Scan Bit" in an S7-300 PLC CPU314. I've created a sequence involving three cylinders, which is detailed in the attached file. You'll notice that there's a first scan bit included at the start. This is my usual practice when working with an S7-1200; however, unfortunately, the S7-300 lacks a native first scan bit. It's necessary to create it manually. I discovered information on using OB100 (warm restart) as the sole method for achieving a "First Scan Bit" in an S7-300. The challenge is that I'm uncertain about what needs to be included within OB100. What should I insert into it? I should add that I'm a beginner, so I appreciate your patience and understanding.
To gain a deeper understanding, it's essential to recognize that the initial Organization Block triggered when the PLC initiates is likely OB100—often associated with a warm restart. Within this block, assign a bit to 1. Subsequently, at the conclusion of OB1, revert this bit back to zero. It's worth noting that OB1 might possess a first-run bit, although this shouldn't be taken as definitive. If you're looking to explore more about PLC functionality and programming, this might be a good starting point.
To optimize search engine visibility while maintaining the original meaning, consider the following rephrased text: In the initial rung of OB1, utilize the same bit address across all instructions: --|/|---------------------(S)--. For the concluding rung in OB1, apply: -----------------------(R)--. Alternatively, you can structure the final rung like this: ---| |--------------------(R)--|---|/|-------. This version integrates relevant keywords like "initial rung," "OB1," and "bit address," which are frequently searched terms in automation and programmable logic controller (PLC) contexts.
In reality, it involves not just a single bit but an entire value in the OB1 parameters. For more detailed information, consider visiting this forum discussion: [Siemens Support Forum on OB1 First Scan](https://support.industry.siemens.com/tf/WW/en/posts/ob1-first-scan/20376?page=0&pageSize=10).
Out of sheer curiosity, why is a first scan bit necessary?
Hey there! It's great that you're diving into using the S7-300, and I totally understand the challenge with the first scan bit. When you're using OB100, you can create your own first scan bit by setting a specific memory bit or a flag to true the first time OB100 runs. You can achieve this by simply adding a block of code at the beginning of OB100 that checks if that memory bit is already set; if not, set it to true and then execute whatever initialization routines you need. Just remember to make sure to reset it in your main program after the first scan is complete to avoid it triggering again on subsequent runs. Don't hesitate to reach out if you need more guidance! Good luck with your project!
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