Multiple data blocks, also known as multiple instance data blocks, are a powerful feature in PLC programming that allows you to reuse the same function block for multiple instances without creating separate data blocks for each. For example, if you call FB10 from OB1 and then call FB1 and FB2 within FB10, you can set the background data block of FB10 as a multiple instance data block. This way, FB1 and FB2 do not need their own instance data blocks, as their interface parameters will be stored directly in the multiple background data block of FB10.
To create a multiple instance data block, simply select “DB of the instance†in the data type option during the setup. This approach is especially useful when you have several similar devices or processes that require the same control logic but different parameter sets, such as controlling multiple motors with identical control functions but distinct operational settings.
For instance, imagine you're using a single function block, FB1, to control two motors. Normally, you'd need separate data blocks for each motor—DB1 for the first and DB2 for the second. However, by using multiple instance data blocks, you can reduce the number of data blocks required. Instead, you can use one background data block (DB10) that contains all necessary parameters for both motors. When you call FB1 from within FB10, each instance of FB1 stores its data in the corresponding section of DB10, eliminating the need for additional data blocks.
This method simplifies program structure and improves efficiency. The key is to define the function block (FB10) with a multiple instance data block (DB10), and then specify the instances of FB1 within it. Each instance will have its own set of parameters stored in the same data block, making it easier to manage and maintain the code.
Here’s how it works step-by-step: First, insert FB10 and DB10 into your project. Then, configure DB10 as a multiple instance data block for FB10. In the local variable window of FB10, define variables under the "STAT" area, selecting the data type as FB. Assign each motor to a separate instance, and the system will automatically allocate memory based on the interface parameters of FB1.
Once this is done, the background data block DB10 will contain all the necessary parameters for the motors. Each motor's data is stored in a specific address range, allowing for easy access and modification. You can then call these instances in your main program, such as OB1, to control each motor independently.
This approach not only reduces the number of data blocks but also makes your program more scalable and easier to modify in the future. Whether you're working with two motors or twenty, multiple instance data blocks offer a clean and efficient solution for managing complex control systems.
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