When a constant of any data type is loaded into accumulator 1 of the S7-300, its original data type attributes are lost, and only binary values (0s and 1s) remain. This raw data is then transferred to a variable that has a defined data type. However, the data type of the variable itself doesn't change automatically, and it's the symbol table that controls this unless modified by the programmer. The symbol table can’t manage these situations on its own—it’s up to the programmer to handle them. If the variable is global, the symbol table defines its data type directly, meaning it controls it. But for local variables within function blocks (FB), function calls (FC), or data blocks, the data types are defined in their respective tables. It’s the programmer’s responsibility to ensure correct usage and management of these types. I mentioned earlier that "Word" is a polysemous term. It can refer to a data type as well as a unit of memory, typically 16 bits. When a 16-bit memory address like MW0 or LW0 isn’t assigned a specific data type, it doesn’t have one. So, when you use such an address without defining its type, you’re working with raw binary data. For example, the data type of OB40_MDL_ADDR is WORD, which means it cannot be directly used with instructions that require an INT type. To use it in integer comparisons, you need to transfer its value to a register like MW10 using the MOVE instruction. After that, MW10 can be used for comparisons. However, MW10 doesn’t automatically inherit the INT data type; the CPU treats it as an INT during operations, even though it's still stored as a 16-bit word. So what exactly is a data type? It's an attribute assigned by the programmer to define how data should be interpreted. Think of it like a label. Imagine a street vendor who dyes a white chicken to make it look colorful to a child. The color is just a temporary cover—when the chicken is put in water, the dye washes off, revealing the original white. In this analogy, the chicken represents the data, and the dye represents the data type. When the data type is removed, the data is left in its raw form, just like the chicken after the dye is washed away. At this point, the same data can be interpreted in different ways depending on how it’s used.
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