When a constant of any data type is loaded into accumulator 1 of the S7-300, its 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 variable’s data type does not automatically change; it remains as specified unless modified through the symbol table.
The symbol table plays a role in managing data types, but it cannot handle all situations on its own. In such cases, the programmer must step in and manage the data type manually. For global variables, the symbol table directly controls the data type, making it easier to track and maintain.
Besides the symbol table, other elements like FB/FC local variables and variables in data blocks also define their own data types. It is up to the programmer to ensure these are properly managed and used according to their intended purpose.
I mentioned that the term "Word" can be ambiguous. It can refer to a data type or act as a quantifier indicating a 16-bit memory unit. When a 16-bit memory address (such as MW0 or LW0) isn’t assigned a specific data type, it doesn’t have one. It simply holds binary data without any semantic meaning.
For example, the OB40_MDL_ADDR variable in OB40 is of the WORD data type. It cannot be directly used with integer comparison instructions that require the INT data type. To use it, you need to move its value to a 16-bit memory location like MW10 using the MOVE instruction. Although MW10 doesn’t automatically gain the INT data type, the CPU treats it as such during operations.
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 dyes a white chicken to make it look colorful to trick a child. But when you put it in water, the color washes off. The chicken itself is still the same, just without the artificial color. Similarly, when data is loaded into an absolute address like MW10 or the accumulator (which acts like a special address), its original data type is stripped away, leaving only the raw bits. At this point, the data can be interpreted as different types based on how the programmer chooses to use it.
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