Using symbols allows you to work with meaningful names instead of absolute addresses, making your code more readable and maintainable. It's recommended to use short, descriptive symbols along with detailed comments to ensure clarity in program creation and documentation.
There are differences between shared symbols and local symbols:
| Symbol Type | Scope | Usage | Uniqueness |
|---|---|---|---|
| Shared Symbol | Valid throughout the entire user program | Can be used by all blocks | Name must be unique across the whole program |
| Local Symbol | Only valid within the block it is defined in | Same symbol can be reused in different blocks depending on context | No uniqueness requirement across the program |
Allowed Characters:
- For shared symbols: letters, numbers, and special characters (e.g., @, #, $). If using special characters, the symbol must be enclosed in quotes.
- For local symbols: only letters, digits, and underscores (_) are allowed.
Examples of Shared Symbols:
- I/O signals (I, IB, IW, ID, Q, QB, QW, QD)
- Peripheral I/O (PI, PQ)
- Bit memory (M, MB, MW, MD)
- Timer (T) / Counter (Z)
- Code blocks (OB, FB, FC, SFB, SFC)
- Data blocks (DB)
- User-defined data types (UDT)
- Variable tables (VAT)
Examples of Local Symbols:
- Block parameters (input, output, and in-out parameters)
- Block static data
- Block temporary data
Important Notes:
In programming, shared symbols are typically enclosed in quotation marks ("..."), while local symbols are prefixed with a hash (#). These identifiers are automatically applied by STEP 7. However, if a symbol exists both as a shared and a local symbol, it's essential to clearly indicate which one is being used to prevent confusion. Additionally, if a shared symbol contains spaces or special characters, it must always be quoted.
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