Glossary · PLC and control programming
Symbol table
Also known as: Tag table, PLC tag table, Symbol list
German: Symboltabelle
In PLC programming, a symbol table is a list that assigns symbolic names, data types and comments to absolute addresses such as inputs, outputs and memory areas, so that programs can use meaningful names instead of addresses.
- PLC programming
- Technical documentation
In one sentence
A symbol table assigns meaningful names, types and comments to PLC addresses, so programs use names instead of raw addresses.
Example
In the symbol table, input %I2.0 is named B12_GuardDoorClosed with the comment Guard door station 2 closed.
How it applies
- Engineering: Symbolic names make programs readable and allow addresses to change without rewriting code. Naming conventions often combine the device tag from the electrical drawing with a readable description.
- Integration: Symbol tables are exported to HMI, SCADA and documentation tools, or imported from electrical CAE systems. A single source for device tags avoids mismatches between drawings, program and HMI.
- Maintenance: Technicians read the symbol table together with the electrical drawings to locate devices. Missing or outdated comments slow down fault finding.
- Documentation: The symbol table and the I/O list are part of the delivered software documentation. They also support terminology work: symbol comments can feed a consistent list of device names used in manuals and on the HMI.
Symbol table vs. cross-reference
The symbol table defines names. The Cross-reference shows where each name is used in the program. Together they provide the Trace link from a physical device to the code that uses it.