Glossary · Automation fundamentals, platforms and components
Symbol
Also known as: Symbolic name, Tag
German: Symbol
In PLC programming, a symbol is a symbolic name assigned to an address, variable or constant, such as "Motor1_Run" for output %Q0.0, so that programs, HMIs and external clients can refer to data by name instead of by memory address. The collection of symbols forms the symbol table.
- Automation components
In one sentence
A symbol is a name for a PLC address or variable, such as Motor1_Run for %Q0.0, so programs and HMIs can refer to data by name.
Example
The HMI accesses the tank level by the symbol "Tank1.Level" instead of a memory address, so the display still works after the PLC memory layout changes.
Explained in context
Context cards connect this term with others to answer one question. Also in British English and German.
How it applies
- Engineering: Symbolic addressing makes code readable and robust against memory changes. A naming convention (for example plant area, equipment, signal) keeps symbols consistent across PLC, HMI and SCADA.
- Integration: OPC UA servers and HMIs often read symbols directly; which symbols are exported can be controlled, for example with a Pragma.
- Documentation: Use the same symbol names in I/O lists, circuit diagrams, programs and manuals. A consistent symbol naming scheme links documentation and code and supports Traceability.
Symbol vs. address
An address such as %I1.3 identifies a physical or memory location. A symbol is a human-readable name for it. IEC 61131-3 calls addresses of the %I, %Q and %M kind directly represented variables.