Glossary · PLC and control programming
Data block (DB)
Also known as: DB, Global data block, Instance data block
German: Datenbaustein
In Siemens STEP 7 and TIA Portal programming, a data block (DB) is a program block that stores data rather than code; a global DB holds data for the whole program, and an instance DB holds the static data of one function block call.
- PLC programming
- Vendor product
In one sentence
In Siemens PLCs, a data block (DB) stores program data: global DBs hold shared data, instance DBs hold the data of one FB call.
Example
Recipe parameters for a mixer are kept in a global data block, while each of the three pump FB calls has its own instance data block.
How it applies
- Engineering: Data blocks structure the memory of a Siemens program. Global DBs are often organized by function (recipes, HMI interface, alarms) and typed with User-defined data type (UDT) definitions. Instance DBs are created automatically when an FB is called as an instance.
- Commissioning: Start values in a DB and the actual values in the controller can differ. Downloading a changed DB can reset actual values to start values, so engineers decide carefully which values are retentive.
- Maintenance: HMI and SCADA systems typically read and write DB variables. Renaming or restructuring a DB can break those connections.
- Documentation: Describe the interface DBs that other systems use as an interface specification: variable name, data type, unit, direction and meaning. Keep it versioned together with the program.
Data block vs. global variable
In IEC 61131-3 terms, the content of a global DB behaves much like a set of Global variable declarations. The DB is a Siemens-specific way of grouping and storing them; other environments declare global variables in global variable lists instead.
Keep in mind: product names and features change. Check block types, memory options and download behavior against the vendor's current documentation.