Online symbolism
The TwinCAT OPC UA Server enables symbol information to be read via the so-called "online symbolism". This is a standardized ADS interface, which is offered by some ADS servers in TwinCAT. This interface is often referred to as "Symbol Upload". The interface enables an ADS client to read all available symbols from the ADS server and process them further if necessary. In addition to the address information of the symbols, further information is also provided, such as data type descriptions and optional attributes.
The optional attributes can be used to explicitly enable symbols or to make further settings for this purpose - the handling is identical to the use of symbol files and runs, for example, via pragmas in the PLC or the TMC Code Generator for TwinCAT 3 C++. This means that you can use all the enables that you have set there both via the symbol file import and the online symbols. Please note that not all real-time environments allow the explicit enabling of symbols. In this case, all symbols are always provided. The following table provides an overview of some ADS servers that provide the online symbolism interface and whether explicit symbol enabling is available.
Type | ADS Port | Symbol enable |
|---|---|---|
TwinCAT 2 PLC | 801, 811, 821, ... | Yes |
TwinCAT 3 PLC | 851, 852, 853, ... | Yes |
TwinCAT 3 C++ / Matlab/Simulink | 350, 351, 352, ... | Yes |
TwinCAT 3 EtherCAT Master | 27905, 27906, ... | No (all symbols are enabled) |
TwinCAT 3 I/O Task with Image | 301, 302, 303, ... | No (all symbols are enabled) |
In the TwinCAT OPC UA Configurator, the online symbol mechanism is available in two forms when configuring a Data Access device:
- TwinCAT Symbol Server: All symbols are imported
- TwinCAT Symbol Server - Filtered: Only enabled symbols are imported
In the structure of its namespace, the TwinCAT OPC UA Server is based on the namespace of the ADS symbolism. As the data is available here, it is also provided via OPC UA.