As explained in Connectivity RTD Server Fundamentals, the RTD function syntax contains a number of topic arguments - at least one, but there can be more. The synatx and semantics of the topic arguments is given by the RTD server being used. The OPC Studio Excel Connector uses its internal Connectivity RTD Server, and the syntax of topic arguments is described here.

Topic Formats

The Connectivity RTD Server supports two formats of topics sequences in the RTD function.

The so-called friendly topic syntax is designed to be easily understandable by humans, and can also be manually typied in without too much effort. This format cannot, however, fully describe all possible requirements, due to the number of properties that can theoretically be set on different points and operation parameters.

For this reason, a second format, XML-based (XML Toipc Syntax) is also supported by the Connectivity RTD Server. This format can describe all requirements without loss, but is much less readable, and not really suitable for manual creation.

Friendly Topic Syntax

With the friendly topic syntax, the overall sequence of topic arguments for Connectivity RTD Server is as follows:

   typeString, pointIdentifier1, pointIdentifier2, ..., subscribeParameter1, subscribeParameter2, ... [,
resultValueSelector]

Where:

• typeString denotes the type of point this RTD function is subscribing to. The type is given by an alias; it can be e.g. "opcdaitem", "opcdaproperty", or "opcuaattribute". See Point Types Reference for details on available point types and their type aliases. Type aliases are case insensitive.
• pointIdentifier1, pointIdentifier2, ... are topics that describe the point to be subscribed to. Their number, syntax and semantics depends on the type of the point. See Point Types Reference for details on point identifiers for each point type.
• subscribeParameter1, subscribeParameter2, ... are topics that describe the parameters of the Subscribe operation on the point. Their number, syntax and semantics depends on the type of the point. See Point Types Reference for details on point identifiers for each point type.
• resultValueSelector is a text string that determines the specific piece (or pieces) of information that will be extracted and displayed from a target object. For details, see Result Value Selector.

Further topics may be appended to the end of this sequence if the Topic Write with RTD Server feature is used.

XML Topic Syntax

The XML topic syntax is capable to fully describe the point and parameters. It is currently not documented, and is subject to change.

Topic Spanning

Some limitations of the RTD function make it problematic to pass the data to it. For example, each topic (string) must not be longer than 255 characters. This is normally not an issue with the friendly topic syntax, but can easily occur with XML topic syntax, where the resulting XML strings tend to be quite long. The Connectivity RTD Server (and the associated integrated functionality in Connectivity Explorer) overcome this limitation by spanning the topics strings over several RTD function arguments. The first topic contains information about how the arguments are laid out in the topic strings. The precise syntax and semantics of this topic spanning feature is currently not documented, and is subject to change.

Examples

Friendly topic syntax example for an OPC-DA item:

   "opcdaitem", "OPCLabs.KitServer.2", "Demo.Ramp"

Friendly topic syntax for OPC Unified Architecture:

   "opcuaattribute", "opc.tcp://opcua.demo-this.com:51210/UA/SampleServer", "nsu=http://test.org/UA/Data/;i=11218"