OPC DA
The OPC DA protocol may require root permissions in some constellations. If you experience problems with running this protocol, please make sure to install your service on an agent with root permissions as described in agent orchestration.
The OPC Data Access (OPC DA) specification is an older protocol specification for real-time data from PLCs and devices, but nowadays this is mostly being superseded by OPC UA.
In OPC DA, the endpoints are called tags and are referenced to by plain strings.
Endpoints can either refer to one single tag, using the property tag
, or multiple tags in one network query, using the property tags
.
Currently read, write and subscribe operations are possible.
Additionally, browsing all available data is available, too, by using the property browse
and sending a message to the req
sub-topic of this endpoint.
Input Format
To write data to a particular tag, first you need to define a write endpoint with a tag
and type
properties defined.
You can see an example of such endpoint on the provided commissioning file below.
Once you have a proper endpoint, you will need to send the following message with the value you want to write to the /set
sub-topic of the endpoint, i.e. if there is an endpoint named myEndpoint, you need to send a message to the topic myEndpoint/set
with the following payload:
Output Format
If data is read from OPC DA the output will be provided as JSON object
The timestamp as given from OPC DA is usually given in ISO 8601 form, which is a string of the following form: Year-Month-Date, then a T
separator, followed by hour:minutes:second, followed by a period, and the milliseconds, and a Z
suffix. Example: 2024-02-15T15:57:36.278Z
The value is in one of two forms, depending on whether this endpoint requested a single tag
or multiple tags
.
If a single tag was requested, the value is just the literal value of the tag, such as an integer number. Example:
If multiple tags were requested, the value is a JSON object with the tag names as keys, and the tag’s values as values. Example:
Browsing the tags
When intending to explore the available tags, the browse
property of the endpoint is useful. To use this, define a read
endpoint with the property browse: 'flat'
as the only addressing property. (Alternatively, browse: 'tree'
can be used, too.) To browse the tag list, send an empty MQTT message to the topic of the endpoint but with the suffix /req
(“request”). The Connectware will run the browsing and return the result on the endpoint topic with the suffix /res
.
For flat browsing, the result value is an array of strings, which are just the tag names. Example:
For tree browsing, the result value is a JSON object tree structure representing the tag tree on the server. Example:
Example Commissioning File
Download:
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