OPC Unified Architecture Data Access Client communication protocol

Protocol specification
Basic concepts
Initiation of communication
Communication line configuration
Protocol configuration on the communication line
Protocol configuration on the communication station
I/O tag configuration
Browser dialog window
Tell commands
Literature
Changes and modifications
Document revisions

Protocol specification

The OPC UA protocol is next generation of OPC standard, which provides cohesive, secure and reliable platform-independent framework for access to the real time data. The main difference in comparison with the previous versions is that the interprocess communication uses TCP/IP communication instead of COM/DCOM technology and therefore is OS (Windows) independent. This protocol supports two methods of data encoding (binary and XML). The existing OPC client implementation supports only binary encoding.

Basic concepts

Endpoint: a physical address on a network, which allows the client to access to one or more of the services provided by the server.

Session: it is an abstract connection of OPC UA server and a client on OSI application layer.

Monitored item: an entity on OPC UA server that is defined by the client. It is used for monitoring the values.

Subscription: an object on OPC UA server that is defined by the client. It returns the notifications about change of monitored items.

Initiation of communication

When initiating the communication, the client exchanges multiple messages with the server. In the case of binary encoding, "Hello Message" is the first message sent from the client to the server. The message defines the size of receiving and sending buffers and the maximum size of messages that may be exchanged during TCP communication between client and server. It also defines the URL address of the endpoint. The server answers by sending "Acknowledge message", in which it confirms the suggested parameters or modifies them according to its limits.

The next message sent by the client is "OpenSecureChannel message". It is used to establish a communication channel to exchange data. In this message, the client and server agree on what type of encrypting and signing mode will be used (either "sign and encrypt" or "encrypt" only). OPCUA client in D2000 System supports only unencrypted mode.

After establishing the communication channel, the client can send the message to create a session, "CreateSession Message". It is the connection on the OSI application layer. After the server confirms the request, the session must be activated by "ActivateSession Message". In this message, the client and server will agree on the algorithm for signing and encrypting if this mode has been agreed on when establishing the communication channel.

After activating the session, all types of messages that are intended for object management in the address space of the OPC UA server can be swapped. In this step, the client creates a subscription with the parameters set on the communication station level for all stations within the communication line.

The monitored items should be then inserted into these subscriptions. They correspond with the instances of I/O tags, which contain the parameters defined in the address dialog window.

From this moment, the server informs the client about the changes in the monitored objects by "Publish message" in the periodic intervals (that have been set in the parameters of subscription). If the monitored objects have not been changed, the server will send a "Publish message" once in (Max KeepAlive Count * Publishing Interval) seconds. The message informs the client that the subscription is still active. A similar check mechanism is also on the client-side - it will send a "Publish message" once every (Max KeepAlive Count * Publishing Interval). If the client does not send the acknowledgment "Publish message" within (LifeTime Count * Publishing Interval), the subscription will expire on the server side.

Communication line configuration

Communication line category: OPC UA Client
Host address: OPC UA server address. You may set the name according to UNC convention (e.g. "\\server" or "server", DNS names (e.g. "domain.com", "example.company.com") or IP address ("196.54.23.113").  In the case of redundant systems, multiple names/addresses separated by commas or semicolons can be entered.
TCP port: TCP port of OPC UA server (e.g. 4840).
EndpointUrl: Endpoint address (e.g. opc.tcp://localhost:4840)
Encoding type: Type of encoding that is used for data exchange (currently only Binary encoding is supported).

Protocol configuration on the communication line

Note: all X509 certificates used in OPC UA communication can be found in the following subdirectories of the kom-opcua directory in the application directory:

• own - a directory with KOM process's own certificate (file cert.der). If this file does not exist, it is generated
  Warning - this automatically generated certificate will only be valid for 1 year, so we recommend replacing it with a certificate valid for a longer period!
• private - a directory with private key for the KOM process own certificate (file private.pem)
• rejected - a directory with rejected certificates
• trusted - a directory with trusted certificates (the first time a connection is established to an OPC UA server, its certificate is stored in this directory)

Protocol configuration on the communication station

The parameters on the level of communication station correspond with the setting of one subscription. It means the one communication station is equivalent to one instance of a subscription within the session.

I/O tag configuration

I/O tag configuration dialog window is used for setting the monitored objects.

Object address setting

Settings of other parameters

Browser dialog window

This dialog window is intended for browsing and inserting the OPC UA objects into the address parameter of the I/O tag. The upper part contains the tree structure of the address space. When clicking on the object, the lower part of the window displays the direct descendants of the object (variables, tags).
Double click on one of the descendants transfers the address parameters of an object to the address dialog window of the I/O tag.

Note: Using Ctrl+C it is possible to copy a list of displayed descentants into Windows clipboard. All descendants will be copied unless a specific descendant is selected.

Note: In versions from 17th December 2018 and newer, the recycling of browser dialog has been implemented. If the dialog is closed by the Close button or after selecting a tag, it is actually only hidden and it is available for browsing by another I/O tag within the same station so that the tree structure of the browsed objects is preserved. Clicking on the close icon at the top right corner will cause the dialog to be really closed.

Checking the "Structured tags" option causes the variables (tags) to appear in the tree structure in addition to the objects, and the KOM process also attempts to read their descendants. This is useful for browsing OPC UA servers that support structured tags. You can also insert a tag into the address dialog window of the I/O tag by double-clicking the tag name in the tree structure.

Tell commands

Literature

OPC Foundation manuals are placed on http://www.opcfoundation.org.

• OPC UA Part 1 - Overview and Concepts 1.01 Specification
• OPC UA Part 2 - Security Model 1.01 Specification
• OPC UA Part 3 - Address Space Model 1.01 Specification
• OPC UA Part 4 - Services 1.01 Specification
• OPC UA Part 5 - Information Model 1.01 Specification
• OPC UA Part 6 - Mappings 1.00 Specification
• OPC UA Part 7 - Profiles 1.00 Specification
• OPC UA Part 8 - Data Access 1.01 Specification

Changes and modifications

• May 10, 2012 - creating the document

Document revisions

• Ver. 1.0 – May 10, 2012
• Ver. 1.1 - December 17, 2018: Added browser dialog recycling and browsing of structured tags