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
| Parameter name | Meaning | Unit | Default value |
|---|---|---|---|
Session Name | Session text identifier. Session identifier should be a unique within the client instance, making it possible to search problems faster in the client or server logs. | String | Kom process |
Requested Channel Lifetime | The channel must be reopened before this time limit elapses. If the time is exceeded, the channel will be closed and unable to change data. | hh:mm:ss | 01:00:00 |
Requested Session Timeout | Any message should be changed between client and server before this time limit elapses. If it is not sent, the sources within the session that are kept on the server are released. The primary work of this parameter is to remove the sessions that became inactive because of some unexpected reason. | mm:ss | 01:00 |
Authentication Type | Type of authentication used with the OPC UA server. Supported types are: | Anonymous / Username | Anonymous |
Token User Name | User name used in authentication if Authentication type = Username. | ||
Token Password | Password used in authentication if Authentication type = Username. | ||
Debug Mode | It changes the number of information about communication. We recommend enabling the mode Extended/Full only when detecting the problems and debugging the communication. | Normal/Extended/Full | Normal |
Debug Threads | The parameter defines the thread(s) that will send the debug info about the communication. | Receiving/Sending/Others treads/All threads | All threads |
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.
| Full name | Description | Unit | Default value |
|---|---|---|---|
Requested Publishing Interval | Defines the time interval for the server to send the information about the change of monitored items within the instance subscription by "Publish message". | mi:ss.mss | 00:05.000 |
Requested LifeTime Count | If the client does not send the request for data till the time defined by (LifeTime Count * Publishing Interval), the subscription expires. The value should be minimally 3 times higher than the "Requested Max KeepAlive Count". | Number | 1000 |
Requested Max KeepAlive Count | If the objects of subscription are not changed, the server will send a keep-alive message after elapsing the time (Max Notifications Per Publish * Publishing Interval). The client will confirm this message when it sends a new request for data. | Number | 5 |
Max Notifications Per Publish | The parameter defines the maximum number of notifications about the object change, which the server can send in one "Publish message". Zero indicates that the number of notifications is unlimited. | Number | 0 |
Publishing Enabled | The parameter enables/disables the publishing within the subscription. | YES/NO | 0 |
Priority | It defines a relative priority of a subscription. If the server should send more notifications, the subscription with higher priority is preferred. | 0-255 | 0 |
Samples Queue Size | This parameter enables to create the object queue with the defined length on the OPC UA server's side for each monitored item in a subscription. | Number | 0 |
Write Timestamp | Timestamps used to write a value:
Note: If the OPC server does not support writing of timestamps, acording to the standard it should return the Bad_WriteNotSupported (2155020288) error code. | - | None |
I/O tag configuration
I/O tag configuration dialog window is used for setting the monitored objects.
Object address setting
| Name | Meaning | Unit | Default value |
|---|---|---|---|
| ID | The identifier in text format, which is, in dependence on ID type, converted to the required native type. | String | |
| ID type | Enumerated types of identifiers. They help to access the objects in OPC UA address space. Numeric-1B ID: Identifier limited to 1-byte value (0-255) Numeric-2B ID: Identifier limited to 2-byte value (0-65535) Numeric-4B ID: 4-byte identifier String: Text identifier Guid -16B ID: 16-byte (128-bit) number that is usually divided into four parts. For example 3F2504E0-4F89-11D3-9A0C-0305E82C3301. ByteString: Identifier that is represented as a sequence of bytes. | Numeric-1B ID / Numeric-2B ID/ Numeric-4B ID/String/Guid -16B ID/ByteString | Undefined |
Namespace | Numerical identifier of namespace of OPC UA server. Each OPC UA server can have N namespaces. However, the object identifier must be unique in one namespace. | Numeric | |
| Variable type | The value type of objects that can be processed by OPC UA client. Variable type should be used only if I/O tag is intended for writing. As regards the reading of the object value, the information about type is sent together with the value. | Undefined / Boolean / Byte / SByte / Integer16 / Unsigned16 / Integer32 / Unsigned32 / Integer64 / Unsigned64 / Float / Double / String / UTC Time / Boolean array / Byte array / SByte array / Integer16 array / Unsigned16 array / Integer32 array / Unsigned32 array / Integer64 array / Unsigned64 array / Float array / Double array / String array / UTC Time array / LocalizedText / LocalizedText array | Undefined |
| Array index | If the object value is represented as a value array (Boolean array / Byte array / SByte array / Integer16 array / Unsigned16 array / Integer32 array / Unsigned32 array / Integer64 array / Unsigned64 array / Float array / Double array / String array / UTC Time array), the parameter defines its range or value of a particular item. A text representation of array index may be in several formats:
| String | |
| Write only | It controls if the I/O tag is a part of subscription. Its value will be sent periodically from the server in "Publish message". | Unchecked/checked | Unchecked |
| Expanded Node ID | If it is checked, it enables to address the ExpandedNodeId. Unlike the classic identifier in the OPC UA address space, ExpandedNodeId is supplemented by NameSpace URI and Server index. | Unchecked/checked | Unchecked |
| NamespaceUri | Text identifier of the namespace of the OPC UA server that is used instead of the numerical representation of a namespace. | String | |
| ServerIndex | A numerical identifier that addresses the server number when using the ExpandedNodeID identifier. | Numeric | 0 |
Settings of other parameters
| Name | Meaning | Unit | Default value |
|---|---|---|---|
| Sampling type | The parameter defines a sampling frequency of monitored object. When using the "Publishing rate", the frequency is equivalent to time Requested Publishing Interval, which is set on the communication station level. "Practical fastest rate" sets the sampling frequency on the maximum value. "Custom rate" enables to specify the custom sampling interval, which may be defined in "Sampling Time". | Publishing rate/Practical fastest rate/Custom rate | Publishing rate |
| Sampling time | The parameter allows you to set the custom sampling frequency if "Sampling type" is "Custom rate". | ss.ms | 0.0 |
| DeadBand type | Deadband is a band in which the change of value does not cause Data Change Notification, which is the part of Publish Message. When using "None", this band is ignored. Otherwise, there is used the relative or absolute value ("Percent"/"Absolute") from "DeadBand value". | None/Absolute/Percent | None |
| DeadBand value | The parameter defines the custom value of a deadband if you chose the relative/absolute value ("Percent"/"Absolute"). | 0.0 | |
| Trigger type | The parameter specifies the condition which causes Data Change Notification. When using "Status", only the status change is reported. Change of value and timestamp are ignored. When using "Status,Value", the change of timestamp is ignored. "Status,Value,Timestamp" ensures the reporting in all options, i.e. when changing the status, value or timestamp. Note: a specific Simatic S7-1500 did not send value changes if this parameter was set to default "Status, Value, Timestamp" - changing it to "Status, Value" helped. | Status/Status,Value/Status,Value,Timestamp | Status,Value,Timestamp |
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.
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: 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
| Command | Syntax | Meaning |
|---|---|---|
| STWATCH | STWATCH StationName | Tell command sends commands for the reading of values of all configured I/O tag |
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
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