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

Next message sent by client is "OpenSecureChannel message" . It is used to establish the 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 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 OPC UA server can be swapped. In this step the client creates a subscriptions with the parameters set on the communication station level for all stations within the communication line.

The monitored items should be then inserted to 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 on 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 "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 "Publish message" once every (Max KeepAlive Count * Publishing Interval). If the client does not send the acknowledgement "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. "", "") or IP address ("").
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 nameMeaningUnitDefault 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.StringKom 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:ss01:00:00
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
Authentication Type
Type of autentication used with OPC UA server. Supported types are:Anonymous / UsernameAnonymous
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 the communication. We recommend to enable the mode Extended/Full only when detecting the problems and debugging the communication.Normal/Extended/FullNormal
Debug Threads
Parameter defines the thread(s) that will send the debug info about the communication.Receiving/Sending/Others treads/All threadsAll 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 subscription within session.

Full nameDescriptionUnitDefault value
Requested Publishing Interval

Defines the time interval for server to send the information about the change of monitored items within the instance subscription by "Publish message".
Note: This parameter defines a proposed value which the OPC UA server can change, e.g. Bernecker-Rainer always returned a value of "Publishing Interval" at least 50 ms, although the requested interval was smaller.

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".
Note: This parameter defines a proposed value which the OPC UA server can change, e.g. Bernecker-Rainer always returned as a value of "LifeTime Count" a maximum of 600, although the requested value was greater.

Requested Max KeepAlive Count

If the objects of subscription are not changed, the server will send 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.
Note: This parameter defines a proposed value which the OPC UA server can change, e.g. Bernecker-Rainer has always returned as a value of "Max KeepAlive Count" a maximum of 200, although the requested value was greater.

Max Notifications Per Publish
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.Number0
Publishing Enabled
Parameter enables/disables the publishing within the subscription.YES/NO0
It defines a relative priority of a subscription. If server should send more notifications, the subscription with higher priority is preferred.0-2550
Samples Queue Size
This parameter enables to create the object queue with the defined length on OPC UA server's side for each monitored item in subscription.Number0

I/O tag configuration

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

Object address setting

NameMeaningUnitDefault value
IDThe identifier in text format, which is, in dependence on ID type, converted to the required native type.String 
ID typeEnumerated types of identifiers. They help to access to the 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/ByteStringUndefined
Numerical identifier of name space of OPC UA server. Each OPC UA server can have N name spaces. However, the object identifier must be unique in one name space.Numeric 
Variable typeValue 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 arrayUndefined
Array indexIf 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:
  • Separate integer, e.g. "6" - when you want to obtain only one value from the array.
  • Two integers separated by a colon, e.g. "6:7", - if you want to obtain the range of values.
  • The expression separated by comma in case of multidimensional array, e.g. "6,7" - when you want to obtain the particular value of the item on 2D array. If you want to define the range, you should use the expression separated by semi-colon, e.g. "6:8,7:10".
Write onlyIt sets if I/O tag is a part of subscription. Its value will be sent periodically from the server in "Publish message".Unchecked/checkedUnchecked
Expanded Node IDIf 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/checkedUnchecked
NamespaceUriText identifier of name space of OPC UA server that is used instead of the numerical representation of a namespace.String 
ServerIndexNumerical identifier that address the server number when using the  ExpandedNodeID identifier.Numeric0

Settings of other parameters

NameMeaningUnitDefault value
Sampling typeParameter defines a sampling frequency of monitored  object. When using "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 ratePublishing rate
Sampling timeParameter allows you to set the custom sampling frequency if "Sampling type" is "Custom rate".ss.ms0.0
DeadBand typeDeadband 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/PercentNone
DeadBand valueParameter defines the custom value of deadband if you chose the relative/absolute value ("Percent"/"Absolute"). 0.0
Trigger typeParameter specifies the condition which causes Data Change Notification. When using "Status", only the status change is reported. Change of value and time stamp is ignored. When using "Status,Value", the change of time stamp is ignored. "Status,Value,Timestamp" ensures the reporting in all options, i.e. when changing the status, value or time stamp.Status/Status,Value/Status,Value,TimestampStatus,Value,Timestamp

Browser dialog window

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

Note: In versions from 17th December 2018 and newer, 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 I/O tag by double-clicking the tag name in the tree structure.

Tell commands

STWATCHSTWATCH StationNameTell command sends commands for reading of values of all configured I/O tag


OPC Foundation manuals are placed on

  • 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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