Reference

OPC UA glossary

One concept per page, defined precisely, with the mistake engineers actually make with it. These are the terms the specification uses and the ones that turn up in error messages, vendor documentation and job descriptions.

Address space 20

BrowseNameIn OPC UA, a BrowseName is a node's machine-readable name: a QualifiedName pairing a namespace index with a non-localised text string. Browse paths an DataTypeIn OPC UA, a DataType is the NodeClass that defines the type of a Variable's value. DataType nodes form a hierarchy rooted at BaseDataType covering th DisplayNameIn OPC UA, DisplayName is a node's human-readable label, held as a LocalizedText with a locale identifier and text. Clients show it in browse trees an HasComponentIn OPC UA, HasComponent is the hierarchical ReferenceType expressing composition: the target node is a constituent part of the source. It is identifie HasPropertyIn OPC UA, HasProperty is the hierarchical ReferenceType linking a node to a Property: a Variable of type PropertyType holding metadata about its pare HasSubtypeIn OPC UA, HasSubtype is the hierarchical ReferenceType that builds type hierarchies: it points from a supertype to a subtype, with the inverse name S HasTypeDefinitionIn OPC UA, HasTypeDefinition is the non-hierarchical ReferenceType linking an instance to its type: every Object points at an ObjectType and every Var MethodIn OPC UA, a Method is the NodeClass representing a callable server-side procedure. It is invoked with the Call service against an object NodeId plus Namespace indexIn OPC UA, a namespace index is the UInt16 position of a namespace URI in a server's NamespaceArray, and it is the first half of every NodeId and Qual Namespace URIIn OPC UA, a namespace URI is the globally unique string that identifies who defined a set of nodes, for example http://opcfoundation.org/UA/Machinery NamespaceArrayIn OPC UA, the NamespaceArray is a String array Variable on the Server object that maps namespace indexes to namespace URIs by array position. Index 0 NodeClassIn OPC UA, NodeClass is the mandatory attribute stating what kind of node you are looking at. There are exactly eight: Object, Variable, Method, Objec NodeIdIn OPC UA, a NodeId is the unique address of a node inside a server's address space. It combines a namespace index with an identifier that is Numeric ObjectIn OPC UA, an Object is the NodeClass that structures the address space: it holds no value of its own but groups Variables, Methods and other Objects,ObjectTypeIn OPC UA, an ObjectType is the NodeClass that defines the shape of Objects: which Variables, Methods, child Objects and events an instance of it has.OrganizesIn OPC UA, Organizes is the hierarchical ReferenceType expressing loose membership: the target node appears under the source, typically a Folder, with ReferenceTypeIn OPC UA, a ReferenceType is the NodeClass that defines the meaning of a reference between two nodes. Every reference is typed, directed and named by VariableIn OPC UA, a Variable is the NodeClass that holds a value. Its Value attribute is a DataValue - value, StatusCode and timestamps - described by the Da VariableTypeIn OPC UA, a VariableType defines the shape of Variables: the DataType, ValueRank and ArrayDimensions its instances take, and the Properties they carr ViewIn OPC UA, a View is the NodeClass that defines a restricted subset of the address space for browsing. A client passes the View's NodeId in a ViewDesc

Modelling 15

Address spaceIn OPC UA, the address space is the complete set of nodes and references a server exposes to clients. It is a directed graph, not a tree, entered at t Companion specificationIn OPC UA, a companion specification is an information model standardised by the OPC Foundation with an industry body, defining the types one machine DataTypeDefinitionIn OPC UA, DataTypeDefinition is the attribute on a DataType node that describes a server-defined structure, union or enumeration field by field. Intr EnumerationIn OPC UA, an Enumeration is a DataType whose values are a fixed set of named integers. Every enumeration is encoded as an Int32 on the wire, whatever Information modelIn OPC UA, an information model is a set of type definitions and standard instances published in its own namespace, so that the same machine concept l Instance declarationIn OPC UA, an instance declaration is a node that belongs to a type definition rather than to a running instance: a child of an ObjectType or Variable InterfaceIn OPC UA, an Interface is a type that adds a named set of children to any node that references it with HasInterface, independent of the node's place Mandatory / OptionalIn OPC UA, Mandatory and Optional are the two modelling rules that decide whether an instance must carry a declared child node. Mandatory children are Modelling ruleIn OPC UA, a modelling rule is the node attached to an instance declaration by a HasModellingRule reference that says what an instantiation of the typ NodeSet2In OPC UA, NodeSet2 is the normative XML serialisation of an address space, defined as UANodeSet in Part 6 Annex F. Every released companion specifica Optional fieldsIn OPC UA, optional fields are Structure fields that may be present or absent per value. A structure declaring them has StructureType StructureWithOpt Structure / ExtensionObjectIn OPC UA, a Structure is a DataType with named fields, and an ExtensionObject is the wire container that carries one: an encoding NodeId plus the enc Type dictionary (legacy)In OPC UA, a type dictionary is the pre-1.04 mechanism for describing server-defined structures: an OPC Binary .bsd or XML schema document published a Type hierarchyIn OPC UA, the type hierarchy is the tree of type nodes built from HasSubtype references. There are four of them, rooted at BaseObjectType, BaseVariab UnionIn OPC UA, a Union is a Structure subtype that carries exactly one of its declared fields at a time. On the wire it is a UInt32 switch value naming th

Connection 19

ApplicationUriThe ApplicationUri is the globally unique identity of one OPC UA application instance, such as urn:edge01:Contoso:Publisher. It appears in the Applica Discovery endpointA discovery endpoint is an endpoint that answers the Discovery Service Set - FindServers and GetEndpoints - without requiring a session, and normally EndpointIn OPC UA, an endpoint is one concrete way to connect to a server: a URL, a security policy, a MessageSecurityMode and a list of accepted user identit EndpointDescriptionAn EndpointDescription is the structure GetEndpoints returns for each endpoint. It carries EndpointUrl, the server's ApplicationDescription, the serve GetEndpointsGetEndpoints is the service that asks a server which endpoints it publishes. It runs on the discovery endpoint without a session and, by convention, o HTTPS transportOPC UA over HTTPS uses opc.https:// or https:// and three profiles. https-uabinary tunnels UA Binary secure-channel messages one chunk per HTTP POST a Keep-aliveKeep-alive in OPC UA means two different mechanisms. The session keep-alive is a periodic cheap read the client makes to prove the session is alive an Local Discovery ServerA Local Discovery Server is a small OPC UA server that runs on a host, listens on port 4840 and keeps a register of the OPC UA servers on that machine opc.tcpopc.tcp:// is the URL scheme of OPC UA's native transport, profile uatcp-uasc-uabinary: UA Binary messages inside a UA Secure Conversation channel on OpenAPI / REST mappingThe OpenAPI mapping is a REST-style surface for the OPC UA service set: POST /<service> with the bare service request as JSON. The OPC Foundation publ Port 48404840/tcp is the registered default port for opc.tcp and for the Local Discovery Server. Related defaults worth knowing: 4843 for secure WebSocket and ProductUriThe ProductUri identifies the software product rather than the running instance: every copy of the same product carries the same ProductUri, assigned Reverse connectReverse connect inverts who dials. The server opens the TCP connection outbound to a client that is listening, sends a ReverseHello, and the client th ReverseHelloReverseHello is the OPC UA TCP message, type code RHE, that a server sends immediately after dialling out to a listening client. It carries the server Secure channelA secure channel is the transport-level security context between a client and a server, created by OpenSecureChannel. The asymmetric handshake authent SessionA session is the application-level context between client and server, created by CreateSession and made usable by ActivateSession. It carries the auth Session timeoutThe session timeout is how long a server keeps a session alive without receiving any request on it. The client proposes RequestedSessionTimeout in Cre Transport profileA transport profile URI names the exact combination of URL scheme, wire encoding and security mechanism an endpoint uses - uatcp-uasc-uabinary for opc WebSocket transportOPC UA over WebSockets uses the opc.wss:// or wss:// scheme, default port 4843, and comes in two shapes. wss-uasc-uabinary carries the ordinary UA Bin

Subscriptions 19

DataChangeFilterA DataChangeFilter is the filter attached to a data MonitoredItem. It has three fields: Trigger (Status, StatusValue, StatusValueTimestamp), DeadbandT DataChangeTriggerDataChangeTrigger is the field of a DataChangeFilter that defines what counts as a change: Status reports only StatusCode changes, StatusValue - the d Deadband (absolute)An absolute deadband suppresses a data change unless the new value differs from the last reported one by at least DeadbandValue, expressed in the valu Deadband (percent)A percent deadband expresses the threshold as a percentage of the item's EURange span, so 1.0 on a 0 to 200 bar range means 2 bar. It requires the nod Discard oldestDiscardOldest is the per-MonitoredItem boolean deciding which notification is thrown away when the queue is full. True, the usual default, discards th EURangeEURange is a Property of an AnalogItemType Variable holding a Range structure with the Low and High engineering-unit limits of the value. It is what a HeartbeatA heartbeat is a client-side or gateway-side watchdog that re-emits a monitored item's last value when the server has reported nothing about it for a KeepAliveCountMaxKeepAliveCount is the number of publishing intervals a subscription may produce no notifications before the server sends an empty keep-alive messag Lifetime countLifetimeCount is the number of publishing intervals a subscription may go without receiving a Publish request before the server deletes it. The server MonitoredItemA MonitoredItem is the server-side object that watches one attribute of one node - usually the Value - samples it at its own sampling interval, applie Monitoring modeMonitoringMode is a per-item switch with three states. Disabled means the server does not sample the item at all. Sampling means it samples and queues Overflow bitWhen a MonitoredItem queue overflows, the server sets an InfoBit inside the surviving DataValue's StatusCode rather than returning an error. The code Publish requestPublish is the long-poll service that carries notifications from server to client. The client keeps several Publish requests outstanding on the sessio Publishing intervalThe publishing interval is the cyclic rate at which a subscription drains its MonitoredItems' queues into one NotificationMessage. It is a delivery ra Queue sizeQueueSize is how many notifications a MonitoredItem may hold between publishing cycles. Size it as the publishing interval divided by the sampling int Sampling intervalThe sampling interval is the rate at which a server samples one MonitoredItem's attribute and evaluates its filter. It is per item, not per subscripti SubscriptionA subscription is the server-side container that groups MonitoredItems and delivers their notifications on one cyclic publishing interval, in response TransferSubscriptionsTransferSubscriptions moves existing subscriptions from a previous session to a new one, so a reconnecting client keeps its MonitoredItems, queues and TriggeringTriggering links MonitoredItems so that when one triggering item reports, a set of linked items release whatever they have queued. Set with SetTrigger

Data 14

AccessLevelIn OPC UA, AccessLevel is a bit-mask attribute on a Variable declaring what the server allows on that node: current read, current write, history read,ArrayDimensionsIn OPC UA, ArrayDimensions is an optional Variable attribute giving the length of each dimension of an array value. It has one entry per dimension dec DataValueIn OPC UA, a DataValue is the container every read, write and subscription notification uses. It holds the Value as a Variant, a StatusCode giving the Engineering unit (UNECE)In OPC UA, engineering units are carried by an EUInformation structure on an AnalogItemType variable, using the code list from UNECE Recommendation 20 IndexRangeIn OPC UA, IndexRange is a string parameter on Read, Write and CreateMonitoredItems that selects part of an array value instead of the whole thing. A Non-reversible JSON encodingIn OPC UA, non-reversible JSON encoding is the compact JSON form that drops type information to produce plain, human-readable values. It cannot be rou Reversible JSON encodingIn OPC UA, reversible JSON encoding is the JSON form that keeps full type information, so a decoder can reconstruct the original Variant, NodeId and s ServerTimestampIn OPC UA, the ServerTimestamp in a DataValue is the time the OPC UA server received or last updated the value in its own address space. It is measure SourceTimestampIn OPC UA, the SourceTimestamp in a DataValue is the time the data source itself produced or last verified the value. It is the timestamp to store in StatusCodeIn OPC UA, a StatusCode is a 32-bit numeric result carried by every service response, operation result and DataValue. Its upper bits give the severity TimestampsToReturnIn OPC UA, TimestampsToReturn is a parameter on Read, HistoryRead and CreateMonitoredItems that tells the server which timestamps to include in each D UA Binary encodingIn OPC UA, UA Binary is the default wire encoding: a compact, little-endian, unpadded serialisation of every service message and data type. It is what ValueRankIn OPC UA, ValueRank is a Variable or VariableType attribute declaring the dimensionality of the value. Minus one means a scalar, one or more means th VariantIn OPC UA, a Variant is the self-describing value container that can hold any of the built-in types, or an array or matrix of them. It carries its own

PubSub 10

DataSetReaderIn OPC UA PubSub, a DataSetReader is the subscriber-side counterpart of a DataSetWriter. It filters arriving NetworkMessages by PublisherId, WriterGro DataSetWriterIn OPC UA PubSub, a DataSetWriter publishes one PublishedDataSet into a WriterGroup. It owns the DataSetWriterId that identifies its messages on the w Delta frameIn OPC UA PubSub, a delta frame is a DataSetMessage that carries only the fields whose values changed since the previous message, each tagged with its Key frameIn OPC UA PubSub, a key frame is a DataSetMessage carrying every field of the PublishedDataSet rather than only the changed ones. It is what lets a su PublishedDataSetIn OPC UA PubSub, a PublishedDataSet defines what a publisher makes available: an ordered list of published fields selected from the address space, pl Security Key ServiceIn OPC UA, a Security Key Service is the component that distributes the symmetric group keys PubSub uses to sign and encrypt messages. PubSub has no s Sparkplug BSparkplug B is an Eclipse Foundation specification that adds a defined topic namespace, a Protobuf payload and strict connection-state handling to pla UADPIn OPC UA, UADP is the compact binary message mapping used by PubSub, defined in Part 14. It carries a NetworkMessage holding one or more DataSetMessa Unified NamespaceA Unified Namespace is an architectural pattern, not an OPC UA feature: one broker-hosted, hierarchically named, always-current view of plant state th WriterGroupIn OPC UA PubSub, a WriterGroup batches DataSetWriters into a single NetworkMessage and owns the timing: PublishingInterval, KeepAliveTime, priority,

Security 18

Anonymous accessIn OPC UA, anonymous access means a session activated with an AnonymousIdentityToken: no user credential is presented. It is independent of channel se Application instance certificateIn OPC UA, an application instance certificate is the X.509v3 certificate that identifies one installed instance of a client or server. Its Subject Al Audit eventIn OPC UA, an audit event is a server-generated event recording a security-relevant action: a channel opened, a session created or activated, a certif Certificate push vs pullIn OPC UA, pull and push are the two Part 12 models for provisioning certificates. In pull, the device asks a Global Discovery Server for a certificat CRLIn OPC UA, a CRL is the Certificate Revocation List a CA publishes listing certificates it has withdrawn before their expiry date. Each CA in your tru Global Discovery ServerIn OPC UA, a Global Discovery Server is the Part 12 infrastructure server that holds an enterprise-wide register of applications and acts as their cer Issuer storeIn OPC UA, the issuer store holds CA certificates needed to complete a certificate chain without granting them trust. It is deliberately separate from MessageSecurityModeIn OPC UA, MessageSecurityMode is the per-endpoint setting that says what is done to every message on a secure channel: None, Sign, or SignAndEncrypt.NonceIn OPC UA, a nonce is a single-use random value exchanged during OpenSecureChannel and ActivateSession. Channel nonces are the input to symmetric key Permission / RolePermissionsIn OPC UA, RolePermissions is the node attribute that lists which roles may do what to that node - browse, read, write, call, read or modify history, Rejected storeIn OPC UA, the rejected store is the PKI directory where an application parks a certificate it just refused. It exists for one purpose: to give an adm Role (Part 18)In OPC UA, a Role is the Part 18 unit of authorisation: a named set of permissions that identities are mapped into, rather than permissions granted to Security policyIn OPC UA, a security policy is a named suite of cryptographic algorithms - identified by a URI such as Basic256Sha256 or Aes256_Sha256_RsaPss - that Self-signed certificateIn OPC UA, a self-signed certificate is an application instance certificate that signs itself instead of being issued by a CA. It authenticates a peer Sign vs SignAndEncryptIn OPC UA, Sign authenticates both applications by certificate and detects tampering, but leaves message contents readable on the wire. SignAndEncrypt Subject Alternative NameIn OPC UA, the Subject Alternative Name is the X.509 extension that carries an application's identity claims. It must contain the ApplicationUri as a Trust listIn OPC UA, the trust list is the set of certificates an application accepts as trust anchors: self-signed peer certificates, or the root and sub-CA ce UserIdentityTokenIn OPC UA, a UserIdentityToken is the credential a client presents in ActivateSession to say who the user is, as distinct from which application is co

Conformance 7

Certification test labAn OPC Foundation Certification Test Lab is an independent laboratory that certifies a product, not a person, against named Profiles and Facets, runni Compliance Test Tool (CTT)The OPC UA Compliance Test Tool (UACTT) is the OPC Foundation's official conformance tester. It is script-driven per conformance unit, exercises every Conformance unitIn OPC UA, a Conformance Unit is the smallest testable requirement in the conformance system: one named capability with defined behaviour that the Com FacetIn OPC UA, a Facet is a reusable bundle of Conformance Units that a Profile composes: Data Access, Methods, Reverse Connect, Historical Raw Data 2022 ProfileIn OPC UA, a Profile is a named, published set of Facets that together define what an application must implement, for example the Standard UA Server P Server capability codeA server capability code is a short string from the normative OPC UA capability list, such as DA, HD, AC, HE, DI, S95, PUB, PACKML, MTC, AUTOID or VRO ServerProfileArrayServerProfileArray is a Property under Server.ServerCapabilities holding the list of Profile and Facet URIs a server claims to implement, such as http

Alarms 7

Acknowledge / ConfirmAcknowledge and Confirm are the two Methods on AcknowledgeableConditionType that move an alarm through its operator-response lifecycle: Acknowledge re ConditionIn OPC UA, a Condition is a stateful event: an object of ConditionType, or a subtype such as AlarmConditionType, whose current state the server retain ConditionClassIdConditionClassId is a Property on every OPC UA Condition holding the NodeId of a ConditionClassType that says what kind of problem this is, independen ConditionRefreshConditionRefresh is a Method on ConditionType that makes a server re-send the current state of every retained Condition to one named subscription. Cal EventFilterAn EventFilter is the filter you attach to an event MonitoredItem created on a node whose EventNotifier attribute is set. It has two parts: SelectClau Select clause / Where clauseIn an OPC UA EventFilter, the select clause lists the event fields to return as SimpleAttributeOperands, each a typeDefinitionId plus a browse path su SeveritySeverity is a mandatory UInt16 field on every OPC UA event, valued 1 to 1000, where 1 is least significant and 1000 most urgent. It is a hint, not a p

History 5

Aggregate functionAn aggregate function is a server-side computation over raw historical values, identified by a NodeId in the server's AggregateFunctions folder: Avera Bounding valueA bounding value is the raw stored value immediately outside a requested history interval, which the server uses to establish or interpolate the value DurationGoodDurationGood is the Part 13 aggregate that returns how much time within each interval a variable held a Good StatusCode, expressed as a Duration in mi PercentGoodPercentGood is the Part 13 aggregate that returns the percentage of each interval during which a variable's StatusCode was Good. It is DurationGood ex TimeAverageTimeAverage is the Part 13 aggregate that computes a time-weighted mean over each interval, weighting every raw value by how long it held rather than

Last verified 7 September 2026 against OPC UA 1.05.07. OPC UA Connect is an educational resource and is not affiliated with the OPC Foundation.