IEC 62541-8:2020 pdf free download – OPC unified architecture
4concepts
Data Access deals with the representation and use of automation data in Servers.
Automation data can be located inside the Server or on l/0 cards directly connected to theServer. lt can also be located in sub-servers or on other devices such as controllers andinputloutput modules,connected by serial links via field buses or other communication links.oPc UA Data Access Servers provide one or more oPC UA Data Access Clients withtransparent access to their automation data.
The links to automation data instances are called Dataltems.The categories of automation dataare provided is completely vendor-specific. Figure 1 illustrates how the AddressSpace of aServer may consist of a broad range of different Dataltems.
Clients may read or write Dataltems, or monitor them for value changes.The Services neededfor these operations are specified in lEC 62541-4.Changes are defined as a change in status(quality) or a change in value that exceeds a client-defined range called a Deadband.To detectthe value change,the difference between the current value and the last reported value iscompared to the Deadband.
5 Model
5.1General
The DataAccess model extends the variable model by defining VariableTypes. TheDataltemType is the base type. ArrayltemType，BaseAnalogType and DiscreteltemType arespecializations.See Figure 2.Each of these VariableTypes can be further extended to formdomain- or server-specific Dataltems.
Annex A specifies the recommended way for mapping the information received from oPC COMData Access (DA) Servers to the model in this document.
lt is important to note that understanding the units of a measurement value is essential for auniform system. In an open system in particular where Servers from different cultures might beused, it is essential to know what the units of measurement are.Based on such knowiedge,values can be converted if necessary before being used. Therefore，although defined asoptional, support of the EngineeringUnits Property is strongly advised.
oPC UA recommends using the “Codes for Units of Measurement”(see UN/CEFACT: UNECERecommendation N° 20).The mapping to the EngineeringUnits Property is specified in 5.6.3.
NOTE 2 Examples for unit mixup: in 1999, the Mars Climate Orbiter crashed into the surface of Mars. The mainreason was a discrepancy over the units used.The navigation software expected data in newton second; the companywho built the orbiter provided data in pound-force seconds.Another, less expensive,disappointment occurs whenpeople used to British pints order a pint in the USA, only to be served what they consider a short measure.
The StatusCode SemanticsChanged bit shall be set if any of the EURange (could change thebehaviour of a Subscription if a PercentDeadband filter is used) or EngineeringUnits (couldcreate problems if the Client uses the value to perform calculations) Properties are changed(see 5.2 for additional information).
EnumValues is an array of EnumValueType. Each entry of the array represents oneenumeration value with its integer notation， a human-readable representation， and helpinformation. This represents enumerations with integers that are not zero-based or have gaps(e.g.1，2,4，8，16).See IEC 62541-3 for the definition of this type.MultiStateValueDiscreteVariables expose the current integer notation in their Value Attribute.Clients will often read theEnumValues Property in advance and cache it to lookup a name or help whenever they receivethe numeric representation.