The following list of terms and abbreviations is specific to OPC usage in NiagaraAX, and covers entries used in this document. For the definitive collection of terms found in OPC publications, refer to the OPC Foundation, at http://www.opcfoundation.org.
For general NiagaraAX terms, see the Glossary in the User Guide.
- Asynchronous vs. Synchronous IO
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Asynchronous Input / Output (that is, communications) in the OPC context means server-initiated change messages sent to the client. Asynchronous (or async) IO is the most efficient and scalable integration option, because the client does not have to poll for changes of value (synchronous)—the server simply sends change-of-value messages as they occur.
- COM and DCOM
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Most OPC specifications are embodied as COM interfaces. COM is Microsoft’s Common Object Model technology. DCOM is distributed COM, where objects in remote processes appear to be local objects.
- OPC Data Access
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There are several OPC specifications. The OPC Data Access specification provides the baseline functionality for reading and writing data from various networked devices via a standard set of interfaces.
- OPC Group
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An OPC Group is an object that an OPC server uses to manage a collection of items. OPC Group objects are created by OPC clients who specify what server items belong in the group. Client grouping does not have to match the organization of the data on the server.
- OPC Item
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OPC refers to individual data points as items.