In This Guide
Specify historical variable values and historical event fields separately when comparing an OPC UA historian proposal.
Name the historical record type
OPC UA Historical Access distinguishes time-series values from historical events with their event fields. Support can cover a subset of a server’s available information. Primary reference: Part 11 historical data architecture.
The word historian does not establish that every alarm, variable or event-specific field is retained. The offered historical capability and the information actually stored must match the project’s requested evidence.
A hypothetical two-record requirement
Imagine a buyer needs a temperature trend and a record of each coded inspection occurrence with its reason field. A stored series of temperature values could satisfy part of the request while leaving the occurrence record unresolved. An event list could likewise omit the continuous trend.
Make distinct rows for variable identity/value/time/status requirements and event type/required fields. Ask what is retained for unsupported event types or omitted fields under the selected implementation. Neither a trend screenshot nor an event-count total alone demonstrates the complete two-record contract.
Review retrieval as well as storage
Preserve the proposed retention scope, historical access capability and representative expected retrieval results. Ask which fields are returned with each selected record and how known collection gaps are represented. Keep this scope separate from a general latest-value connection.
No historian or event interface is assigned to HM equipment. This guide contains no database migration or retention-setting instructions. Its result is a concrete list of historical evidence the supplier must demonstrate, allowing a buyer to distinguish time-series availability from event completeness instead of accepting a single historian label for both.
Customer Questions
Does a historian label prove every event field is stored?
Verify the offered scope.
Can a trend replace an occurrence reason field?
They answer different record questions.
Is latest-value access complete historical access?
The capability and retention contract must be specified.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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