AVEVA PI

Reference for the AVEVA PI driver: PI Web API over HTTPS, bearer or Basic authentication, AF and PI point addressing, reads and writes.

The AVEVA PI driver reads and writes an AVEVA PI System through PI Web API, the system's REST interface (JSON over HTTP). It is a client-mode driver — Ganter Lab connects to the PI Web API service — and the transport is HTTPS by default: the driver puts the device's credentials on every request it makes, so an unencrypted connection would publish them. Plain HTTP exists only as an explicit per-device choice.

A tag addresses one AF attribute, one PI point, or a raw PI Web API stream, and reads its snapshot (current) value.

Connection fields

On connect the driver verifies the service root (HEAD, falling back to GET). Over HTTPS, the service's TLS certificate must pass normal Windows certificate validation. The poll interval is configurable: the device default (1000 ms) applies to every tag that does not state its own.

Tag addressing

The tag's PI path field accepts four forms:

Path lookups are cached for 12 hours, so steady-state polling does not repeat them. For the first three forms the read GETs the stream's snapshot value (streams/{webId}/value) and extracts the JSON Value field; a relative URL is read literally, so point it at an endpoint that answers with a value document. The result is converted to the tag's declared data type.

The archive's own verdict on the sample is read with it. A value the archive marks as not good, as questionable, or as substituted (a number entered by hand rather than measured) is not a reading of the equipment, so it is not published as one: the tag goes to bad quality and the reason says which of the three it was. The same happens when the response carries no value, or answers with a digital state rather than a single value, in which case the state's name is part of the reason. Nothing is invented in their place, and no document arrives as if it were the measurement.

Examples:

• \\PI-SRV01\FURNACE.TEMP — a classic PI point.
• \\AF-SRV\Plant\Line 3\Furnace|Temperature — an AF attribute.
• streams/F1DPmNQx2kqBk0qbIVMoxAVBJw/value — a raw stream URL, useful when you already hold the WebId from another tool.

Writes

Tags with a writable access mode write through PI Web API's stream endpoint: a POST of {"Timestamp":"*","Value":…} (timestamp * means now) to streams/{webId}/value. The write needs a resolvable WebId, so it works for tags addressed by AF path, PI point path, WebId, and by a relative streams/… URL as well: the WebId is read out of that URL and the write lands on that stream's value endpoint, whatever read-side sub-resource (recorded, say) the address carried past it. Three relative forms are refused, because each answers with a document rather than with one stream's value and a write would address the wrong resource: streamsets/…, attributes?… and points?…. The refusal names the accepted forms. Whether the write lands also depends on the PI Web API being configured for writes and the account having write access to the point.

The value you type in the tag's write box is the engineering value; the tag's conversion stages are reversed before the raw value goes on the wire, as described on the Connector page.

Supported data types

Boolean, Int32, Int64, Float, Double and String. Numeric PI points are typically Float or Double. A digital state is not a single value, so a point that answers with one reads as bad quality naming the state, whatever type the tag declares.

Commands

Beyond reading and writing points, a PI Web API endpoint can be asked to do something, and which requests it accepts is the service's own business. Declare each one on the device, in the Commands card of its page: a name, the relative URL it is POSTed to under the service root, and optionally the name of the one value it takes, which travels as the request body. The row's Run verb sends it and reports what the service answered; the command also becomes a method on the device in this station's own OPC UA address space.

Discovery

The Discover button reads one service. Put its address and the AF element the walk starts from in the driver page's Scan scope card, separated by a #: https://pi-host/piwebapi#\\AF-SRV\Plant\Line 3. The scan resolves that element, walks every element under it, and offers the device with one point per attribute, each addressed by its own stream. The scan reads the service without credentials, because it runs before the device that would hold them exists, so a PI Web API that authenticates every request answers nothing and the device is added by hand instead: add the service by host and add tags by their AF or PI paths, as the add-a-device flow on the Connector page describes.