Binding & subscribing

Binding a property to a variable is the whole subscription. Point a field at a tag with $var, and the server subscribes on your behalf and streams updates to the screen. Writable tags flow the other way.

How the live pipeline works

Three hops, two protocols:

A browser only ever subscribes to the tags actually on screen. A viewport-aware fast / slow window refreshes visible tags first, so what the operator is looking at stays the freshest.

Bind a property to a tag

1. Select the widget and its field — e.g. a gauge's Value. Click the field's source pill and choose Variable.
2. Pick the tag from the tree — The variable picker shows your datasources' trees. Choose a leaf — say LinePLC:Motor1/Speed. For an array, also choose the index to bind one element; leave it off to bind the whole array.
3. Watch it go live — The canvas immediately reflects streamed values. That's the subscription — there's nothing else to register.

// a scalar tag
"value": { "$var": { "path": "LinePLC:Motor1/Speed" } }

// one element of an array
"value": { "$var": { "path": "LinePLC:Readings", "index": 0 } }

// a whole struct member reached by slash-path
"value": { "$var": { "path": "LinePLC:Motor1/Torque" } }

The path is always datasource:location, with the location slash-separated. The prefix before the colon names the connection.

Write back to the machine

Mark a variable writable when browsing, and controls can push values to it. A Button binds a struct with bVisible, bEnabled and a writable bValue — press it and the runtime sends a write_field message that sets the tag. A numeric setpoint written from a numpad works the same way.

What happens when data goes bad

The runtime turns failure into a defined visual state — never a stale value passed off as live, and never a crash.