Multistate BACnet properties mapping

[egekorkan]

As seen in BrickSchema/Brick#566, a property can have 3 values i.e. State_Text = ("Off", "On_Low", "On_High") but these are not what is sent via the protocol. Based on the implementation, these "meanings" can be mapped to different numbers. E.g. one implementation says that "Off" is 0 where the other has that as "2". We can solve this with TDs

[fennibay]

So, BACnet's approach is that only the numeric values (unsigned, enum, boolean) are transferred over the wire. Strings remain on the state-text property on multistate objects (and active-text, inactive-text properties on binary objects), and BACnet clients can use these strings to show the actual value of the object as a string instead of the numeric value.

In WoT-BACnet protocol binding we mimic this behavior with the ValueMap. Data schema is based on strings, which is then mapped to protocol-specific numeric values in the Form using the ValueMap.

Example:

wot-binding-templates/bindings/protocols/bacnet/index.html

Lines 1045 to 1086 in a6d657c

	<pre id="example-form-variables" class="example" title="value mapping for an Enumeration">
	{
	"@context": ["https://www.w3.org/2022/wot/td/v1.1",
	{
	"bacv": "https://example.org/bacnet"
	}],
	...
	"properties": {
	"multistate1": {
	"type": "string",
	"enum": ["on", "off", "auto", "manual"]
	"readOnly": true,
	"forms": [{
	"op": [ "readproperty" ],
	"href": "bacnet://5/14,1/85",
	"bacv:usesService": "ReadProperty",
	"bacv:hasDataType": {
	"@type": "bacv:Enumerated",
	"bacv:hasValueMap": [
	{
	"bacv:protocolVal": 1,
	"bacv:logicalVal": "on"
	},
	{
	"bacv:protocolVal": 2,
	"bacv:logicalVal": "off"
	},
	{
	"bacv:protocolVal": 3,
	"bacv:logicalVal": "auto"
	},
	{
	"bacv:protocolVal": 4,
	"bacv:logicalVal": "normal"
	}
	]
	}
	}]
	}
	}
	}
	</pre>