Unclear modbus datatypes in TMs

[Ostrzyciel]

The TMs currently include modbus:type definitions like:

thingmodels/siemens/siemens/3nacom-fuse/v1.0.0-20240802121832-3b18ae135fbc.tm.json

Line 983 in 50ef5ff

"modbus:type": "number",

So far, I've found that integer, number, and integer types are used, which sounds like datatypes in JSON.

However, it's really not clear how exactly should these types be interpreted on the binary level. I found the Siemens manual for the relevant device, and there is a section on Siemens datatypes in Modbus, so I can figure it out manually. But, it's not possible to do that automatically.

This document uses instead XSD datatypes: https://w3c.github.io/wot-binding-templates/bindings/protocols/modbus/#payloaddatatype

Which honestly creates even more problems (e.g., "how is xsd:decimal encoded???") and does not answer the questions about binary encodings.

This document also uses XSD datatypes, but with completely different indentifiers: https://w3c.github.io/wot-binding-templates/bindings/protocols/modbus/ontology.html#payloaddatatype

I currently have implemented a workaround which guesses the binary layout and datatype based on the generic datatype given in the TM and its length in registers.

[Ostrzyciel]

Note: right now I must use modbus:quantity to infer the datatype (e.g., distinguish between FP32 and FP64 in Siemens nomenclature), so it's not possible to describe numerical arrays with this arrangement.

This:

{
  "modbus:quantity": 8,
  "modbus:type": "number"
}

could mean either a 2-element array of FP64, or a 4-element array of FP32.

I did not find such ambiguities in the TMs in this repo, but I'm noting this here as a general motivation for having less ambiguous datatypes.