Modbus TCP (REST)

Not JSON-RPC. The FastAPI app exposes:

• POST /modbus/read_registers

Implementation: bacpypes_server/modbus_routes.py + modbus_service.py. Reads run in asyncio.to_thread so the event loop stays responsive while pyModbusTCP performs synchronous I/O.

Request body (ModbusReadRequestBody)

Field	Type	Notes
host	string	Modbus TCP host.
port	int	Default 502.
unit_id	int	Unit / slave id 0–255.
timeout	float	Seconds 0.5–60.
registers	array	1–32 operations per HTTP request.

Each register operation (ModbusRegisterOp):

Field	Notes
address	0–65535 (wire vs documented addressing is device-specific).
count	1–125 words.
function	"holding" (FC03) or "input" (FC04).
decode	Optional: raw, uint16, int16, uint32, int32, float32 (32-bit types need count >= 2; big-endian, first word = high 16 bits).
scale / offset	Applied after decode.
label	Echoed in the response.

Authentication

When BACNET_RPC_API_KEY is set, use the same Bearer rules as JSON-RPC (see JSON-RPC), except paths exempted by the auth middleware.

Example

curl -sS -X POST "http://localhost:8080/modbus/read_registers" \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $BACNET_RPC_API_KEY" \
  -d '{
    "host": "10.200.200.170",
    "port": 502,
    "unit_id": 1,
    "timeout": 5.0,
    "registers": [
      {"address": 184, "count": 1, "function": "holding", "decode": "uint16", "label": "Battery SoC %"},
      {"address": 500, "count": 2, "function": "input", "decode": "float32", "label": "Example L1-N V"}
    ]
  }'