Read the data registers of a Vaisala HMP110 probe

viasala_hmp110.py

import struct

# pip install pymodbus[serial]
from pymodbus.client import ModbusSerialClient
from pymodbus.exceptions import ModbusIOException


def read_holding_registers():
    def unpack(r, i):
        a, b = r.registers[i-1:i+1]
        return struct.unpack('<f', struct.pack('<HH', a, b))[0]

    while True:
        response = probe.read_holding_registers(address=0, count=28, slave=240)
        if not isinstance(response, ModbusIOException):
            return {
                'relative_humidity': unpack(response, 1),
                'temperature': unpack(response, 3),
                'dewpoint': unpack(response, 9),
                'absolute_humidity': unpack(response, 15),
                'mixing_ratio': unpack(response, 17),
                'wetbulb_temperature': unpack(response, 19),
                'enthalpy': unpack(response, 27),
            }


probe = ModbusSerialClient('COM10', method='rtu', baudrate=19200, stopbits=2, parity='N', timeout=1)
probe.connect()
print(read_holding_registers())