Simple modbus code in python.

#!/usr/bin/python3
import serial
import crcmod  # pip install --user crcmod
import struct
import binascii
import time


_modbus_crc = crc = crcmod.Crc(poly=0x18005)
def modbus_crc(frame) :
    '''Generate the modbus CRC for given frame (without the CRC) as bytes().'''
    crc = _modbus_crc.new()
    crc.update(frame)
    # CRC returned in *LITTLE* endian, unless usual modbus
    return struct.pack('<H',crc.crcValue)

def request_frame(unit, function, register, length) :
    '''Generate a modbus request frame.'''
    payload = struct.pack('>BBHH', unit, function, register, length)
    return payload + modbus_crc(payload)

def error(what, msg, ret) :
    def ascii(raw) :
        return str(binascii.hexlify(raw), 'ascii')
    print(what)
    print('Frame sent:      (%2d byte) %s'%(len(msg),ascii(msg)))
    print('Answer received: (%2d byte) %s'%(len(ret),ascii(ret)))

def modbus_request(dev, unit, fct, reg, nregs) :
    # generate request frame, send to serial port
    msg = request_frame(unit, fct, reg, nregs)
    dev.write(msg)

    # request max of 1024 bytes, but only wait up to timeout (0.2sec)
    ret = dev.read(1024)

    # check returned packet for
    if len(ret) < 5 or (ret[-2:] != modbus_crc(ret[0:-2])) :
        error('Short message, or wrong CRC in answer.', ret, msg)
        return None

    if ret[0] != unit or ret[1] != fct :
        error('Reply has wrong unit of function.', ret, msg)
        return None

    return ret[3:-2] # only return payload

# open serial port
dev = serial.Serial('/dev/ttyUSB0', 9600, stopbits=2, parity='N', timeout=0.2)

# query these registers, for a 1 phase meter, returns 2 16-bit values, which,
# when concatenated, correspond to a 32bit IEEE float.
for reg in 0x00, 0x06, 0x0c, 0x12, 0x18, 0x1e, 0x24, 0x46 :

    payload = modbus_request(dev, unit=1, fct=4, reg=reg, nregs=2)
    if payload is None :
        continue

    v = struct.unpack('>f', payload)[0]
    print('Float register %04x is %f.'%(reg, v))