Modbus, python and minimal modbus

Question

I'm using the python library Minimal Modbus to read an energy meter (RS-485).

I'm trying to use the function to read multiple registers at once

read_registers(registeraddress, numberOfRegisters, functioncode=3)

Here is the code of the python library:

def read_registers(self, registeraddress, numberOfRegisters, functioncode=3):
        """Read integers from 16-bit registers in the slave.

        The slave registers can hold integer values in the range 0 to 65535 ("Unsigned INT16").

        Args:
            * registeraddress (int): The slave register start address (use decimal numbers, not hex).
            * numberOfRegisters (int): The number of registers to read.
            * functioncode (int): Modbus function code. Can be 3 or 4.

        Any scaling of the register data, or converting it to negative number (two's complement)
        must be done manually.

        Returns:
            The register data (a list of int).

        Raises:
            ValueError, TypeError, IOError

        """
        _checkFunctioncode(functioncode, [3, 4])
        _checkInt(numberOfRegisters, minvalue=1, description='number of registers')
        return self._genericCommand(functioncode, registeraddress, \
            numberOfRegisters=numberOfRegisters, payloadformat='registers')

The problem I have got is that the registers hold the data as Longbut this function returns them as a list of int and looking at the values it doesn't seem correct.

This is my script at the moment:

##!/usr/bin/env python

import minimalmodbus
import time
import glob
import sys
import MySQLdb

instrument = minimalmodbus.Instrument('/dev/ttyUSB1', 7)

#Debug Options

#instrument.debug = True #debug modbus

read = False

def convert(value):
        data=chr(value)
        return data

def read_registers_ime():
        data = instrument.read_registers(4096,20)
        return data

while True:
        try:
                while read == False:
                        data = read_registers_ime()
                        print data
                        time.sleep(0.11)
                        read = True

                break

        except KeyboardInterrupt:
                print "Stopped"
                sys.exit()

        except TypeError:
                print "TypeError"

        except IOError:
                print "IOError"

AT the moment it returns the following:

[3, 39192, 3, 44592, 3, 44592, 0, 423, 0, 0, 0, 0, 0, 0, 6, 19884, 6, 24584, 6, 19884]

Everything I have tried so far to convert the data back to the original format has failed. I would really appreciate some help here.

Many thanks,

Bryan

Answer 1

I already encountered this kind of problem working with modbus using umodbus instead of minimal modbus. I solved it by:

• getting the registers by pairs
• using struct to convert the pairs to ints

see example below (Python 2.7):

import struct

registers = [3, 39192, 3, 44592, 3, 44592, 0, 423, 0, 0, 0, 0, 0, 0, 6, 19884, 6, 24584, 6, 19884]

def grouped(iterable, group_size):
    """iterates on iterable, yielding groups of group_size elements"""
    it = iter(iterable)
    while True:
        group = [next(it) for _ in range(group_size)]
        yield group

for group in grouped(registers, 2):
    bytes = b""
    for word in group:
        word_bytes = struct.pack("H", word)
        bytes += word_bytes
    bytes_hex = "0x" + "".join(["{:>02X}".format(ord(byte_)) for byte_ in bytes])
    print("{} -> {} -> {}".format(group, bytes_hex, struct.unpack("<i", bytes)[0]))

prints:

[3, 39192] -> 0x03001899 -> -1726480381
[3, 44592] -> 0x030030AE -> -1372585981
[3, 44592] -> 0x030030AE -> -1372585981
[0, 423] -> 0x0000A701 -> 27721728
[0, 0] -> 0x00000000 -> 0
[0, 0] -> 0x00000000 -> 0
[0, 0] -> 0x00000000 -> 0
[6, 19884] -> 0x0600AC4D -> 1303117830
[6, 24584] -> 0x06000860 -> 1611137030
[6, 19884] -> 0x0600AC4D -> 1303117830

This example may need some tweaks according to your exact needs, but it may help.