CODEWITHSUNDEEP
pythonmodbuspymodbus
Benmo751
Benmo751

Reputation: 83

Python Modbus String decoding issue

I have an issue with the pymodbus decoder with strings. For example, when I try to read 'abcdefg' pymodbus gives me 'badcfehg'. The byteorder and the wordorder don't change the result.

Here is my code: 

result=client.read_holding_registers(25000,4)
decoder = BinaryPayloadDecoder.fromRegisters(result.registers,byteorder=Endian.Little,wordorder=Endian.Big)
decoder.decode_string(8)

Can someone explain why the order does not change the result? I try with the builder and it's the same problem. However, I don't have this problem with 32 bits floats for example.

I also tried with an older version of pymodbus and it works:

decoder = BinaryPayloadDecoder.fromRegisters(registers,endian=Endian.Little)

Note: I already read the following topic: pymodbus: Issue reading String & multiple type of data from Modbus device but I don't have any access to the modbus server.

Upvotes: 1

Views: 2111

Answers (2)

mattroy10
mattroy10

Reputation: 11

I've encountered this same issue with byteorder not working (wordorder seems to be fine). The solution I came up with is to use Struct:

import struct

Then:

count = 4 #Read 4 16bit registers
    
    result = client.read_holding_registers(25000,count)
    
    for i in range(count):
        result.registers[i] = struct.unpack("<H", struct.pack(">H", result.registers[i]))[0]
    
    decoder = BinaryPayloadDecoder.fromRegisters(result.registers)
    print(decoder.decode_string(7)) #Since string is 7 characters long

This uses Struct to unpack and pack as an unsigned short integer. The endianness does not matter since all you're doing is swapping bytes. The result overwrites the registers so you can then use the BinaryPayloadDecoder as you normally would.

I would have preferred to iterate through the responses instead of using range(count), but couldn't find a way to do it and wanted to post this workaround. If I figure it out, I will let you know.

Upvotes: 1

from56
from56

Reputation: 4137

The problem is that Modbus specs does not define in what order the two bytes for char strings are sent or even in what order 16-bit words are sent for 32-bit types.

Then some Modbus devices send bytes or words in an order and others do the opposite.

If you are writing a Modbus client then you should add the option in the configuration to be able to invert the order of both bytes and 16-bit words in 32-bit data types.

https://en.wikipedia.org/wiki/Endianness

Upvotes: 1

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