Why can't read the correct data in python with modbus?

Question

I am trying to get data from a power network meter (https://www.lumel.com.pl/resources/Pliki%20do%20pobrania/ND25/ND25_service_manual_Interface.pdf), but the data I get isn't "understandable" for me. I get some UINT16 values (checked in wireshark), but they aren't good values.

Here is the code:

import time
from pyModbusTCP.client import ModbusClient

slave_adress = '192.168.1.10'
port = 502
unit_id = 2

modbus_clinet = ModbusClient(host=slave_adress, port=port, unit_id=unit_id, auto_open=True)

if __name__ == '__main__':
    while True:
        regs = modbus_clinet.read_holding_registers(reg_addr=0x1772, reg_nb=16)
        if regs:
            print(regs)
        else:
            print("baj van")
        time.sleep(2)

here are the print values:

[16640, 0, 16384, 0, 17359, 32768, 16544, 0, 16448, 0, 17096, 0, 0, 0, 16384, 0]

from the documentation writes something else.

Tried difference python modbus modules

Which values did you expect to get instead?

mkrieger1
– mkrieger1

2024-03-08 20:34:55 +00:00
Commented Mar 8, 2024 at 20:34 — mkrieger1
– mkrieger1, Commented Mar 8, 2024 at 20:34
8, 2, 415, 5, 3, 100, 0, 2

Péter
– Péter

2024-03-08 21:02:16 +00:00
Commented Mar 8, 2024 at 21:02 — Péter
– Péter, Commented Mar 8, 2024 at 21:02

Tim Roberts · Accepted Answer · 2024-03-08 21:02:15Z

3

Those actually ARE the correct values. The information you've missed is that these are single-precision floating point values, being sent to you as pairs of 16-bit integers, which is all ModBus supports. The first two values in hex are 0x4100 and 0, and 0x41000000 is the floating point value 8.0, which is the default for register 46003 as you have requested.

Try this:

>>> x = [16640, 0, 16384, 0, 17359, 32768, 16544, 0, 16448, 0, 17096, 0, 0, 0, 16384, 0]
>>> import struct
>>> print(struct.unpack('>8f',struct.pack('>16H',*x)))
(8.0, 2.0, 415.0, 5.0, 3.0, 100.0, 0.0, 2.0)
>>>

Those match the documentation. Note that both the encoding and decoding need to be big-endian.

answered Mar 8, 2024 at 21:02

Tim Roberts

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3 Comments

Péter Over a year ago

Can i get an example for the ohter 3 method of encoding ([Big endian with byte swap], [Little endian with byte swap], [Low order byte first])? I checked the struct documentation, but i didn't get the answer. Really thank you, for your help

Tim Roberts Over a year ago

I'm not sure what you mean. The DATA you have is big-endian. Interpreting it as little endian will just give you garbage.

Tim Roberts Over a year ago

That link is invalid. However, MODBUS is a big-endian protocol. Any MODBUS device you have should produce the same kind of data. And if you read the struct documentation, you'll see that the ">" triggers big-endian, and "<" triggers little-endian.

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