convert_from_registers byteorder

[lorithai]

Hi

I am facing some issues with the new method for converting registers to float. The documention for the device i am communicating with says the word order and byte order is in the little endian format.

This example has the two registers [0xDA77, 0xFB41] which is supposed to be read as 31.43

I have been using the following method to convert registers to floats (being deprecated)

from pymodbus.payload import BinaryPayloadDecoder, BinaryPayloadBuilder
from pymodbus.constants import Endian
temperature_registers = [0xDA77, 0xFB41]  # DA 77 FB 41 (temperature)
values = {}
values["raw_value"] = [hex(val) for val in temperature_registers]
values["decoded_value"] = BinaryPayloadDecoder.fromRegisters(temperature_registers,byteorder=Endian.LITTLE, wordorder=Endian.LITTLE).decode_32bit_float()      

This has been working fine and gives the expected output of
{'raw_value': ['0xda77', '0xfb41'], 'decoded_value': 31.433521270751953}

I get a message that this method will be deprecated after version 3.9.0 so i am trying to convert to the new method of using convert_from_registers()

My attempt is as follows

from pymodbus.client.mixin import ModbusClientMixin
DATATYPE = ModbusClientMixin.DATATYPE
temperature_registers = [0xDA77, 0xFB41]  # DA 77 FB 41 (temperature)
values = {}
values["raw_value"] = [hex(val) for val in temperature_registers]
values["decoded_value"] = ModbusClientMixin.convert_from_registers(temperature_registers,data_type=DATATYPE.FLOAT32,word_order="little")

But this gives an unexpexted result
{'raw_value': ['0xda77', '0xfb41'], 'decoded_value': -1.006544287142157e+36}

Shuffling the bytes around to a reverse order (little endian byte order) and selecting word_order="big" in the following way

from pymodbus.client.mixin import ModbusClientMixin
DATATYPE = ModbusClientMixin.DATATYPE
#temperature_registers = [0xDA77, 0xFB41]  # DA 77 FB 41 (temperature)
temperature_registers = [0x41FB,0x77DA] # reverse byte order
values = {}
values["raw_value"] = [hex(val) for val in temperature_registers]
values["decoded_value"] = ModbusClientMixin.convert_from_registers(temperature_registers,data_type=DATATYPE.FLOAT32,word_order="big")

This gives the expected result
{'raw_value': ['0x41fb', '0x77da'], 'decoded_value': 31.433521270751953}

From what i can tell from the documentation of the modbus device the order needs to be reversed from A B C D (DA 77 FB 41) to D C B A (41 FB 77 DA). To me this would require a little endian word_order and little endian byte order.
Somewhat confused by the stark difference in inputs and outputs in these two methods and that the convert_from_registers doesn't seem to have byteorder option.

I guess i can create a byte order swap function for myself though.

Regards

[janiversen]

The modbus standard defines byte order to be big, therefore the pymodbus library only supports changing the word order.

Python have standard methods to switch the byte order, so use those.