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Add little-endian support for register data #1

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6043383
feat: Add little-endian support for register data
google-labs-jules[bot] Aug 14, 2025
1f70ebe
feat: Add little-endian support for register data
google-labs-jules[bot] Aug 21, 2025
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27 changes: 19 additions & 8 deletions umodbus/common.py
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Original file line number Diff line number Diff line change
Expand Up @@ -110,8 +110,13 @@ def __init__(self, function_code: int, exception_code: int) -> None:

class CommonModbusFunctions(object):
"""Common Modbus functions"""
def __init__(self):
pass
def __init__(self, byte_order: str = 'big'):
if byte_order == 'big':
self._byte_order = '>'
elif byte_order == 'little':
self._byte_order = '<'
else:
raise ValueError("byte_order must be 'big' or 'little'")

def read_coils(self,
slave_addr: int,
Expand Down Expand Up @@ -200,7 +205,8 @@ def read_holding_registers(self,
modbus_pdu=modbus_pdu,
count=True)

register_value = functions.to_short(byte_array=response, signed=signed)
register_value = functions.to_short(byte_array=response, signed=signed,
byte_order=self._byte_order)

return register_value

Expand Down Expand Up @@ -232,7 +238,8 @@ def read_input_registers(self,
modbus_pdu=modbus_pdu,
count=True)

register_value = functions.to_short(byte_array=response, signed=signed)
register_value = functions.to_short(byte_array=response, signed=signed,
byte_order=self._byte_order)

return register_value

Expand Down Expand Up @@ -295,7 +302,8 @@ def write_single_register(self,
modbus_pdu = functions.write_single_register(
register_address=register_address,
register_value=register_value,
signed=signed)
signed=signed,
byte_order=self._byte_order)

response = self._send_receive(slave_addr=slave_addr,
modbus_pdu=modbus_pdu,
Expand All @@ -309,7 +317,8 @@ def write_single_register(self,
function_code=Const.WRITE_SINGLE_REGISTER,
address=register_address,
value=register_value,
signed=signed)
signed=signed,
byte_order=self._byte_order)

return operation_status

Expand Down Expand Up @@ -372,7 +381,8 @@ def write_multiple_registers(self,
modbus_pdu = functions.write_multiple_registers(
starting_address=starting_address,
register_values=register_values,
signed=signed)
signed=signed,
byte_order=self._byte_order)

response = self._send_receive(slave_addr=slave_addr,
modbus_pdu=modbus_pdu,
Expand All @@ -386,7 +396,8 @@ def write_multiple_registers(self,
function_code=Const.WRITE_MULTIPLE_REGISTERS,
address=starting_address,
quantity=len(register_values),
signed=signed
signed=signed,
byte_order=self._byte_order
)

return operation_status
59 changes: 37 additions & 22 deletions umodbus/functions.py
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Original file line number Diff line number Diff line change
Expand Up @@ -129,7 +129,8 @@ def write_single_coil(output_address: int,

def write_single_register(register_address: int,
register_value: int,
signed: bool = True) -> bytes:
signed: bool = True,
byte_order: str = '>') -> bytes:
"""
Create Modbus message to writes a single register

Expand All @@ -139,16 +140,18 @@ def write_single_register(register_address: int,
:type register_value: int
:param signed: Flag whether data is signed or not
:type signed: bool
:param byte_order: The byte order
:type byte_order: str

:returns: Packed Modbus message
:rtype: bytes
"""
fmt = 'h' if signed else 'H'

return struct.pack('>BH' + fmt,
pdu = struct.pack('>BH',
Const.WRITE_SINGLE_REGISTER,
register_address,
register_value)
register_address)
pdu += struct.pack(byte_order + fmt, register_value)
return pdu


def write_multiple_coils(starting_address: int,
Expand Down Expand Up @@ -194,7 +197,8 @@ def write_multiple_coils(starting_address: int,

def write_multiple_registers(starting_address: int,
register_values: List[int],
signed: bool = True) -> bytes:
signed: bool = True,
byte_order: str = '>') -> bytes:
"""
Create Modbus message to update multiple coils

Expand All @@ -204,6 +208,8 @@ def write_multiple_registers(starting_address: int,
:type register_values: List[int, bool]
:param signed: Flag whether data is signed or not
:type signed: bool
:param byte_order: The byte order
:type byte_order: str

:returns: Packed Modbus message
:rtype: bytes
Expand All @@ -215,20 +221,23 @@ def write_multiple_registers(starting_address: int,
byte_count = quantity * 2
fmt = ('h' if signed else 'H') * quantity

return struct.pack('>BHHB' + fmt,
pdu = struct.pack('>BHHB',
Const.WRITE_MULTIPLE_REGISTERS,
starting_address,
quantity,
byte_count,
*register_values)
byte_count)
pdu += struct.pack(byte_order + fmt, *register_values)

return pdu


def validate_resp_data(data: bytes,
function_code: int,
address: int,
value: int = None,
quantity: int = None,
signed: bool = True) -> bool:
signed: bool = True,
byte_order: str = '>') -> bool:
"""
Validate the response data.

Expand All @@ -244,14 +253,15 @@ def validate_resp_data(data: bytes,
:type quantity: int
:param signed: Indicates if signed
:type signed: bool
:param byte_order: The byte order
:type byte_order: str

:returns: True if valid, False otherwise
:rtype: bool
"""
fmt = '>H' + ('h' if signed else 'H')

if function_code in [Const.WRITE_SINGLE_COIL, Const.WRITE_SINGLE_REGISTER]:
resp_addr, resp_value = struct.unpack(fmt, data)
fmt = 'h' if signed else 'H'
resp_addr, resp_value = struct.unpack(byte_order + 'H' + fmt, data)

# if bool(True) or int(1) is used as "output_value" of
# "write_single_coil" it will be internally converted to int(0xFF00),
Expand All @@ -267,7 +277,7 @@ def validate_resp_data(data: bytes,
return True
elif function_code in [Const.WRITE_MULTIPLE_COILS,
Const.WRITE_MULTIPLE_REGISTERS]:
resp_addr, resp_qty = struct.unpack(fmt, data)
resp_addr, resp_qty = struct.unpack(byte_order + 'HH', data)

if (address == resp_addr) and (quantity == resp_qty):
return True
Expand All @@ -280,7 +290,8 @@ def response(function_code: int,
request_register_qty: int,
request_data: list,
value_list: Optional[list] = None,
signed: bool = True) -> bytes:
signed: bool = True,
byte_order: str = '>') -> bytes:
"""
Construct a Modbus response Protocol Data Unit

Expand All @@ -296,6 +307,8 @@ def response(function_code: int,
:type value_list: Optional[list]
:param signed: Indicates if signed
:type signed: bool
:param byte_order: The byte order
:type byte_order: str

:returns: Protocol data unit
:rtype: bytes
Expand Down Expand Up @@ -333,10 +346,10 @@ def response(function_code: int,
for s in signed:
fmt += 'h' if s else 'H'

return struct.pack('>BB' + fmt,
function_code,
quantity * 2,
*value_list)
pdu = struct.pack('>BB', function_code, quantity * 2)
pdu += struct.pack(byte_order + fmt, *value_list)

return pdu

elif function_code in [Const.WRITE_SINGLE_COIL,
Const.WRITE_SINGLE_REGISTER]:
Expand Down Expand Up @@ -398,22 +411,24 @@ def bytes_to_bool(byte_list: bytes, bit_qty: Optional[int] = 1) -> List[bool]:
return bool_list


def to_short(byte_array: bytes, signed: bool = True) -> bytes:
def to_short(byte_array: bytes, signed: bool = True, byte_order: str = '>') -> bytes:
"""
Convert bytes to tuple of integer values

:param byte_array: The byte array
:type byte_array: bytes
:param signed: Indicates if signed
:type signed: bool
:param byte_order: The byte order
:type byte_order: str

:returns: Integer representation
:rtype: bytes
"""
response_quantity = int(len(byte_array) / 2)
fmt = '>' + (('h' if signed else 'H') * response_quantity)
fmt = (('h' if signed else 'H') * response_quantity)

return struct.unpack(fmt, byte_array)
return struct.unpack(byte_order + fmt, byte_array)


def float_to_bin(num: float) -> bin:
Expand Down
11 changes: 9 additions & 2 deletions umodbus/modbus.py
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Original file line number Diff line number Diff line change
Expand Up @@ -33,9 +33,15 @@ class Modbus(object):
:param addr_list: List of addresses
:type addr_list: List[int]
"""
def __init__(self, itf, addr_list: List[int]) -> None:
def __init__(self, itf, addr_list: List[int], byte_order: str = 'big') -> None:
self._itf = itf
self._addr_list = addr_list
if byte_order == 'big':
self._byte_order = '>'
elif byte_order == 'little':
self._byte_order = '<'
else:
raise ValueError("byte_order must be 'big' or 'little'")

# modbus register types with their default value
self._available_register_types = ['COILS', 'HREGS', 'IREGS', 'ISTS']
Expand Down Expand Up @@ -233,7 +239,8 @@ def _process_write_access(self, request: Request, reg_type: str) -> None:
elif reg_type == 'HREGS':
valid_register = True
val = list(functions.to_short(byte_array=request.data,
signed=False))
signed=False,
byte_order=self._byte_order))

if request.function in [Const.WRITE_SINGLE_REGISTER,
Const.WRITE_MULTIPLE_REGISTERS]:
Expand Down
26 changes: 16 additions & 10 deletions umodbus/serial.py
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Original file line number Diff line number Diff line change
Expand Up @@ -54,17 +54,20 @@ def __init__(self,
parity: Optional[int] = None,
pins: List[Union[int, Pin], Union[int, Pin]] = None,
ctrl_pin: int = None,
uart_id: int = 1):
uart_id: int = 1,
byte_order: str = 'big'):
super().__init__(
# set itf to Serial object, addr_list to [addr]
Serial(uart_id=uart_id,
baudrate=baudrate,
data_bits=data_bits,
stop_bits=stop_bits,
parity=parity,
pins=pins,
ctrl_pin=ctrl_pin),
[addr]
itf=Serial(uart_id=uart_id,
baudrate=baudrate,
byte_order=byte_order,
data_bits=data_bits,
stop_bits=stop_bits,
parity=parity,
pins=pins,
ctrl_pin=ctrl_pin),
addr_list=[addr],
byte_order=byte_order
)


Expand All @@ -76,7 +79,9 @@ def __init__(self,
stop_bits: int = 1,
parity=None,
pins: List[Union[int, Pin], Union[int, Pin]] = None,
ctrl_pin: int = None):
ctrl_pin: int = None,
byte_order: str = 'big'):
super().__init__(byte_order=byte_order)
"""
Setup Serial/RTU Modbus

Expand All @@ -95,6 +100,7 @@ def __init__(self,
:param ctrl_pin: The control pin
:type ctrl_pin: int
"""
super().__init__(byte_order=byte_order)
# UART flush function is introduced in Micropython v1.20.0
self._has_uart_flush = callable(getattr(UART, "flush", None))
self._uart = UART(uart_id,
Expand Down
9 changes: 6 additions & 3 deletions umodbus/tcp.py
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Original file line number Diff line number Diff line change
Expand Up @@ -27,11 +27,12 @@

class ModbusTCP(Modbus):
"""Modbus TCP client class"""
def __init__(self):
def __init__(self, byte_order: str = 'big'):
super().__init__(
# set itf to TCPServer object, addr_list to None
TCPServer(),
None
None,
byte_order=byte_order
)

def bind(self,
Expand Down Expand Up @@ -77,7 +78,9 @@ class TCP(CommonModbusFunctions):
def __init__(self,
slave_ip: str,
slave_port: int = 502,
timeout: float = 5.0):
timeout: float = 5.0,
byte_order: str = 'big'):
super().__init__(byte_order=byte_order)
self._sock = socket.socket()
self.trans_id_ctr = 0

Expand Down