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Merged
janiversen merged 1 commit into from
Aug 13, 2026
+18 −6
Merged

Control count in bit/register PDUs. #2997

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12 changes: 9 additions & 3 deletions pymodbus/pdu/bit_message.py
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Original file line number Diff line number Diff line change
Expand Up @@ -16,16 +16,18 @@ class ReadCoilsRequest(ModbusPDU):

rtu_frame_size = 8
function_code = 1
MAX_COUNT = 2000

def encode(self) -> bytes:
"""Encode a request pdu."""
self.verifyCount(2000)
self.verifyCount(self.MAX_COUNT)
self.verifyAddress()
return struct.pack(">HH", self.address, self.count)

def decode(self, data: bytes) -> None:
"""Decode a request pdu."""
self.address, self.count = struct.unpack(">HH", data[:4])
self.verifyCount(self.MAX_COUNT)

def get_response_pdu_size(self) -> int:
"""Get response pdu size.
Expand Down Expand Up @@ -139,6 +141,7 @@ class WriteMultipleCoilsRequest(ModbusPDU):
count: int
byte_count: int | None
data_byte_count: int
MAX_COUNT = 2000

def __init__(
self,
Expand All @@ -162,7 +165,7 @@ def encode(self) -> bytes:
"""Encode write coils request."""
self.count = len(self.bits)
self.verifyAddress()
self.verifyCount(2000)
self.verifyCount(self.MAX_COUNT)
byte_count = (self.count + 7) // 8
return struct.pack(
">HHB", self.address, self.count, byte_count
Expand All @@ -171,8 +174,11 @@ def encode(self) -> bytes:
def decode(self, data: bytes) -> None:
"""Decode a write coils request."""
self.address, self.count, self.byte_count = struct.unpack(">HHB", data[0:5])
self.verifyCount(self.MAX_COUNT)
self.data_byte_count = len(data) - 5
self.bits = unpack_bitstring(data[5 : 5 + self.byte_count])[: self.count]
self.bits = unpack_bitstring(data[5 : 5 + cast(int, self.byte_count)])[
: self.count
]

async def datastore_update(
self, context: ModbusServerContext, device_id: int
Expand Down
12 changes: 9 additions & 3 deletions pymodbus/pdu/register_message.py
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Original file line number Diff line number Diff line change
Expand Up @@ -19,16 +19,18 @@ class ReadHoldingRegistersRequest(ModbusPDU):

function_code = 3
rtu_frame_size = 8
MAX_COUNT = 125

def encode(self) -> bytes:
"""Encode the request packet."""
self.verifyAddress()
self.verifyCount(125)
self.verifyCount(self.MAX_COUNT)
return struct.pack(">HH", self.address, self.count)

def decode(self, data: bytes) -> None:
"""Decode a register request packet."""
self.address, self.count = struct.unpack(">HH", data[:4])
self.verifyCount(self.MAX_COUNT)

def get_response_pdu_size(self) -> int:
"""Get response pdu size.
Expand Down Expand Up @@ -100,6 +102,8 @@ class ReadWriteMultipleRegistersRequest(ModbusPDU):

function_code = 23
rtu_byte_count_pos = 10
MAX_READ_COUNT = 125
MAX_WRITE_COUNT = 121

def __init__(
self,
Expand All @@ -126,8 +130,8 @@ def encode(self) -> bytes:
"""Encode the request packet."""
self.verifyAddress(address=self.read_address)
self.verifyAddress(address=self.write_address)
self.verifyCount(125, count=self.read_count)
self.verifyCount(121, count=self.write_count)
self.verifyCount(self.MAX_READ_COUNT, count=self.read_count)
self.verifyCount(self.MAX_READ_COUNT, count=self.write_count)
result = struct.pack(
">HHHHB",
self.read_address,
Expand All @@ -149,6 +153,8 @@ def decode(self, data: bytes) -> None:
self.write_count,
self.write_byte_count,
) = struct.unpack(">HHHHB", data[:9])
self.verifyCount(self.MAX_READ_COUNT, count=self.read_count)
self.verifyCount(self.MAX_READ_COUNT, count=self.write_count)
self._payload_byte_count = len(data) - 9
self.write_registers = [
struct.unpack(">H", data[i : i + 2])[0]
Expand Down