write_registers() throws "unpack requires a buffer of 4 bytes"

[Frankstar]

• Python: 3.12.4
• OS: Fedora 39
• Pymodbus: 3.6.9 and 3.7.0
• Modbus Hardware: Solarman Stick Logger LSE-3 (Modbus TCP)

sample code

from pymodbus.client import ModbusTcpClient
client = ModbusTcpClient(host='10.0.100.34', port=8899)
try:
    response = client.write_registers(0x1110, 0, slave=1)
except Exception as e:
    print(f"General exception: {e}")
finally:
    client.close()

Im trying to write a single register (with write_registers since the Hardware needs it that way).
The example code is throwing the error unpack requires a buffer of 4 bytes.
The interesting thing is that the register gets set correctly.
So something might be wrong with the response.
reading via read_holding_registers() is working fine.

3.6.9 throwed:

unpack requires a buffer of 4 bytes
Modbus IO exception Modbus Error: [Input/Output] Unable to decode request
Error writing to register 4368: Modbus Error: [Input/Output] Unable to decode request

3.7.0 is throwing:

unpack requires a buffer of 4 bytes

more simplified Code:

from pymodbus.client import ModbusTcpClient
client = ModbusTcpClient(host='10.0.100.34', port=8899)
response = client.write_registers(0x1110, 0, slave=1)
print(response)
client.close()

Traceback:

(most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/username/.local/lib/python3.12/site-packages/pymodbus/client/mixin.py", line 334, in write_registers
    return self.execute(
           ^^^^^^^^^^^^^
  File "/home/username/.local/lib/python3.12/site-packages/pymodbus/client/base.py", line 237, in execute
    return self.transaction.execute(request)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/username/.local/lib/python3.12/site-packages/pymodbus/transaction.py", line 267, in execute
    self.client.framer.processIncomingPacket(
  File "/home/username/.local/lib/python3.12/site-packages/pymodbus/framer/old_framer_base.py", line 154, in processIncomingPacket
    self.frameProcessIncomingPacket(single, callback, slave, tid=tid)
  File "/home/username/.local/lib/python3.12/site-packages/pymodbus/framer/old_framer_socket.py", line 87, in frameProcessIncomingPacket
    if (result := self.decoder.decode(data)) is None:
                  ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/username/.local/lib/python3.12/site-packages/pymodbus/factory.py", line 236, in decode
    return self._helper(message)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/home/username/.local/lib/python3.12/site-packages/pymodbus/factory.py", line 266, in _helper
    response.decode(data[1:])
  File "/home/username/.local/lib/python3.12/site-packages/pymodbus/pdu/register_write_message.py", line 264, in decode
    self.address, self.count = struct.unpack(">HH", data)
                               ^^^^^^^^^^^^^^^^^^^^^^^^^^
struct.error: unpack requires a buffer of 4 bytes
>>> print(response)

output with debug logging:

DEBUG:pymodbus.logging:Connection to Modbus server established. Socket ('10.0.100.132', 49226)
DEBUG:root:Sending value: 0 to register: 4368
DEBUG:pymodbus.logging:[0]
DEBUG:pymodbus.logging:Current transaction state - TRANSACTION_COMPLETE
DEBUG:pymodbus.logging:Running transaction 2
DEBUG:pymodbus.logging:SEND: 0x0 0x2 0x0 0x0 0x0 0x9 0x1 0x10 0x11 0x10 0x0 0x1 0x2 0x0 0x0
DEBUG:pymodbus.logging:New Transaction state "SENDING"
DEBUG:pymodbus.logging:Changing transaction state from "SENDING" to "WAITING FOR REPLY"
DEBUG:pymodbus.logging:Changing transaction state from "WAITING FOR REPLY" to "PROCESSING REPLY"
DEBUG:pymodbus.logging:RECV: 0x0 0x2 0x0 0x0 0x0 0x14 0x1 0x10 0x11 0x10 0x0 0x1 0x5 0x30 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0
DEBUG:pymodbus.logging:Got response!!!
DEBUG:pymodbus.logging:Processing: 0x0 0x2 0x0 0x0 0x0 0x14 0x1 0x10 0x11 0x10 0x0 0x1 0x5 0x30 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0
DEBUG:pymodbus.logging:Factory Response[WriteMultipleRegistersResponse': 16]
ERROR:root:General exception: unpack requires a buffer of 4 bytes
Traceback (most recent call last):
  File "<stdin>", line 8, in write
  File "/home/username/.local/lib/python3.12/site-packages/pymodbus/client/mixin.py", line 334, in write_registers
    return self.execute(
           ^^^^^^^^^^^^^
  File "/home/username/.local/lib/python3.12/site-packages/pymodbus/client/base.py", line 237, in execute
    return self.transaction.execute(request)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/username/.local/lib/python3.12/site-packages/pymodbus/transaction.py", line 267, in execute
    self.client.framer.processIncomingPacket(
  File "/home/username/.local/lib/python3.12/site-packages/pymodbus/framer/old_framer_base.py", line 154, in processIncomingPacket
    self.frameProcessIncomingPacket(single, callback, slave, tid=tid)
  File "/home/username/.local/lib/python3.12/site-packages/pymodbus/framer/old_framer_socket.py", line 87, in frameProcessIncomingPacket
    if (result := self.decoder.decode(data)) is None:
                  ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/username/.local/lib/python3.12/site-packages/pymodbus/factory.py", line 236, in decode
    return self._helper(message)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/home/username/.local/lib/python3.12/site-packages/pymodbus/factory.py", line 266, in _helper
    response.decode(data[1:])
  File "/home/username/.local/lib/python3.12/site-packages/pymodbus/pdu/register_write_message.py", line 264, in decode
    self.address, self.count = struct.unpack(">HH", data)
                               ^^^^^^^^^^^^^^^^^^^^^^^^^^
struct.error: unpack requires a buffer of 4 bytes
>>> 

Any Ideas?
I cant see the "real" error but i'm not sure if its a bug/issue of pymodbus.

[janiversen]

Your call to write_registers is not correct, please read the documentation or at least follow the parameter definitions.

[Frankstar]

sorry jan - but whats wrong? im even more confused now
maybe i can't see the forest for the trees...

documentation states:

write_registers(address: int, values: list[int] | int, slave: int = 1, skip_encode: bool = False)→ T[]
Parameters:
address – Start address to write to
values – List of values to write, or a single value to write
slave – (optional) Modbus slave ID
skip_encode – (optional)

I could pass the values as a list (values: list[int] | int)
tried that with client.write_registers(0x1110, [0], slave=1) -> same error.

Than i tried with skip_encode=True and passed my value formatted with encoded_value = struct.pack('>H', 0) -> same error.
What else i'm missing?

[Frankstar]

ok than i'm misunderstanding the documentation.
I read it as "you can pass a list or just an int value" or even a list with 1 value.

i know about write_register but:
a) the device is not allowing function code 0x06
b) it would be cleaner for me personally just using one function for all.
c) it "works" with write_registers but somehow the response code is broken for pymodbus

anyway -> sry for wasting your time than and thanks for helping!
maybe i will find a solution for this problem.

[janiversen]

I am just controlling your different statements:

• The example code of 3.7.0 works perfectly (client_async_calls), and sends the request correctly, and gets a correct response.
• I added a couple of extra tests e.g. [10] as value, and it works nicely.
• The parameter description was wrong, that's corrected.

Please make a proper debug log (pymodbus_apply_debug) with 3.7.0, I think your problem might be that your device responds wrongly.

[janiversen]

just checked your traceback, it fails in "self.decoder.decode", that is when the response is received, so now it is very interesting to see what you actually receive.

[janiversen]

Just to avoid misunderstandings, I would like to see the test with 3.7.0, since I cannot make bug finding in 3.6.9

[Frankstar]

the traceback was with 3.7.0
3.6.9 just throwed 2 more errors earlier that day - thats why i was mentioning it.
Thought it might help understanding my problem.

[janiversen]

The response:

DEBUG:pymodbus.logging:RECV: 0x0 0x2 0x0 0x0 0x0 0x14 0x1 0x10 0x11 0x10 0x0 0x1 0x5 0x30 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0
DEBUG:pymodbus.logging:Got response!!!

Is by far too large ! The standard defines 7 bytes header + 5 bytes response.

[janiversen]

There are no byte count in the response (see your own attachment).

[Frankstar]

i also tried communicate with the device directly via rs485 - but got some weird timeouts.
Sofar Support suggest than the LSE-3 usb to ethernet adapter and stated "This one is talking ModBus TCP natively over the port 8899."

anyway - you helped alot.
Thanks!

[Frankstar]

There are no byte count in the response (see your own attachment).

yes - and still i got some data ....
but i couldn't have troubleshooted this far without you. thanks for your time!
Now i know its no pymodbus problem - i dont know how i can workaround it - but thats another story.

[Frankstar]

Is by far too large ! The standard defines 7 bytes header + 5 bytes response.

just to clarify it -> the wrong (too much) response data is than leading to

struct.error: unpack requires a buffer of 4 bytes)

?
(so i can mark this question/discussion as answered )