No response received, expected at least 8 bytes (0 received)

[JimmyTang999]

This is very similar to a post from stack overflow that was never answered like 2 years ago

its working with radzio

just not with python- pymodbus==3.7.4

from pymodbus.client import ModbusTcpClient
client = ModbusTcpClient('192.168.1.195', port=26)  # Replace with your server's IP and port
client.connect()
result = client.read_holding_registers(address=4000, count=10, slave=1) 
if result.isError():
    print("Error reading registers:", result)
else:
    print("Register values:", result.registers)
client.close()

[janiversen]

Please add a debug log, as the issue template showed you how to do, so we can see what happens.

We do not follow stack overflow, and apart from that the library was very different 2 years ago.

The typical problem in this case, is that your device do not respond. A modbus socket packet is at least 8 bytes, but we received 0 within the timeout period. Are you sure slave=1 is correct for your device ?

[JimmyTang999]

and this is whats happening when i run pymodbus

0020   01 c3 d9 2e 00 1a 66 a1 56 6d 00 00 34 49 50 10   ......f.Vm..4IP.
0030   fa f0 85 0f 00 00                                 ......

0020   01 c3 d9 2e 00 1a 66 a1 56 6d 00 00 34 49 50 18   ......f.Vm..4IP.
0030   fa f0 85 1b 00 00 00 01 00 00 00 06 01 03 0f a0   ................
0040   00 0a    

pretty different structure :(

[janiversen]

Of course, you need to use rtu over tcp, the structure you see is for socket.

[JimmyTang999]

changed to

client = ModbusTcpClient('192.168.1.195', port=26,framer=FramerType.RTU)
but now getting a slightly different error:
Error reading registers: Modbus Error: [Input/Output] Modbus Error: [Invalid Message] No response received, expected at least 4 bytes (0 received)

[JimmyTang999]

so that got me closer

0020   01 c3 eb a7 00 1a 7d ce f1 bd 00 00 40 2b 50 18   ......}.....@+P.
0030   fa f0 85 17 00 00 01 03 0f a0 00 0a c6 fb         ..............

i noticed it closely matched radzio.
01 03 0f a0 00 0a c6 fb
the address looked too large so, i modified the syntax
result = client.read_holding_registers(address=0, count=10, slave=1)

i guess the library is smart enough to know when you say holding_register, it automatically starts you at 4000
so if you want to start with the first holding register, specify 0

heres my working code if it helps someone else in the future

from pymodbus.client import ModbusTcpClient
from pymodbus import (ExceptionResponse,FramerType,ModbusException,pymodbus_apply_logging_config)
client = ModbusTcpClient('192.168.1.195', port=26,framer=FramerType.RTU)
if client.connect():
    print("connected")
else:
    print("not connectd")
result = client.read_holding_registers(address=0, count=10, slave=1)
if result.isError():
    print("Error reading registers:", result)
else:
    print("Register values:", result.registers)
client.close()

[janiversen]

No pymodbus do not assume holding registers start at a specific address, because different devices uses different addresses.

Depending on how your vendor specifies the addresses is might be necessary to add +1.

address 4000 is correct as 0x0F 0xA0, but you might need to ask for 4001.

[janiversen]

The picture shows RTU over tcp/ip but your code uses socket, so maybe that is the problem,

Please refer to our documentation.