Python Modbus RTU over TCP

Question

I am trying to read and write data over Modbus TCP with python. When I am using ModbusPoll with the following setup everything works.

ModbusPoll

I try to read the data now with python and I am using the pymodbus library for this. My code looks like this:

from pymodbus.client.sync import ModbusTcpClient
from pymodbus.transaction import ModbusRtuFramer as ModbusFramer
client = ModbusTcpClient("192.168.0.7", port=502, framer=ModbusFramer)
success = client.connect()
read = client.read_holding_registers(address=4000)
read.registers

But I am always getting the following error:

ModbusIOException(InvalidMessageReceivedException('No response received, expected at least 2 bytes (0 received)'), 1)

Answer 1

Read holding register needs a unit to correctly read the message

read = client.read_holding_registers(address=4000, unit=1)

Answer 2

This is what helped me.

Check slave_ID of your device (unit=slave_ID). It is not nessecary 1 or 0. In my case - 240, for example.

result = client.read_holding_registers(address=0x0010, count = 2, **unit=240**)

Documentation says:

 def write_register(self, address, value, **kwargs):
 '''
 :param address: The starting address to write to
 :param value: The value to write to the specified address
 **:param unit: The slave unit this request is targeting**
 :returns: A deferred response handle
 '''

And:

 def read_holding_registers(self, address, count=1, **kwargs):
 '''
 :param address: The starting address to read from
 :param count: The number of registers to read
 **:param unit: The slave unit this request is targeting**
 :returns: A deferred response handle
 '''

Look file "common.py" or press Ctrl+B while cursor is on function.