TCP server example on Raspberry Pi

[haiop...@gmail.com]

Hello,

At the moment I want to apply the TCP server example on my needs. I want to read registers from a SMA photovoltaic inverter via MODBUS. The occuring protocol can be found here. The most important parameters are port = 502 and Unit ID = 3.

One registers of interest is 30535 which is an U32 value which means to read two registers.

I tried the following implementation in Python. I tried both to use holding registers and analog inputs.

#!/usr/bin/env python
# -*- coding: utf_8 -*-

import sys

import modbus_tk
import modbus_tk.defines as cst
import modbus_tk.modbus as modbus
import modbus_tk.modbus_tcp as modbus_tcp

if __name__ == '__main__':
    try:
        #Create the server
        server = modbus_tcp.TcpServer(address='192.168.178.26', port=502)

        server.start()

        slave = server.add_slave(3)
        slave.add_block('0', cst.HOLDING_REGISTERS, 30535, 2)
        slave.add_block('1', cst.ANALOG_INPUTS, 30535, 2)
 
        values = slave.get_values('0', 30535, 2)
        sys.stdout.write(str(values))

        values = slave.get_values('1', 30535, 2)
        sys.stdout.write(str(values))

    finally:
        server.stop()

Unfortunately I only can read the values (0, 0) for both cases.

On the other hand by using QModMaster I can read the correct values for the register. The reading is succesfully for both Read Input Registers and Read Holding Registers.See the following screenshots for more details:

The python script was running on a raspberry pi with the operating system raspbian. The QModMaster was executed on windows 7.

Is there any mistake in my Python script?


Thank you for your help!

[Luc JEAN]

Hello,

I am not sure to understand correctly but it seems to me that your python script should not be a Modbus slave but a Modbus master.

If you want that your script collect data from a device: you must write a master. If you want to make data available : you can write a slave and use the TcpServer.

You can look at this example for writing a modbus master: https://code.google.com/p/modbus-tk/source/browse/examples/tcpmaster_example.py

I hope it helps

Best

luc

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[haiop...@gmail.com]

Hello Luc,

yes I need a Master. Thank you very much for your hint. Now my application works!

Best regards,
Thomas

[Luc JEAN]

Hello Thomas,

Thanks for your message. I am happy to know that everything goes well now.

Best

luc

[Roel Nagels]

Thomas, I'm trying to connect to my SMA TriPower, but can't figure out how to do that.

Any chance in getting your code so I can modify it for my needs?

Kind regards, Roel

Op vrijdag 27 februari 2015 13:26:46 UTC+1 schreef luc: