[Python3] A very simple MODBUS TCP server supporting only 3: Read Holding Registers and sending zeros.

modbus-rtu-server.py

import socket
import struct
import threading

class SocketThread(threading.Thread):
    def __init__(self, sock):
        super().__init__()
        self.socket = socket

    def recv_all(self, length: int) -> bytearray:
      data = bytearray()
      while len(data) < length:
        s = self.socket.recv(length - len(data))
        if not s:
            raise ValueError('End of connection')
        data += s
      return data
      
    def run(self):
      try:
        self._run()
      finally:
        self.socket.close()
      
    def _run(self):
      while True:
        t_id = self.recv_all(2)   # Grab the transaction ID
        self.recv_all(2)          # Skip the protocol
        length, = struct.unpack('>H', self.recv_all(2))       # Get packet length
        packet = self.recv_all(length)
        code = packet[1]
        if code != 3:
            raise ValueError(f'Unknown code {code}')
        addr, code, starting_reg, amount_regs = struct.unpack('>BBHH', packet)
        resp = struct.pack('>BBB', addr, code, amount_regs*2) + struct.pack('>' + ('H'*amount_regs), *([0]*amount_regs))
        response = t_id + struct.pack('>HH', 0, len(resp)) + resp
        self.socket.sendall(response)
        print(f'Sent back {repr(response)}')
      

if __name__ == '__main__':
  s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
  s.bind(('0.0.0.0', 502))
  s.listen(20)
  while True:
      client, addr = s.accept()
      print(f'Accepted {addr}')
      st = SocketThread(client)
      st.start()