server.py

#!/usr/bin/python3
import socket
import _thread
from array import array
from math import ceil
from struct import unpack
import numpy as np

coils = [0] * 65536
discreteInputs = [0] * 65536
holdingRegisters = [0] * 65536
inputRegisters = [0] * 65536


def TCP(conn, addr):
    """
## This Modbus server accepts connections from multiple clients.
## Supported Function Codes are:
    * 1 = Read Coils or Digital Outputs
    * 2 = Read Digital Inputs
    * 3 = Read Holding Registers or Analog Outputs
    * 4 = Read Input Registers or Analog Inputs
    * 5 = Write Single Coil
    * 6 = Write Single Register or Analog Output
    * 15 = Write Coils or Digital Outputs
    * 16 = Write Holding Registers or Analog Outputs
"""
    buffer = array('B', [0] * 300)
    cnt = 0
    while True:
        try:
            if cnt < 60000: cnt = cnt + 1
            else: cnt = 1

            conn.recv_into(buffer)

            TID0 = buffer[0]   #Transaction ID  to sync
            TID1 = buffer[1]   #Transaction ID
            ID = buffer[6]     #Unit ID
            FC = buffer[7]
            mADR = buffer[8]
            lADR = buffer[9]
            ADR = mADR * 256 + lADR
            LEN = buffer[10] * 256 + buffer[11]
            BYT = LEN * 2
            print("Received = ", buffer[0:13 + buffer[12]])
            if (FC in [1, 2, 3, 4]):  # Read Inputs or Registers
                DAT = array('B')
                if FC < 3:
                    BYT = ceil(LEN / 8)  # Round off the no. of bytes
                    v = 85  # send 85,86.. for bytes.
                    for i in range(BYT):
                        DAT.append(v)
                        v = (lambda x: x + 1 if (x < 255) else 85)(v)
                else:
                    DAT = array('B', np.arange(cnt, LEN+cnt, dtype=np.dtype('>i2')).tobytes())
                print("TID = %d, ID= %d, Fun.Code= %d, Address= %d, Length= %d" \
                      %((TID0 * 256 + TID1), ID, FC, ADR, LEN))
                conn.send(
                    array('B', [TID0, TID1, 0, 0, 0, BYT + 3, ID, FC, BYT]) + DAT)
            elif (FC in [5, 6, 15, 16]):  # Write Registers
                BYT = buffer[12]
                conn.send(
                    array('B', [TID0, TID1, 0, 0, 0, 6, ID, FC, mADR, lADR, buffer[10], buffer[11]]))
                buf = buffer[13:(13 + BYT)]
                print("TID = %d, ID= %d, Fun.Code= %d, Address= %d, Length= %d," \
                    "Bytes= %d" % ((TID0 * 256 + TID1), ID, FC, ADR, LEN, BYT))

                if FC == 5 or FC == 15:
                    message = 'bytes: ' + str(unpack('B' * BYT, buf))
                elif FC == 6 or FC == 16:
                    message = str(unpack('>' + 'H' * int(BYT / 2), buf))
                print("Received Write Values =", message)
            else:
                print("Funtion Code %d Not Supported" % FC)
                exit()
        except Exception as e:
            print(e, "\nConnection with Client terminated")
            conn.close()
            exit()

if __name__ == '__main__':
    try:
        s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        s.bind(('', 502))
        s.listen(1)
        print("Start server")
        while True:
            conn, addr = s.accept()
            print("Connected by", addr[0])
            _thread.start_new_thread(TCP, (conn, addr))
    except Exception as err:
        print(err)
        s.close()