https://github.com/gitalecu/pymodbus  
https://code.google.com/p/pymodbus/

$sudo bash
$sudo apt-get update
$sudo apt-get install python-dev

$git clone git://github.com/bashwork/pymodbus.git
$cd pymodbus
$python setup.py install

$cd pymodbus
$cd examples
$cd common
$nano test.py


paste this in file, write your RPI IP at host='raspberry IP'test.py  PORT=503
=======================================================py script
#!/usr/bin/env python

from pymodbus.client.sync import ModbusTcpClient as ModbusClient

#assign modbus server connection
TRIPLC = ModbusClient(host='10.0.0.4', port=503)

#write_valuee = input ("Value to write: ")
#print 'Output: ', write_valuee

import time
a = 0
print 'wait to start ethernet'
time.sleep(5)
while a < 15:
 a += 1
 print '          cycle: ', a
 time.sleep(5)
 #send read command to server
 read_reg0 = 0
 read_qty = 10
 rr = TRIPLC.read_input_registers(0, 10)
#rr = TRIPLC.read_holding_registers(read_register, read_qty)
 print 'Input0: ', rr.getRegister(0)
 print 'Input1: ', rr.getRegister(1)
 print 'Input2: ', rr.getRegister(2)
 print 'Input3: ', rr.getRegister(3)
 print 'Input4: ', rr.getRegister(4)
 print 'Input5: ', rr.getRegister(5)
 print 'Input6: ', rr.getRegister(6)
 print 'Input7: ', rr.getRegister(7)
 print 'Input8: ', rr.getRegister(8)
 print 'Input9: ', rr.getRegister(9)
 # ============================= >> write to txt file
 g0 = rr.getRegister(0)
 g1 = rr.getRegister(1)
 g2 = rr.getRegister(2)
 g3 = rr.getRegister(3)
 g4 = rr.getRegister(4)
 g5 = rr.getRegister(5)
 g6 = rr.getRegister(6)
 g7 = rr.getRegister(7)
 g8 = rr.getRegister(8)
 g9 = rr.getRegister(9)

 if g0 > 0 and g1 > 0 and g2 > 0 and g3 > 0 and g4 > 0 and g5 > 0 and g6 > 0 and g7 > 0 and g8 > 0 and g9 > 0:
   pos = [g0 , g1 , g2 , g3 , g4 , g5 , g6 , g7 , g8 , g9]
   with open('vars.txt','w') as f:
      f.write("%s %s %s %s %s %s %s %s %s %s" % (pos[0], pos[1], pos[2], pos[3], pos[4], pos[5], pos[6], pos[7], pos[8], pos[9]))
      f.close()

    #f.write("%s %s" % (x,y))
 #else:

 # ---------------------------------->
 #===================================> read text  file
 w = 0
 file_path = 'vars.txt'
 try:
     fp = open(file_path)
 except IOError:
     # If not exist, create the file
     fp = open(file_path, 'w+').close()
     w = 1
 if w == 1:
     with open ('vars.txt', 'w') as fil:
       fil.write("%s %s %s %s %s %s %s %s %s %s" % (0, 0, 0, 0, 0, 0, 0, 0, 0, 1))
       fil.close()
       w = 0

 with open ('vars.txt', 'r') as myfile:
     position=myfile.read().split()

 position = map(int, position)
 #myfile.close()
 print 'line0: ', position[0]
 print 'line1: ', position[1]
 print 'line2: ', position[2]
 print 'line3: ', position[3]
 print 'line4: ', position[4]
 print 'line5: ', position[5]
 print 'line6: ', position[6]
 print 'line7: ', position[7]
 print 'line8: ', position[8]
 print 'line9: ', position[9]
 #------------------------------------->
 #send write command to modbus server

 # write_value= rr.getRegister(0)
 #TRIPLC.write_register(write_register, write_value)
 TRIPLC.write_registers(0, position[0])
 TRIPLC.write_registers(1, position[1])
 TRIPLC.write_registers(2, position[2])
 TRIPLC.write_registers(3, position[3])
 TRIPLC.write_registers(4, position[4])
 TRIPLC.write_registers(5, position[5])
 TRIPLC.write_registers(6, position[6])
 TRIPLC.write_registers(7, position[7])
 TRIPLC.write_registers(8, position[8])
 TRIPLC.write_registers(9, position[9])

 if a>= 10:
   a = 1

 #================>

 #x=1.123
 #y=2.123

 #pos = [143, write_value]

 #with open('vars.txt','w') as f:
 #    f.write("%s %s" % (pos[0], pos[1]))
    #f.write("%s %s" % (x,y))

 #with open ('vars.txt', 'r') as myfile:
 #    position=myfile.read().split()

 #position = map(int, position)

 #print 'position 0: ', position[0]
 #print 'position 1: ', position[1]
 # position = [1.123, 2.123]
---------------------------------------------------py script
pi@raspberrypi:~/Documents/pymodbus/examples/common $

Ctr + x
y
Enter

$chmod -R 777 vars.txt
$sudo python test.py
$cat vars.txt
12428 12429 12430 12431 12432 12433 12434 12435 12436 12437


http://jacekhryniewicz.wix.com/website#!raspberry-pi-with-modbus-tcp/c1d2y