title: PyModbus RTU response received even if there is no connection
body: <p>I'm using PyModbus on Linkit MK7688 that is connected to Arduino UNO using MAX485. Following is my code for writing to coil.</p>
  
  <pre><code>from pymodbus.client.sync import ModbusSerialClient as ModbusClient
  import logging
  
  FORMAT = ('%(asctime)-15s %(threadName)-15s '
            '%(levelname)-8s %(module)-15s:%(lineno)-8s %(message)s')
  logging.basicConfig(format=FORMAT)
  log = logging.getLogger()
  log.setLevel(logging.DEBUG)
  
  client = ModbusClient(method='rtu', port='/dev/ttyS1', timeout=2000, baudrate=9600)
  
  client.debug=True
  print(client)
  response = client.write_coil(1, True, unit=1)
  </code></pre>
  
  <p>I am trying this out with nothing connected on the Modbus. In this setup I have turned off Arduino and expect no response for the write request. However, in the debug log I see that transaction is complete. This is wrong as I should not receive any response for the <code>.write_coil()</code>. If there is loopback that is happening somewhere, how do I detect it? </p>
  
  <p>Appreciate your response.</p>
  
  <pre><code>2018-06-26 03:56:53,331 MainThread      DEBUG    sync           :405      Serial client intialising
  2018-06-26 03:56:53,337 MainThread      DEBUG    sync           :42       Initializing...
  ModbusSerialClient(rtu baud[9600])
  2018-06-26 03:56:53,344 MainThread      DEBUG    transaction    :107      Current transaction state - IDLE
  2018-06-26 03:56:53,347 MainThread      DEBUG    transaction    :111      Running transaction 1
  2018-06-26 03:56:53,350 MainThread      DEBUG    transaction    :201      SEND: 0x1 0x5 0x0 0x1 0xff 0x0 0xdd 0xfa
  2018-06-26 03:56:53,352 MainThread      DEBUG    sync           :77       New Transaction state 'SENDING'
  2018-06-26 03:56:53,358 MainThread      DEBUG    transaction    :204      Changing transaction state from 'SENDING' to 'WAITING FOR REPLY'
  2018-06-26 03:56:53,371 MainThread      DEBUG    sync           :554      Finished reading socket....
  2018-06-26 03:56:53,374 MainThread      DEBUG    sync           :554      Finished reading socket....
  2018-06-26 03:56:53,376 MainThread      DEBUG    transaction    :279      Changing transaction state from 'WAITING FOR REPLY' to 'PROCESSING REPLY'
  2018-06-26 03:56:53,379 MainThread      DEBUG    transaction    :209      RECV: 0x1 0x5 0x0 0x1 0xff 0x0 0xdd 0xfa
  2018-06-26 03:56:53,382 MainThread      DEBUG    rtu_framer     :175      Getting Frame - 0x5 0x0 0x1 0xff 0x0
  2018-06-26 03:56:53,385 MainThread      DEBUG    factory        :246      Factory Response[WriteSingleCoilResponse: 5]
  2018-06-26 03:56:53,388 MainThread      DEBUG    rtu_framer     :110      Frame advanced, resetting header!!
  2018-06-26 03:56:53,390 MainThread      DEBUG    transaction    :410      Adding transaction 1
  2018-06-26 03:56:53,393 MainThread      DEBUG    transaction    :420      Getting transaction 1
  2018-06-26 03:56:53,397 MainThread      DEBUG    transaction    :175      Changing transaction state from 'PROCESSING REPLY' to 'TRANSACTION_COMPLETE'
  </code></pre>