title: pymodbus USB connection issue
body: <p>I have a Click PLC that I am trying to read coil data from.  I am using pymodbus, but I am getting a connection error.  The only issue I can come up with is that I don't know if the port entry is correct.  I am using an RS-232 converter to USB.  The USB is on com4.  This there a reason I can't get a connection.  Simple code is below.</p>
  
  <p>Thanks,
  Marc</p>
  
  <pre><code>from pymodbus.client.sync import ModbusSerialClient
  
  click = ModbusSerialClient(method='rtu',port='com4',baudrate=9600,parity='O')
  
  outputLight = click.read_coils(8193,1,unit=1)
  
  print outputLight
  </code></pre>