title: Python PyVISA USB connection issue
body: <p>I am trying to program a semi-automatic probing station in my lab. Specifically, the P200L from Micromanipulator, which is controlled by a ESP301 Motion Controller.</p>
  
  <p>I have been trying to use PyVISA to communicate with the device but am running into problems simply establishing a basic connection.</p>
  
  <pre><code>In [1]: import visa
  In [2]: rm = visa.ResourceManager()
  In [3]: rm.list_resources()
  Out [3]:(u'ASRL1::INSTR', u'ASRL3::INSTR', u'ASRL4::INSTR')
  </code></pre>
  
  <p>However, the P200L is using a USB to USB connection that I am not seeing on the actual list of resources. However, when I disconnect the USB <code>u'ASRL4::INSTR'</code>disappears so I assuming that's what the connection is being read as?.</p>
  
  <p>I am relatively new to Python, any help/advice would be great.</p>