title: Timeout error with PyVISA -- communicating with Agilent 34970A throug RS232 (USB)
body: <p>This is my first time i try to use <code>Pyvisa</code>, in order to communicate with an <strong><code>Agilent 34970A</code></strong>, using a <code>RS232</code> connection(using an USB port).</p>
  
  <p>This is what's happening to me, inserting the basic first lines:</p>
  
  <pre><code>IN: import visa
  
  IN: rm=visa.ResourceManager()
  
  IN: print rm.list_resources()
  
  (u'ASRL4::INSTR',)
  
  IN: inst=rm.open_resource("ASRL4::INSTR")
  
  IN: print inst.query("*IDN?")
  
  Traceback (most recent call last):
  
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
  
    File "C:\Anaconda2\lib\site-packages\pyvisa-1.8-py2.7.egg\pyvisa\resources\messagebased.py", line 407, in query
      return self.read()
  
    File "C:\Anaconda2\lib\site-packages\pyvisa-1.8-py2.7.egg\pyvisa\resources\messagebased.py", line 332, in read
      message = self.read_raw().decode(enco)
  
    File "C:\Anaconda2\lib\site-packages\pyvisa-1.8-py2.7.egg\pyvisa\resources\messagebased.py", line 306, in read_raw
      chunk, status = self.visalib.read(self.session, size)
  
    File "C:\Anaconda2\lib\site-packages\pyvisa-1.8-py2.7.egg\pyvisa\ctwrapper\functions.py", line 1582, in read
      ret = library.viRead(session, buffer, count, byref(return_count))
  
    File "C:\Anaconda2\lib\site-packages\pyvisa-1.8-py2.7.egg\pyvisa\ctwrapper\highlevel.py", line 188, in _return_handler
  
      raise errors.VisaIOError(ret_value)
  </code></pre>
  
  <blockquote>
    <p><code>pyvisa.errors.VisaIOError: VI_ERROR_TMO (-1073807339): Timeout expired before operation completed.</code></p>
  </blockquote>
  
  <p>This timeout error happens everytime i try to read a value. I imposed also a larger timeout but nothing, i waited some minutes in vain. Any idea for this problem?</p>