code: def sn(self):
          
          string = []
  
          # Send the command byte and sleep for 9 ms
          self.cnxn.xfer([0x10])
          sleep(9e-3)
  
          # Read the info string by sending 60 empty bytes
          for i in range(60):
              resp = self.cnxn.xfer([0x00])[0]
              string.append(chr(resp))
  
          sleep(0.1)
  
          return ''.join(string)
docs: Read the Serial Number string. This method is only available on OPC-N2
          firmware versions 18+.
  
          :rtype: string
  
          :Example:
  
          >>> alpha.sn()
          'OPC-N2 123456789'