Instantiate hpib port on first use

camrasdevices.py

 import hpib
 from hpib import HpibDevice
-
-_port = hpib.PrologixGpibUsb()
+import threading
 
 class CamrasHpibDevice(HpibDevice):
     """Wrapper around HPIB commands"""
+
+    _CamrasHpibDevice__port = None
+    _CamrasHpibDevice__port_lock = threading.Lock()
+
     def __init__(self, address):
+        if not CamrasHpibDevice._CamrasHpibDevice__port:
+            with CamrasHpibDevice._CamrasHpibDevice__port_lock:
+                _port = hpib.PrologixGpibUsb()
         super(CamrasHpibDevice, self).__init__(address, _port)
         self.__frequency = 0