diff --git a/README.md b/README.md
index ed36416..0e65162 100644
--- a/README.md
+++ b/README.md
@@ -35,3 +35,6 @@ dc=True
 peak_max=False
 peak_min=False
 ```
+
+# Source
+https://github.com/tobiasfalk/UIN-T-USB-Reader
diff --git a/readDMM.py b/readDMM.py
index 7609921..ddeff2e 100644
--- a/readDMM.py
+++ b/readDMM.py
@@ -1,15 +1,18 @@
 
 import logging
+from enum import Enum
+
 from ut61eplus import UT61EPLUS
+from ut61eplus import SERIAL_NUMBERS
 
 log = logging.getLogger(__name__)
 
 logging.basicConfig(level=logging.DEBUG)
 
-dmm = UT61EPLUS()
+dmm = UT61EPLUS(serial = str(SERIAL_NUMBERS.A))
 
 log.info('name=%s', dmm.getName())
 dmm.sendCommand('lamp')
 m = dmm.takeMeasurement()
 log.info('measurent=%s', m)
-
+dmm.listDevices()
diff --git a/ut61eplus/__init__.py b/ut61eplus/__init__.py
index 3c357f6..0de4bc9 100644
--- a/ut61eplus/__init__.py
+++ b/ut61eplus/__init__.py
@@ -1 +1,2 @@
-from .ut61eplus import UT61EPLUS
\ No newline at end of file
+from .ut61eplus import UT61EPLUS
+from .ut61eplus import SERIAL_NUMBERS
\ No newline at end of file
diff --git a/ut61eplus/ut61eplus.py b/ut61eplus/ut61eplus.py
index 0f4ccf7..dc190ce 100644
--- a/ut61eplus/ut61eplus.py
+++ b/ut61eplus/ut61eplus.py
@@ -35,7 +35,10 @@
 8d . => sum over all - LSB
 
 """
-
+class SERIAL_NUMBERS:
+    A = '006C2895'
+    B = '006C3EE8'
+    C = '006C2FB6'
 
 class Measurement:
 
@@ -308,9 +311,14 @@ class UT61EPLUS:
         'not_peak': 78,
     }
 
-    def __init__(self):
+    def __init__(self, serial = None):
         """open device"""
-        self.dev = hid.Device(vid=self.CP2110_VID, pid=self.CP2110_PID)
+        if not serial == None:
+            print(serial)
+            print(type(serial))
+            self.dev = hid.Device(vid=self.CP2110_VID, pid=self.CP2110_PID, serial = serial)
+        else:
+            self.dev = hid.Device(vid=self.CP2110_VID, pid=self.CP2110_PID)
         #self.dev.nonblocking = 1
         self._send_feature_report([0x41, 0x01])  # enable uart
         self._send_feature_report([0x50, 0x00, 0x00, 0x25, 0x80, 0x00, 0x00, 0x03, 0x00, 0x00])  # 9600 8N1 - from USB trace
@@ -398,6 +406,15 @@ def sendCommand(self, cmd)->None:
         self._write(seq)
         # pylint: disable=unused-variable
         unknown = self._readResponse()
+        
+    def listDevices(self):
+        for device_dict in hid.enumerate():
+            keys = list(device_dict.keys())
+            keys.sort()
+            if device_dict['vendor_id'] == self.CP2110_VID:
+                for key in keys:
+                    print("%s : %s" % (key, device_dict[key]))
+                print()
 
     def _test(self):
         self._write(self._SEQUENCE_GET_NAME)