diff --git a/myhdl/Cosimulation.py b/myhdl/Cosimulation.py
index 1bf9966a..001399c2 100644
--- a/myhdl/Cosimulation.py
+++ b/myhdl/Cosimulation.py
@@ -31,6 +31,7 @@ import exceptions
 from Signal import Signal
 import myhdl
 import _simulator
+from myhdl import intbv
 
 
 _flag = 0
@@ -75,8 +76,8 @@ class Cosimulation(object):
         _flag = 1
         _simulator._cosim = self
 
-        self.rt, self.wt = rt, wt = os.pipe()
-        self.rf, self.wf = rf, wf = os.pipe()
+        self._rt, self._wt = rt, wt = os.pipe()
+        self._rf, self._wf = rf, wf = os.pipe()
 
         self._fromSignames = fromSignames = []
         self._fromSizes = fromSizes = []
@@ -139,20 +140,25 @@ class Cosimulation(object):
                 else:
                     raise Error, "Unexpected cosim input"
 
-    def get(self):
-        s = os.read(self.rt, MAXLINE)
+    def _get(self):
+        s = os.read(self._rt, _MAXLINE)
+        if not s:
+            raise SimulationEndError
         e = s.split()
         vals = e[1:]
-        for s, v in zip(self.toSigs, vals):
-            s.next = long(v, 16)
+        for s, v in zip(self._toSigs, vals):
+            try:
+                s.next = long(v, 16)
+            except ValueError:
+                s.next = intbv(None)
 
-    def put(self):
-        buf = hex(_simulator._time)
+    def _put(self):
+        buf = hex(_simulator._time)[2:]
         buf += " "
-        for s in self.fromSigs:
-            buf += hex(s)
+        for s in self._fromSigs:
+            buf += hex(s)[2:]
             buf += " "
-        os.write(wf, buf)
+        os.write(self._wf, buf)
             
     def __del__(self):
         """ Clear flag when this object destroyed - to suite unittest. """
diff --git a/myhdl/test_Cosimulation.py b/myhdl/test_Cosimulation.py
index ee9089da..6b7ca40f 100644
--- a/myhdl/test_Cosimulation.py
+++ b/myhdl/test_Cosimulation.py
@@ -50,14 +50,17 @@ exe = "python test_Cosimulation.py CosimulationTest"
 
 fromSignames = ['a', 'bb', 'ccc']
 fromSizes = [1, 11, 63]
+fromVals = [0x2, 0x43, 0x24]
 fromSigs = {}
-for s in fromSignames:
-    fromSigs[s] = Signal(0)
+for s, v in zip(fromSignames, fromVals):
+    fromSigs[s] = Signal(v)
 toSignames = ['d', 'ee', 'fff', 'g']
 toSizes = [32, 12, 3, 6]
 toSigs = {}
 for s in toSignames:
     toSigs[s] = Signal(0)
+toVals = [0x3, 0x45, 0x14, 0x12]
+toXVals = ["X00", "FZ3", "34XZ", "56U"]
 allSigs = fromSigs.copy()
 allSigs.update(toSigs)
 
@@ -195,6 +198,68 @@ class CosimulationTest(TestCase):
         buf += "fff 6"
         os.write(wt, buf)
 
+    def testFromSignalVals(self):
+        cosim = Cosimulation(exe + ".cosimFromSignalVals", **allSigs)
+        os.read(cosim._rt, MAXLINE)
+        cosim._put()
+
+    def cosimFromSignalVals(self):
+        wt = int(os.environ['MYHDL_TO_PIPE'])
+        rf = int(os.environ['MYHDL_FROM_PIPE'])
+        buf = "FROM 00 "
+        for s, w in zip(fromSignames, fromSizes):
+            buf += "%s %s " % (s, w)
+        os.write(wt, buf)
+        os.read(rf, MAXLINE)
+        os.write(wt, "TO 0000 a 1")
+        os.read(rf, MAXLINE)
+        os.write(wt, "START")
+        os.read(rf, MAXLINE)
+        os.write(wt, "DUMMY")
+        s = os.read(rf, MAXLINE)
+        vals = [long(e, 16) for e in s.split()[1:]]
+        self.assertEqual(vals, fromVals)
+
+    def testToSignalVals(self):
+        cosim = Cosimulation(exe + ".cosimToSignalVals", **allSigs)
+        for n in toSignames:
+            self.assertEqual(toSigs[n].next, 0)
+        cosim._get()
+        for n, v in zip(toSignames, toVals):
+            self.assertEqual(toSigs[n].next, v)
+        os.write(cosim._wf, "DUMMY")
+        cosim._get()
+        for n in toSignames:
+            self.assertEqual(toSigs[n].next, None)
+        
+
+    def cosimToSignalVals(self):
+        wt = int(os.environ['MYHDL_TO_PIPE'])
+        rf = int(os.environ['MYHDL_FROM_PIPE'])
+        buf = "FROM 00 "
+        for s, w in zip(fromSignames, fromSizes):
+            buf += "%s %s " % (s, w)
+        os.write(wt, buf)
+        os.read(rf, MAXLINE)
+        buf = "TO 00 "
+        for s, w in zip(toSignames, toSizes):
+            buf += "%s %s " % (s, w)
+        os.write(wt, buf)
+        os.read(rf, MAXLINE)
+        os.write(wt, "START")
+        os.read(rf, MAXLINE)
+        buf = "0 "
+        for v in toVals:
+            buf += hex(v)[2:]
+            buf += " "
+        os.write(wt, buf)
+        os.read(rf, MAXLINE)
+        buf = "0 "
+        for v in toXVals:
+            buf += v
+            buf += " "
+        os.write(wt, buf)
+
                    
 if __name__ == "__main__":
     unittest.main()