autoMaxMin.diff

diff --git a/canmatrix/canmatrix.py b/canmatrix/canmatrix.py
index 78cda25..8199023 100644
--- a/canmatrix/canmatrix.py
+++ b/canmatrix/canmatrix.py
@@ -149,16 +149,6 @@ class Signal(object):
         else:
             self._offset = float(0)
  
-        if 'min' in kwargs:
-            self._min = float(kwargs["min"])
-        else:
-            self._min = float(0)
-
-        if 'max' in kwargs:
-            self._max = float(kwargs["max"])
-        else:
-            self._max = float(0)
-
         if 'unit' in kwargs:
             self._unit = kwargs["unit"]
         else:
@@ -183,6 +173,26 @@ class Signal(object):
         else:
             self._multiplex = None
 
+        if 'min' in kwargs:
+            min = kwargs["min"]
+        else:
+            min = None
+
+        if min is None:
+            self.setMin()
+        else:
+            self._min = float(min)
+
+        if 'max' in kwargs:
+            max = kwargs["max"]
+        else:
+            max = None
+
+        if max is None:
+            self.setMax()
+        else:
+            self._max = float(max)
+
         self._name = name
         self._attributes = {}
         self._values = {}
@@ -272,7 +282,29 @@ class Signal(object):
 
     def getLsbStartbit(self):
         return int(self._startbit)
-       
+
+    def calculateRawRange(self):
+        rawRange = 2 ** self._signalsize
+
+        if self._is_signed:
+            rawRange /= 2
+
+        return (-rawRange if self._is_signed else 0,
+                rawRange - 1)
+
+    def setMin(self, min=None):
+        self._min = min
+
+        if self._min is None:
+            rawMin = self.calculateRawRange()[0]
+            self._min = self._offset + (rawMin * self._factor)
+
+    def setMax(self, max=None):
+        self._max = max
+
+        if self._max is None:
+            rawMax = self.calculateRawRange()[1]
+            self._max = self._offset + (rawMax * self._factor)
 
 
 class SignalGroup(object):
diff --git a/canmatrix/importarxml.py b/canmatrix/importarxml.py
index f555ac9..091d670 100644
--- a/canmatrix/importarxml.py
+++ b/canmatrix/importarxml.py
@@ -82,8 +82,8 @@ def getSignals(signalarray, Bo, arDict, ns, multiplexId):
             length = arGetChild(syssignal,  "LENGTH", arDict, ns)
         name = arGetChild(syssignal,  "SHORT-NAME", arDict, ns)
   
-        Min = 0
-        Max = 1
+        Min = None
+        Max = None
         factor = 1.0
         offset = 0
         Unit = ""
diff --git a/canmatrix/importkcd.py b/canmatrix/importkcd.py
index 250600a..9b5167c 100644
--- a/canmatrix/importkcd.py
+++ b/canmatrix/importkcd.py
@@ -52,8 +52,8 @@ def parseSignal(signal, mux, namespace, nodelist):
     unit = ""
     offset = 0
     factor = 1
-    min = 0
-    max = 1
+    min = None
+    max = None
     is_signed = False
 
     values = signal.find('./' + namespace + 'Value')
@@ -163,8 +163,8 @@ def importKcd(filename):
 
             is_little_endian = True
   
-            min = 0
-            max = 1
+            min = None
+            max = None
             values = multiplex.find('./' + namespace + 'Value')
             if values is not None:
                 if 'min' in values.attrib:
diff --git a/canmatrix/importsym.py b/canmatrix/importsym.py
index 1a85e3c..663c966 100644
--- a/canmatrix/importsym.py
+++ b/canmatrix/importsym.py
@@ -139,8 +139,8 @@ def importSym(filename, **options):
                 intel = 1
                 unit = ""
                 factor = 1
-                max = 1
-                min = 0
+                max = None
+                min = None
                 longName = None
                 startValue = 0
                 offset = 0
diff --git a/canmatrix/importxls.py b/canmatrix/importxls.py
index 559aa94..ef40c6a 100644
--- a/canmatrix/importxls.py
+++ b/canmatrix/importxls.py
@@ -260,8 +260,8 @@ def importXls(filename, **options):
                 newSig._max = float(maxi)
             except:
                 newSig._offset = 0
-                newSig._min = 0
-                newSig._max = 1
+                newSig._min = None
+                newSig._max = None
 
 
         elif valueName.__len__() > 0:
@@ -272,8 +272,8 @@ def importXls(filename, **options):
             newSig._max = float(maxi)
         else:
             newSig._offset = 0
-            newSig._min = 0
-            newSig._max = 1
+            newSig._min = None
+            newSig._max = None
 
 
     for frame in db._fl._list:
diff --git a/canmatrix/importxlsx.py b/canmatrix/importxlsx.py
index 09d5943..67ff854 100644
--- a/canmatrix/importxlsx.py
+++ b/canmatrix/importxlsx.py
@@ -339,8 +339,8 @@ def importXlsx(filename, **options):
                 newSig._max = float(maxi)
             except:
                 newSig._offset = 0
-                newSig._min = 0
-                newSig._max = 1
+                newSig._min = None
+                newSig._max = None
 
 
         elif valueName.__len__() > 0:
@@ -351,8 +351,8 @@ def importXlsx(filename, **options):
             newSig._max = float(maxi)
         else:
             newSig._offset = 0
-            newSig._min = 0
-            newSig._max = 1
+            newSig._min = None
+            newSig._max = None
 
     # dlc-estimation / dlc is not in xls, thus calculate a minimum-dlc:
     for frame in db._fl._list: