diff --git a/secop/datatypes.py b/secop/datatypes.py
index 03847ba..571c93c 100644
--- a/secop/datatypes.py
+++ b/secop/datatypes.py
@@ -102,7 +102,7 @@ class DataType(HasProperties):
                 self.setProperty(k, v)
             self.checkProperties()
         except Exception as e:
-            raise ProgrammingError(str(e))
+            raise ProgrammingError(str(e)) from None
 
     def get_info(self, **kwds):
         """prepare dict for export or repr
@@ -194,7 +194,7 @@ class FloatRange(DataType):
         try:
             value = float(value)
         except Exception:
-            raise BadValueError('Can not convert %r to float' % value)
+            raise BadValueError('Can not convert %r to float' % value) from None
         # map +/-infty to +/-max possible number
         value = clamp(-sys.float_info.max, value, sys.float_info.max)
 
@@ -268,7 +268,7 @@ class IntRange(DataType):
             fvalue = float(value)
             value = int(value)
         except Exception:
-            raise BadValueError('Can not convert %r to int' % value)
+            raise BadValueError('Can not convert %r to int' % value) from None
         if not self.min <= value <= self.max or round(fvalue) != fvalue:
             raise BadValueError('%r should be an int between %d and %d' %
                                 (value, self.min, self.max))
@@ -371,7 +371,7 @@ class ScaledInteger(DataType):
         try:
             value = float(value)
         except Exception:
-            raise BadValueError('Can not convert %r to float' % value)
+            raise BadValueError('Can not convert %r to float' % value) from None
         prec = max(self.scale, abs(value * self.relative_resolution),
                    self.absolute_resolution)
         if self.min - prec <= value <= self.max + prec:
@@ -454,7 +454,7 @@ class EnumType(DataType):
         try:
             return self._enum[value]
         except (KeyError, TypeError):  # TypeError will be raised when value is not hashable
-            raise BadValueError('%r is not a member of enum %r' % (value, self._enum))
+            raise BadValueError('%r is not a member of enum %r' % (value, self._enum)) from None
 
     def from_string(self, text):
         return self(text)
@@ -533,7 +533,7 @@ class BLOBType(DataType):
             if self.minbytes < other.minbytes or self.maxbytes > other.maxbytes:
                 raise BadValueError('incompatible datatypes')
         except AttributeError:
-            raise BadValueError('incompatible datatypes')
+            raise BadValueError('incompatible datatypes') from None
 
 
 class StringType(DataType):
@@ -572,7 +572,7 @@ class StringType(DataType):
             try:
                 value.encode('ascii')
             except UnicodeEncodeError:
-                raise BadValueError('%r contains non-ascii character!' % value)
+                raise BadValueError('%r contains non-ascii character!' % value) from None
         size = len(value)
         if size < self.minchars:
             raise BadValueError(
@@ -606,7 +606,7 @@ class StringType(DataType):
                     self.isUTF8 > other.isUTF8:
                 raise BadValueError('incompatible datatypes')
         except AttributeError:
-            raise BadValueError('incompatible datatypes')
+            raise BadValueError('incompatible datatypes') from None
 
 
 # TextType is a special StringType intended for longer texts (i.e. embedding \n),
@@ -618,7 +618,7 @@ class TextType(StringType):
     def __init__(self, maxchars=None):
         if maxchars is None:
             maxchars = UNLIMITED
-        super(TextType, self).__init__(0, maxchars)
+        super().__init__(0, maxchars)
 
     def __repr__(self):
         if self.maxchars == UNLIMITED:
@@ -771,7 +771,7 @@ class ArrayOf(DataType):
                 raise BadValueError('incompatible datatypes')
             self.members.compatible(other.members)
         except AttributeError:
-            raise BadValueError('incompatible datatypes')
+            raise BadValueError('incompatible datatypes') from None
 
 
 class TupleOf(DataType):
@@ -811,7 +811,7 @@ class TupleOf(DataType):
             return tuple(sub(elem)
                          for sub, elem in zip(self.members, value))
         except Exception as exc:
-            raise BadValueError('Can not validate:', str(exc))
+            raise BadValueError('Can not validate:', str(exc)) from None
 
     def export_value(self, value):
         """returns a python object fit for serialisation"""
@@ -894,7 +894,7 @@ class StructOf(DataType):
             return ImmutableDict((str(k), self.members[k](v))
                                  for k, v in list(value.items()))
         except Exception as exc:
-            raise BadValueError('Can not validate %s: %s' % (repr(value), str(exc)))
+            raise BadValueError('Can not validate %s: %s' % (repr(value), str(exc))) from None
 
     def export_value(self, value):
         """returns a python object fit for serialisation"""
@@ -924,7 +924,7 @@ class StructOf(DataType):
             if mandatory:
                 raise BadValueError('incompatible datatypes')
         except (AttributeError, TypeError, KeyError):
-            raise BadValueError('incompatible datatypes')
+            raise BadValueError('incompatible datatypes') from None
 
 
 class CommandType(DataType):
@@ -958,7 +958,7 @@ class CommandType(DataType):
         argstr = repr(self.argument) if self.argument else ''
         if self.result is None:
             return 'CommandType(%s)' % argstr
-        return 'CommandType(%s)->%s' % (argstr, repr(self.result))
+        return 'CommandType(%s, %s)' % (argstr, repr(self.result))
 
     def __call__(self, value):
         """return the validated argument value or raise"""
@@ -987,7 +987,7 @@ class CommandType(DataType):
             if self.result != other.result:  # not both are None
                 other.result.compatible(self.result)
         except AttributeError:
-            raise BadValueError('incompatible datatypes')
+            raise BadValueError('incompatible datatypes') from None
 
 
 # internally used datatypes (i.e. only for programming the SEC-node)
@@ -999,7 +999,10 @@ class DataTypeType(DataType):
         returns the value or raises an appropriate exception"""
         if isinstance(value, DataType):
             return value
-        raise ProgrammingError('%r should be a DataType!' % value)
+        try:
+            return get_datatype(value)
+        except Exception as e:
+            raise ProgrammingError(e) from None
 
     def export_value(self, value):
         """if needed, reformat value for transport"""
@@ -1034,6 +1037,13 @@ class ValueType(DataType):
         """
         raise NotImplementedError
 
+    def setProperty(self, key, value):
+        """silently ignored
+
+        as ValueType is used for the datatype default, this makes code
+        shorter for cases, where the datatype may not yet be defined
+        """
+
 
 class NoneOr(DataType):
     """validates a None or smth. else"""
@@ -1080,7 +1090,7 @@ UInt64 = IntRange(0, (1 << 64) - 1)
 # Goodie: Convenience Datatypes for Programming
 class LimitsType(TupleOf):
     def __init__(self, members):
-        TupleOf.__init__(self, members, members)
+        super().__init__(members, members)
 
     def __call__(self, value):
         limits = TupleOf.__call__(self, value)
@@ -1092,7 +1102,7 @@ class LimitsType(TupleOf):
 class StatusType(TupleOf):
     # shorten initialisation and allow access to status enumMembers from status values
     def __init__(self, enum):
-        TupleOf.__init__(self, EnumType(enum), StringType())
+        super().__init__(EnumType(enum), StringType())
         self._enum = enum
 
     def __getattr__(self, key):
@@ -1164,9 +1174,9 @@ def get_datatype(json, pname=''):
             kwargs = json.copy()
             base = kwargs.pop('type')
         except (TypeError, KeyError, AttributeError):
-            raise BadValueError('a data descriptor must be a dict containing a "type" key, not %r' % json)
+            raise BadValueError('a data descriptor must be a dict containing a "type" key, not %r' % json) from None
 
     try:
         return DATATYPES[base](pname=pname, **kwargs)
     except Exception as e:
-        raise BadValueError('invalid data descriptor: %r (%s)' % (json, str(e)))
+        raise BadValueError('invalid data descriptor: %r (%s)' % (json, str(e))) from None
diff --git a/secop/modules.py b/secop/modules.py
index 1883e76..d63cb93 100644
--- a/secop/modules.py
+++ b/secop/modules.py
@@ -25,9 +25,10 @@
 
 import sys
 import time
+from collections import OrderedDict
 
 from secop.datatypes import ArrayOf, BoolType, EnumType, FloatRange, \
-    IntRange, StatusType, StringType, TextType, TupleOf, get_datatype
+    IntRange, StatusType, StringType, TextType, TupleOf
 from secop.errors import BadValueError, ConfigError, InternalError, \
     ProgrammingError, SECoPError, SilentError, secop_error
 from secop.lib import formatException, mkthread
@@ -40,7 +41,7 @@ Done = object()  #: a special return value for a read/write function indicating
 
 
 class HasAccessibles(HasProperties):
-    """base class of module
+    """base class of Module
 
     joining the class's properties, parameters and commands dicts with
     those of base classes.
@@ -52,40 +53,43 @@ class HasAccessibles(HasProperties):
         super().__init_subclass__()
         # merge accessibles from all sub-classes, treat overrides
         # for now, allow to use also the old syntax (parameters/commands dict)
-        accessibles = {}
-        for base in reversed(cls.__bases__):
-            accessibles.update(getattr(base, 'accessibles', {}))
-        newaccessibles = {k: v for k, v in cls.__dict__.items() if isinstance(v, Accessible)}
-        for aname, aobj in list(accessibles.items()):
-            value = getattr(cls, aname, None)
-            if not isinstance(value, Accessible):  # else override is already done in __set_name__
-                if value is None:
-                    accessibles.pop(aname)
-                else:
-                    # this is either a method overwriting a command
-                    # or a value overwriting a property value or parameter default
-                    anew = aobj.override(value)
-                    newaccessibles[aname] = anew
-                    setattr(cls, aname, anew)
-                    anew.__set_name__(cls, aname)
-
-        ordered = {}
-        for aname in cls.__dict__.get('paramOrder', ()):
+        accessibles = OrderedDict()  # dict of accessibles
+        merged_properties = {}  # dict of dict of merged properties
+        new_names = []  # list of names of new accessibles
+        for base in reversed(cls.__mro__):
+            for key, value in base.__dict__.items():
+                if isinstance(value, Accessible):
+                    value.updateProperties(merged_properties.setdefault(key, {}))
+                    if base == cls and key not in accessibles:
+                        new_names.append(key)
+                    accessibles[key] = value
+                elif key in accessibles:
+                    # either a bare value overriding a parameter
+                    # or a method overriding a command
+                    aobj = aobj.copy()
+                    aobj.override(value)
+                    accessibles[key] = aobj
+        for aname, aobj in accessibles.items():
+            if aobj != getattr(cls, aname, None):
+                aobj = aobj.copy()
+                setattr(cls, aname, aobj)
+            aobj.merge(merged_properties[aname])
+            accessibles[aname] = aobj
+        # rebuild order: (1) inherited items, (2) items from paramOrder, (3) new accessibles
+        # move (2) to the end
+        for aname in list(cls.__dict__.get('paramOrder', ())):
             if aname in accessibles:
-                ordered[aname] = accessibles.pop(aname)
-            elif aname in newaccessibles:
-                ordered[aname] = newaccessibles.pop(aname)
-            # ignore unknown names
-        # starting from old accessibles not mentioned, append items from 'order'
-        accessibles.update(ordered)
-        # then new accessibles not mentioned
-        accessibles.update(newaccessibles)
+                accessibles.move_to_end(aname)
+                # ignore unknown names
+        # move (3) to the end
+        for aname in new_names:
+            accessibles.move_to_end(aname)
+        # note: for python < 3.6 the order of inherited items is not ensured between
+        # declarations within the same class
         cls.accessibles = accessibles
 
         # Correct naming of EnumTypes
-        for k, v in accessibles.items():
-            if isinstance(v, Parameter) and isinstance(v.datatype, EnumType):
-                v.datatype.set_name(k)
+        # moved to Parameter.__set_name__
 
         # check validity of Parameter entries
         for pname, pobj in accessibles.items():
@@ -318,8 +322,9 @@ class Module(HasAccessibles):
                 paramobj = self.accessibles.get(paramname, None)
                 # paramobj might also be a command (not sure if this is needed)
                 if paramobj:
-                    if propname == 'datatype':
-                        propvalue = get_datatype(propvalue, k)
+                    # no longer needed, this conversion is done by DataTypeType.__call__:
+                    # if propname == 'datatype':
+                    #     propvalue = get_datatype(propvalue, k)
                     try:
                         paramobj.setProperty(propname, propvalue)
                     except KeyError:
@@ -347,6 +352,10 @@ class Module(HasAccessibles):
             self.valueCallbacks[pname] = []
             self.errorCallbacks[pname] = []
 
+            if not pobj.hasDatatype():
+                errors.append('%s needs a datatype' % pname)
+                continue
+
             if pname in cfgdict:
                 if not pobj.readonly and pobj.initwrite is not False:
                     # parameters given in cfgdict have to call write_<pname>
@@ -393,11 +402,15 @@ class Module(HasAccessibles):
                     cfgdict.pop(k)
             except (ValueError, TypeError) as e:
                 # self.log.exception(formatExtendedStack())
-                errors.append('module %s, parameter %s: %s' % (self.name, k, e))
+                errors.append('parameter %s: %s' % (k, e))
+
+        # ensure consistency
+        for aobj in self.accessibles.values():
+            aobj.finish()
 
         # Modify units AFTER applying the cfgdict
-        for k, v in self.parameters.items():
-            dt = v.datatype
+        for pname, pobj in self.parameters.items():
+            dt = pobj.datatype
             if '$' in dt.unit:
                 dt.setProperty('unit', dt.unit.replace('$', self.parameters['value'].datatype.unit))
 
@@ -410,7 +423,7 @@ class Module(HasAccessibles):
             for pname, p in self.parameters.items():
                 try:
                     p.checkProperties()
-                except ConfigError:
+                except ConfigError as e:
                     errors.append('%s: %s' % (pname, e))
         if errors:
             raise ConfigError(errors)
@@ -426,7 +439,7 @@ class Module(HasAccessibles):
         """announce a changed value or readerror"""
         pobj = self.parameters[pname]
         timestamp = timestamp or time.time()
-        changed = pobj.value != value or timestamp > (pobj.timestamp or 0) + 1
+        changed = pobj.value != value
         if value is not None:
             pobj.value = value  # store the value even in case of error
         if err:
@@ -439,8 +452,10 @@ class Module(HasAccessibles):
                 pobj.value = pobj.datatype(value)
             except Exception as e:
                 err = secop_error(e)
-            if not changed:
-                return  # experimental: do not update unchanged values within 1 sec
+            if not changed and timestamp < ((pobj.timestamp or 0)
+                                            + self.DISPATCHER.OMIT_UNCHANGED_WITHIN):
+                # no change within short time -> omit
+                return
         pobj.timestamp = timestamp
         pobj.readerror = err
         if pobj.export:
diff --git a/secop/params.py b/secop/params.py
index 6fe9274..72d0455 100644
--- a/secop/params.py
+++ b/secop/params.py
@@ -35,9 +35,17 @@ UNSET = object()  # an argument not given, not even None
 
 
 class Accessible(HasProperties):
-    """base class for Parameter and Command"""
+    """base class for Parameter and Command
 
-    kwds = None  # is a dict if it might be used as Override
+    Inheritance mechanism:
+
+    param.propertyValues contains the properties, which will be used when the
+      owner class will be instantiated
+
+    param.ownProperties contains the properties to be used for inheritance
+    """
+
+    ownProperties = None
 
     def init(self, kwds):
         # do not use self.propertyValues.update here, as no invalid values should be
@@ -45,43 +53,47 @@ class Accessible(HasProperties):
         for k, v in kwds.items():
             self.setProperty(k, v)
 
-    def inherit(self, cls, owner):
-        for base in owner.__bases__:
-            if hasattr(base, self.name):
-                aobj = getattr(base, 'accessibles', {}).get(self.name)
-                if aobj:
-                    if not isinstance(aobj, cls):
-                        raise ProgrammingError('%s %s.%s can not inherit from a %s' %
-                                               (cls.__name__, owner.__name__, self.name, aobj.__class__.__name__))
-                    # inherit from aobj
-                    for pname, value in aobj.propertyValues.items():
-                        if pname not in self.propertyValues:
-                            self.propertyValues[pname] = value
-                break
-
     def as_dict(self):
         return self.propertyValues
 
-    def override(self, value=UNSET, **kwds):
-        """return a copy, overridden by a bare attribute
-
-        and/or some properties"""
+    def override(self, value):
+        """override with a bare value"""
         raise NotImplementedError
 
-    def copy(self):
-        """return a (deep) copy of ourselfs"""
+    def copy(self, **kwds):
+        """return a (deep) copy of ourselfs
+
+        :param kwds: override given properties
+        """
+        raise NotImplementedError
+
+    def updateProperties(self, merged_properties):
+        """update merged_properties with our own properties"""
+        raise NotImplementedError
+
+    def merge(self, merged_properties):
+        """merge with inherited properties
+
+        :param merged_properties: dict of properties to be updated
+        note: merged_properties may be modified
+        """
+        raise NotImplementedError
+
+    def finish(self):
+        """ensure consistency"""
         raise NotImplementedError
 
     def for_export(self):
         """prepare for serialisation"""
         raise NotImplementedError
 
+    def hasDatatype(self):
+        return 'datatype' in self.propertyValues
+
     def __repr__(self):
         props = []
-        for k, prop in sorted(self.propertyDict.items()):
-            v = self.propertyValues.get(k, prop.default)
-            if v != prop.default:
-                props.append('%s=%r' % (k, v))
+        for k, v in sorted(self.propertyValues.items()):
+            props.append('%s=%r' % (k, v))
         return '%s(%s)' % (self.__class__.__name__, ', '.join(props))
 
 
@@ -100,7 +112,7 @@ class Parameter(Accessible):
         extname='description', mandatory=True, export='always')
     datatype = Property(
         'datatype of the Parameter (SECoP datainfo)', DataTypeType(),
-        extname='datainfo', mandatory=True, export='always')
+        extname='datainfo', mandatory=True, export='always', default=ValueType())
     readonly = Property(
         'not changeable via SECoP (default True)', BoolType(),
         extname='readonly', default=True, export='always')
@@ -160,9 +172,13 @@ class Parameter(Accessible):
     timestamp = 0
     readerror = None
 
-    def __init__(self, description=None, datatype=None, inherit=True, *, unit=None, constant=None, **kwds):
+    def __init__(self, description=None, datatype=None, inherit=True, **kwds):
         super().__init__()
-        if datatype is not None:
+        if datatype is None:
+            # collect datatype properties. these are not applied, as we have no datatype
+            self.ownProperties = {k: kwds.pop(k) for k in list(kwds) if k not in self.propertyDict}
+        else:
+            self.ownProperties = {}
             if not isinstance(datatype, DataType):
                 if isinstance(datatype, type) and issubclass(datatype, DataType):
                     # goodie: make an instance from a class (forgotten ()???)
@@ -174,15 +190,15 @@ class Parameter(Accessible):
             if 'default' in kwds:
                 self.default = datatype(kwds['default'])
 
-        self.init(kwds)  # datatype must be defined before we can treat dataset properties like fmtstr or unit
-
         if description is not None:
-            self.description = inspect.cleandoc(description)
+            kwds['description'] = inspect.cleandoc(description)
 
-        # save for __set_name__
-        self._inherit = inherit
-        self._unit = unit  # for legacy code only
-        self._constant = constant
+        self.init(kwds)
+
+        if inherit:
+            self.ownProperties.update(self.propertyValues)
+        else:
+            self.ownProperties = {k: getattr(self, k) for k in self.propertyDict}
 
     def __get__(self, instance, owner):
         # not used yet
@@ -195,57 +211,74 @@ class Parameter(Accessible):
 
     def __set_name__(self, owner, name):
         self.name = name
+        if isinstance(self.datatype, EnumType):
+            self.datatype.set_name(name)
 
-        if self._inherit:
-            self.inherit(Parameter, owner)
+        if self.export is True:
+            predefined_cls = PREDEFINED_ACCESSIBLES.get(self.name, None)
+            if predefined_cls is Parameter:
+                self.export = self.name
+            elif predefined_cls is None:
+                self.export = '_' + self.name
+            else:
+                raise ProgrammingError('can not use %r as name of a Parameter' % self.name)
 
-        # check for completeness
-        missing_properties = [pname for pname in ('description', 'datatype') if pname not in self.propertyValues]
-        if missing_properties:
-            raise ProgrammingError('Parameter %s.%s needs a %s' %
-                                   (owner.__name__, name, ' and a '.join(missing_properties)))
-        if self._unit is not None:
-            self.datatype.setProperty('unit', self._unit)
+    def copy(self, **kwds):
+        """return a (deep) copy of ourselfs
 
-        if self._constant is not None:
-            constant = self.datatype(self._constant)
+        :param kwds: override given properties
+        """
+        res = type(self)()
+        res.name = self.name
+        res.init(self.propertyValues)
+        res.init(kwds)
+        if 'datatype' in self.propertyValues:
+            res.datatype = res.datatype.copy()
+        return res
+
+    def updateProperties(self, merged_properties):
+        """update merged_properties with our own properties"""
+        datatype = self.ownProperties.get('datatype')
+        if datatype is not None:
+            # clear datatype properties, as they are overriden by datatype
+            for key in list(merged_properties):
+                if key not in self.propertyDict:
+                    merged_properties.pop(key)
+        merged_properties.update(self.ownProperties)
+
+    def override(self, value):
+        """override default"""
+        self.default = self.datatype(value)
+
+    def merge(self, merged_properties):
+        """merge with inherited properties
+
+        :param merged_properties: dict of properties to be updated
+        note: merged_properties may be modified
+        """
+        datatype = merged_properties.pop('datatype', None)
+        if datatype is not None:
+            self.datatype = datatype.copy()
+        self.init(merged_properties)
+        self.finish()
+
+    def finish(self):
+        """ensure consistency"""
+
+        if self.constant is not None:
+            constant = self.datatype(self.constant)
             # The value of the `constant` property should be the
             # serialised version of the constant, or unset
             self.constant = self.datatype.export_value(constant)
             self.readonly = True
-
         if 'default' in self.propertyValues:
             # fixes in case datatype has changed
             try:
-                self.datatype(self.default)
+                self.default = self.datatype(self.default)
             except BadValueError:
                 # clear default, if it does not match datatype
                 self.propertyValues.pop('default')
 
-        if self.export is True:
-            predefined_cls = PREDEFINED_ACCESSIBLES.get(name, None)
-            if predefined_cls is Parameter:
-                self.export = name
-            elif predefined_cls is None:
-                self.export = '_' + name
-            else:
-                raise ProgrammingError('can not use %r as name of a Parameter' % name)
-
-    def copy(self):
-        # deep copy, as datatype might be altered from config
-        res = type(self)()
-        res.name = self.name
-        res.init(self.propertyValues)
-        res.datatype = res.datatype.copy()
-        return res
-
-    def override(self, value=UNSET, **kwds):
-        res = self.copy()
-        res.init(kwds)
-        if value is not UNSET:
-            res.value = res.datatype(value)
-        return res
-
     def export_value(self):
         return self.datatype.export_value(self.value)
 
@@ -255,15 +288,23 @@ class Parameter(Accessible):
     def getProperties(self):
         """get also properties of datatype"""
         super_prop = super().getProperties().copy()
-        super_prop.update(self.datatype.getProperties())
+        if self.datatype:
+            super_prop.update(self.datatype.getProperties())
         return super_prop
 
     def setProperty(self, key, value):
         """set also properties of datatype"""
-        if key in self.propertyDict:
-            super().setProperty(key, value)
-        else:
-            self.datatype.setProperty(key, value)
+        try:
+            if key in self.propertyDict:
+                super().setProperty(key, value)
+            else:
+                try:
+                    self.datatype.setProperty(key, value)
+                except KeyError:
+                    raise ProgrammingError('cannot set %s on parameter with datatype %s'
+                                           % (key, type(self.datatype).__name__)) from None
+        except ValueError as e:
+            raise ProgrammingError('property %s: %s' % (key, str(e))) from None
 
     def checkProperties(self):
         super().checkProperties()
@@ -336,10 +377,9 @@ class Command(Accessible):
         if self.func is None:
             raise ProgrammingError('Command %s.%s must be used as a method decorator' %
                                    (owner.__name__, name))
-        if self._inherit:
-            self.inherit(Command, owner)
 
         self.datatype = CommandType(self.argument, self.result)
+        self.ownProperties = self.propertyValues.copy()
         if self.export is True:
             predefined_cls = PREDEFINED_ACCESSIBLES.get(name, None)
             if predefined_cls is Command:
@@ -347,39 +387,77 @@ class Command(Accessible):
             elif predefined_cls is None:
                 self.export = '_' + name
             else:
-                raise ProgrammingError('can not use %r as name of a Command' % name)
+                raise ProgrammingError('can not use %r as name of a Command' % name) from None
+        if not self._inherit:
+            for key, pobj in self.properties.items():
+                if key not in self.propertyValues:
+                    self.propertyValues[key] = pobj.default
 
     def __get__(self, obj, owner=None):
         if obj is None:
             return self
         if not self.func:
-            raise ProgrammingError('Command %s not properly configured' % self.name)
+            raise ProgrammingError('Command %s not properly configured' % self.name) from None
         return self.func.__get__(obj, owner)
 
     def __call__(self, func):
+        """called when used as decorator"""
         if 'description' not in self.propertyValues and func.__doc__:
             self.description = inspect.cleandoc(func.__doc__)
         self.func = func
         return self
 
-    def copy(self):
+    def copy(self, **kwds):
+        """return a (deep) copy of ourselfs
+
+        :param kwds: override given properties
+        """
         res = type(self)()
         res.name = self.name
         res.func = self.func
         res.init(self.propertyValues)
+        res.init(kwds)
         if res.argument:
             res.argument = res.argument.copy()
         if res.result:
             res.result = res.result.copy()
-        res.datatype = CommandType(res.argument, res.result)
+        self.finish()
         return res
 
-    def override(self, value=UNSET, **kwds):
-        res = self.copy()
-        res.init(kwds)
-        if value is not UNSET:
-            res.func = value
-        return res
+    def updateProperties(self, merged_properties):
+        """update merged_properties with our own properties"""
+        merged_properties.update(self.ownProperties)
+
+    def override(self, value):
+        """override method
+
+        this is needed when the @Command is missing on a method overriding a command"""
+        if not callable(value):
+            raise ProgrammingError('%s = %r is overriding a Command' % (self.name, value))
+        self.func = value
+
+    def merge(self, merged_properties):
+        """merge with inherited properties
+
+        :param merged_properties: dict of properties to be updated
+        """
+        self.init(merged_properties)
+        self.finish()
+
+    def finish(self):
+        """ensure consistency"""
+        self.datatype = CommandType(self.argument, self.result)
+
+    def setProperty(self, key, value):
+        """special treatment of datatype"""
+        try:
+            if key == 'datatype':
+                command = DataTypeType()(value)
+                super().setProperty('argument', command.argument)
+                super().setProperty('result', command.result)
+            super().setProperty(key, value)
+        except ValueError as e:
+            raise ProgrammingError('property %s: %s' % (key, str(e))) from None
 
     def do(self, module_obj, argument):
         """perform function call
diff --git a/secop/protocol/dispatcher.py b/secop/protocol/dispatcher.py
index 2485004..0273865 100644
--- a/secop/protocol/dispatcher.py
+++ b/secop/protocol/dispatcher.py
@@ -61,6 +61,8 @@ def make_update(modulename, pobj):
 
 class Dispatcher:
 
+    OMIT_UNCHANGED_WITHIN = 1  # do not send unchanged updates within 1 sec
+
     def __init__(self, name, logger, options, srv):
         # to avoid errors, we want to eat all options here
         self.equipment_id = options.pop('id', name)
diff --git a/test/test_iohandler.py b/test/test_iohandler.py
index 03dd95b..21012ce 100644
--- a/test/test_iohandler.py
+++ b/test/test_iohandler.py
@@ -71,6 +71,8 @@ class Data:
 
 
 class DispatcherStub:
+    OMIT_UNCHANGED_WITHIN = 0
+
     def __init__(self, updates):
         self.updates = updates
 
@@ -106,7 +108,6 @@ def test_IOHandler():
     group1 = Hdl('group1', 'SIMPLE?', '%g')
     group2 = Hdl('group2', 'CMD?%(channel)d', '%g,%s,%d')
 
-
     class Module1(Module):
         channel = Property('the channel', IntRange(), default=3)
         loop = Property('the loop', IntRange(), default=2)
diff --git a/test/test_modules.py b/test/test_modules.py
index b082e0d..cc6582a 100644
--- a/test/test_modules.py
+++ b/test/test_modules.py
@@ -26,14 +26,16 @@ import threading
 
 import pytest
 
-from secop.datatypes import BoolType, FloatRange, StringType
-from secop.errors import ProgrammingError
-from secop.modules import Communicator, Drivable, Module
+from secop.datatypes import BoolType, FloatRange, StringType, IntRange, CommandType
+from secop.errors import ProgrammingError, ConfigError
+from secop.modules import Communicator, Drivable, Readable, Module
 from secop.params import Command, Parameter
 from secop.poller import BasicPoller
 
 
 class DispatcherStub:
+    OMIT_UNCHANGED_WITHIN = 0
+
     def __init__(self, updates):
         self.updates = updates
 
@@ -51,6 +53,9 @@ class LoggerStub:
     info = warning = exception = debug
 
 
+logger = LoggerStub()
+
+
 class ServerStub:
     def __init__(self, updates):
         self.dispatcher = DispatcherStub(updates)
@@ -125,7 +130,7 @@ def test_ModuleMagic():
         value = Parameter(datatype=FloatRange(unit='deg'))
         a1 = Parameter(datatype=FloatRange(unit='$/s'), readonly=False)
         b2 = Parameter('<b2>', datatype=BoolType(), default=True,
-                            poll=True, readonly=False, initwrite=True)
+                       poll=True, readonly=False, initwrite=True)
 
         def write_a1(self, value):
             self._a1_written = value
@@ -142,7 +147,6 @@ def test_ModuleMagic():
     sortcheck2 = ['status', 'pollinterval', 'target', 'stop',
                   'a1', 'a2', 'cmd2', 'param1', 'param2', 'cmd', 'value', 'b2']
 
-    logger = LoggerStub()
     updates = {}
     srv = ServerStub(updates)
 
@@ -228,3 +232,103 @@ def test_ModuleMagic():
         o.earlyInit()
     for o in objects:
         o.initModule()
+
+
+def test_param_inheritance():
+    srv = ServerStub({})
+
+    class Base(Module):
+        param = Parameter()
+
+    class MissingDatatype(Base):
+        param = Parameter('param')
+
+    class MissingDescription(Base):
+        param = Parameter(datatype=FloatRange(), default=0)
+
+    # missing datatype and/or description of a parameter has to be detected
+    # at instantation and only then
+    with pytest.raises(ConfigError) as e_info:
+        MissingDatatype('o', logger, {'description': ''}, srv)
+    assert 'datatype' in repr(e_info.value)
+
+    with pytest.raises(ConfigError) as e_info:
+        MissingDescription('o', logger, {'description': ''}, srv)
+    assert 'description' in repr(e_info.value)
+
+    with pytest.raises(ConfigError) as e_info:
+        Base('o', logger, {'description': ''}, srv)
+
+
+def test_mixin():
+    # srv = ServerStub({})
+
+    class Mixin:  # no need to inherit from Module or HasAccessible
+        value = Parameter(unit='K')  # missing datatype and description acceptable in mixins
+        param1 = Parameter('no datatype yet', fmtstr='%.5f')
+        param2 = Parameter('no datatype yet', default=1)
+
+    class MixedReadable(Mixin, Readable):
+        pass
+
+    class MixedDrivable(MixedReadable, Drivable):
+        value = Parameter(unit='Ohm', fmtstr='%.3f')
+        param1 = Parameter(datatype=FloatRange())
+
+    with pytest.raises(ProgrammingError):
+        class MixedModule(Mixin):
+            param1 = Parameter('', FloatRange(), fmtstr=0)  # fmtstr must be a string
+
+    assert repr(MixedDrivable.status.datatype) == repr(Drivable.status.datatype)
+    assert repr(MixedReadable.status.datatype) == repr(Readable.status.datatype)
+    assert MixedReadable.value.datatype.unit == 'K'
+    assert MixedDrivable.value.datatype.unit == 'Ohm'
+    assert MixedDrivable.value.datatype.fmtstr == '%.3f'
+    # when datatype is overridden, fmtstr falls back to default:
+    assert MixedDrivable.param1.datatype.fmtstr == '%g'
+
+    srv = ServerStub({})
+
+    MixedDrivable('o', logger, {
+        'description': '',
+        'param1.description': 'param 1',
+        'param1': 0,
+        'param2.datatype': {"type": "double"},
+    }, srv)
+
+    with pytest.raises(ConfigError):
+        MixedReadable('o', logger, {
+            'description': '',
+            'param1.description': 'param 1',
+            'param1': 0,
+            'param2.datatype': {"type": "double"},
+        }, srv)
+
+
+def test_command_config():
+    class Mod(Module):
+        @Command(IntRange(0, 1), result=IntRange(0, 1))
+        def convert(self, value):
+            return value
+
+    srv = ServerStub({})
+    mod = Mod('o', logger, {
+        'description': '',
+        'convert.argument': {'type': 'bool'},
+    }, srv)
+    assert mod.commands['convert'].datatype.export_datatype() == {
+        'type': 'command',
+        'argument': {'type': 'bool'},
+        'result': {'type': 'int', 'min': 0, 'max': 1},
+    }
+
+    mod = Mod('o', logger, {
+        'description': '',
+        'convert.datatype': {'type': 'command', 'argument': {'type': 'bool'}, 'result': {'type': 'bool'}},
+    }, srv)
+    assert mod.commands['convert'].datatype.export_datatype() == {
+        'type': 'command',
+        'argument': {'type': 'bool'},
+        'result': {'type': 'bool'},
+    }
+
diff --git a/test/test_params.py b/test/test_params.py
index 0976f8a..bf08cd7 100644
--- a/test/test_params.py
+++ b/test/test_params.py
@@ -88,15 +88,18 @@ def test_Override():
         p1 = Parameter(default=True)
         p2 = Parameter()  # override without change
 
-    assert Mod.p1 != Base.p1
-    assert Mod.p2 != Base.p2
-    assert Mod.p3 == Base.p3
-
-    assert id(Mod.p2) != id(Base.p2)  # must be a new object
-    assert repr(Mod.p2) == repr(Base.p2)  # but must be a clone
+    assert id(Mod.p1) != id(Base.p1)
+    assert id(Mod.p2) != id(Base.p2)
+    assert id(Mod.p3) == id(Base.p3)
+    assert repr(Mod.p2) == repr(Base.p2)  # must be a clone
+    assert repr(Mod.p3) == repr(Base.p3)  # must be a clone
+    assert Mod.p1.default == True
+    # manipulating default makes Base.p1 and Mod.p1 match
+    Mod.p1.default = False
+    assert repr(Mod.p1) == repr(Base.p1)
 
 
 def test_Export():
-    class Mod:
+    class Mod(HasAccessibles):
         param = Parameter('description1', datatype=BoolType, default=False)
     assert Mod.param.export == '_param'
diff --git a/test/test_properties.py b/test/test_properties.py
index a7dbb6f..9e68765 100644
--- a/test/test_properties.py
+++ b/test/test_properties.py
@@ -159,3 +159,26 @@ def test_Property_override():
             a = 's'
 
     assert 'can not set' in str(e.value)
+
+
+def test_Properties_mro():
+    class A(HasProperties):
+        p = Property('base', StringType(), 'base', export='always')
+
+    class B(A):
+        pass
+
+    class C(A):
+        p = Property('sub', FloatRange(), extname='p')
+
+    class D(C, B):
+        p = 1
+
+    class E(B, C):
+        p = 2
+
+    assert B().exportProperties() == {'_p': 'base'}
+    assert D().exportProperties() == {'p': 1.0}
+    # in an older implementation the following would fail, as B.p is constructed first
+    # and then B.p overrides C.p
+    assert E().exportProperties() == {'p': 2.0}