diff --git a/secop/lib/asynconn.py b/secop/lib/asynconn.py
index 8ef9ff4..9d5df16 100644
--- a/secop/lib/asynconn.py
+++ b/secop/lib/asynconn.py
@@ -48,6 +48,7 @@ class ConnectionClosed(ConnectionError):
 
 class AsynConn:
     timeout = 1  # inter byte timeout
+    scheme = None
     SCHEME_MAP = {}
     connection = None  # is not None, if connected
     defaultport = None
@@ -62,11 +63,11 @@ class AsynConn:
             except (ValueError, TypeError, AssertionError):
                 if 'COM' in uri:
                     raise ValueError("the correct uri for a COM port is: "
-                                     "'serial://COM<i>[?<option>=<value>[+<option>=value ...]]'")
+                                     "'serial://COM<i>[?<option>=<value>[+<option>=value ...]]'") from None
                 if '/dev' in uri:
                     raise ValueError("the correct uri for a serial port is: "
-                                     "'serial:///dev/<tty>[?<option>=<value>[+<option>=value ...]]'")
-                raise ValueError('invalid uri: %s' % uri)
+                                     "'serial:///dev/<tty>[?<option>=<value>[+<option>=value ...]]'") from None
+                raise ValueError('invalid uri: %s' % uri) from None
             iocls = cls.SCHEME_MAP['tcp']
             uri = 'tcp://%s:%d' % host_port
         return object.__new__(iocls)
@@ -80,7 +81,9 @@ class AsynConn:
 
     @classmethod
     def __init_subclass__(cls):
-        cls.SCHEME_MAP[cls.scheme] = cls
+        """register subclass to scheme, if available"""
+        if cls.scheme:
+            cls.SCHEME_MAP[cls.scheme] = cls
 
     def disconnect(self):
         raise NotImplementedError
@@ -166,7 +169,7 @@ class AsynTcp(AsynConn):
             self.connection = tcpSocket(uri, self.defaultport, self.timeout)
         except (ConnectionRefusedError, socket.gaierror) as e:
             # indicate that retrying might make sense
-            raise CommunicationFailedError(str(e))
+            raise CommunicationFailedError(str(e)) from None
 
     def disconnect(self):
         if self.connection:
@@ -237,8 +240,8 @@ class AsynSerial(AsynConn):
             options = dict((kv.split('=') for kv in uri[1].split('+')))
         except IndexError:  # no uri[1], no options
             options = {}
-        except ValueError:
-            raise ConfigError('illegal serial options')
+        except ValueError as e:
+            raise ConfigError('illegal serial options') from e
         parity = options.pop('parity', None)  # only parity is to be treated as text
         for k, v in options.items():
             try:
@@ -251,14 +254,12 @@ class AsynSerial(AsynConn):
             if not fullname.startswith(name):
                 raise ConfigError('illegal parity: %s' % parity)
             options['parity'] = name[0]
-        if 'timeout' in options:
-            options['timeout'] = float(self.timeout)
-        else:
+        if 'timeout' not in options:
             options['timeout'] = self.timeout
         try:
             self.connection = Serial(dev, **options)
         except ValueError as e:
-            raise ConfigError(e)
+            raise ConfigError(e) from None
         # TODO: turn exceptions into ConnectionFailedError, where a retry makes sense
 
     def disconnect(self):
diff --git a/secop/lib/py35compat.py b/secop/lib/py35compat.py
new file mode 100644
index 0000000..fa612ea
--- /dev/null
+++ b/secop/lib/py35compat.py
@@ -0,0 +1,58 @@
+#  -*- coding: utf-8 -*-
+# *****************************************************************************
+#
+# This program is free software; you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free Software
+# Foundation; either version 2 of the License, or (at your option) any later
+# version.
+#
+# This program is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+# FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
+# details.
+#
+# You should have received a copy of the GNU General Public License along with
+# this program; if not, write to the Free Software Foundation, Inc.,
+# 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+#
+# Module authors:
+#   Markus Zolliker <markus.zolliker@psi.ch>
+#
+# *****************************************************************************
+"""workaround for python versions older than 3.6
+
+``Object`` must be inherited for classes needing support for
+__init_subclass__ and __set_name__
+"""
+
+
+if hasattr(object, '__init_subclass__'):
+    class Object:
+        pass
+else:
+    class PEP487Metaclass(type):
+        # support for __set_name__ and __init_subclass__ for older python versions
+        # slightly modified from PEP487 doc
+        def __new__(cls, *args, **kwargs):
+            if len(args) != 3:
+                return super().__new__(cls, *args)
+            name, bases, ns = args
+            init = ns.get('__init_subclass__')
+            if callable(init):
+                ns['__init_subclass__'] = classmethod(init)
+            newtype = super().__new__(cls, name, bases, ns)
+            for k, v in newtype.__dict__.items():
+                func = getattr(v, '__set_name__', None)
+                if func is not None:
+                    func(newtype, k)
+            if bases:
+                super(newtype, newtype).__init_subclass__(**kwargs)  # pylint: disable=bad-super-call
+            return newtype
+
+        def __init__(cls, name, bases, ns, **kwargs):
+            super().__init__(name, bases, ns)
+
+    class Object(metaclass=PEP487Metaclass):
+        @classmethod
+        def __init_subclass__(cls, *args, **kwargs):
+            pass
diff --git a/secop/modules.py b/secop/modules.py
index 35c4d5e..39aeec1 100644
--- a/secop/modules.py
+++ b/secop/modules.py
@@ -29,7 +29,7 @@ from collections import OrderedDict
 
 from secop.datatypes import ArrayOf, BoolType, EnumType, FloatRange, \
     IntRange, StatusType, StringType, TextType, TupleOf
-from secop.errors import BadValueError, ConfigError, InternalError, \
+from secop.errors import BadValueError, ConfigError, \
     ProgrammingError, SECoPError, SilentError, secop_error
 from secop.lib import formatException, mkthread
 from secop.lib.enum import Enum
@@ -57,6 +57,7 @@ class HasAccessibles(HasProperties):
         merged_properties = {}  # dict of dict of merged properties
         new_names = []  # list of names of new accessibles
         override_values = {}  # bare values overriding a parameter and methods overriding a command
+
         for base in reversed(cls.__mro__):
             for key, value in base.__dict__.items():
                 if isinstance(value, Accessible):
@@ -66,8 +67,6 @@ class HasAccessibles(HasProperties):
                     accessibles[key] = value
                     override_values.pop(key, None)
                 elif key in accessibles:
-                    # either a bare value overriding a parameter
-                    # or a method overriding a command
                     override_values[key] = value
         for aname, aobj in accessibles.items():
             if aname in override_values:
@@ -244,12 +243,12 @@ class Module(HasAccessibles):
 
     # reference to the dispatcher (used for sending async updates)
     DISPATCHER = None
-
     pollerClass = Poller  #: default poller used
 
     def __init__(self, name, logger, cfgdict, srv):
         # remember the dispatcher object (for the async callbacks)
         self.DISPATCHER = srv.dispatcher
+        self.omit_unchanged_within = getattr(self.DISPATCHER, 'omit_unchanged_within', 0.1)
         self.log = logger
         self.name = name
         self.valueCallbacks = {}
@@ -444,23 +443,20 @@ class Module(HasAccessibles):
         pobj = self.parameters[pname]
         timestamp = timestamp or time.time()
         changed = pobj.value != value
-        if value is not None:
-            pobj.value = value  # store the value even in case of error
+        try:
+            # store the value even in case of error
+            pobj.value = pobj.datatype(value)
+        except Exception as e:
+            if not err:  # do not overwrite given error
+                err = e
         if err:
-            if not isinstance(err, SECoPError):
-                err = InternalError(err)
+            err = secop_error(err)
             if str(err) == str(pobj.readerror):
                 return  # do call updates for repeated errors
-        else:
-            try:
-                pobj.value = pobj.datatype(value)
-            except Exception as e:
-                err = secop_error(e)
-            if not changed and timestamp < ((pobj.timestamp or 0)
-                                            + self.DISPATCHER.OMIT_UNCHANGED_WITHIN):
-                # no change within short time -> omit
-                return
-        pobj.timestamp = timestamp
+        elif not changed and timestamp < (pobj.timestamp or 0) + self.omit_unchanged_within:
+            # no change within short time -> omit
+            return
+        pobj.timestamp = timestamp or time.time()
         pobj.readerror = err
         if pobj.export:
             self.DISPATCHER.announce_update(self.name, pname, pobj)
@@ -495,15 +491,15 @@ class Module(HasAccessibles):
         for pname in self.parameters:
             errfunc = getattr(modobj, 'error_update_' + pname, None)
             if errfunc:
-                def errcb(err, p=pname, m=modobj, efunc=errfunc):
+                def errcb(err, p=pname, efunc=errfunc):
                     try:
                         efunc(err)
                     except Exception as e:
-                        m.announceUpdate(p, err=e)
+                        modobj.announceUpdate(p, err=e)
                 self.errorCallbacks[pname].append(errcb)
             else:
-                def errcb(err, p=pname, m=modobj):
-                    m.announceUpdate(p, err=err)
+                def errcb(err, p=pname):
+                    modobj.announceUpdate(p, err=err)
                 if pname in autoupdate:
                     self.errorCallbacks[pname].append(errcb)
 
@@ -516,8 +512,8 @@ class Module(HasAccessibles):
                         efunc(e)
                 self.valueCallbacks[pname].append(cb)
             elif pname in autoupdate:
-                def cb(value, p=pname, m=modobj):
-                    m.announceUpdate(p, value)
+                def cb(value, p=pname):
+                    modobj.announceUpdate(p, value)
                 self.valueCallbacks[pname].append(cb)
 
     def isBusy(self, status=None):