diff --git a/secop/client/__init__.py b/secop/client/__init__.py
index f0b54c4..d3c9729 100644
--- a/secop/client/__init__.py
+++ b/secop/client/__init__.py
@@ -145,6 +145,21 @@ class ProxyClient:
         if kwds:
             raise TypeError('unknown callback: %s' % (', '.join(kwds)))
 
+    def unregister_callback(self, key, *args, **kwds):
+        """unregister a callback
+
+        for the arguments see register_callback
+        """
+        for cbfunc in args:
+            kwds[cbfunc.__name__] = cbfunc
+        for cbname, func in kwds.items():
+            cblist = self.callbacks[cbname][key]
+            if func in cblist:
+                cblist.remove(func)
+            if not cblist:
+                self.callbacks[cbname].pop(key)
+
+
     def callback(self, key, cbname, *args):
         """perform callbacks
 
@@ -332,6 +347,8 @@ class SecopClient(ProxyClient):
 
         self._rxthread = None
         self.disconnect(False)
+        if self._shutdown:
+            return
         if self.activate:
             self.log.info('try to reconnect to %s', self.uri)
             self._connthread = mkthread(self._reconnect)
@@ -440,10 +457,11 @@ class SecopClient(ProxyClient):
             self.modules[modname] = dict(accessibles=accessibles, parameters=parameters,
                                          commands=commands, properties=properties)
         if changed_modules is not None:
-            done = self.callback(None, 'descriptiveDataChange', None, self)
+            done = done_main = self.callback(None, 'descriptiveDataChange', None, self)
             for mname in changed_modules:
                 if not self.callback(mname, 'descriptiveDataChange', mname, self):
-                    self.log.warning('descriptive data changed on module %r', mname)
+                    if not done_main:
+                        self.log.warning('descriptive data changed on module %r', mname)
                     done = True
             if not done:
                 self.log.warning('descriptive data of %r changed', self.nodename)
diff --git a/secop/lib/asynconn.py b/secop/lib/asynconn.py
index 5569fe2..8e68624 100644
--- a/secop/lib/asynconn.py
+++ b/secop/lib/asynconn.py
@@ -23,8 +23,9 @@
 """asynchronous connections
 
 generic class for byte oriented communication
-includes implementation for TCP connections
-support for asynchronous communication, but may be used also for StringIO
+includes implementation for TCP and Serial connections
+support for asynchronous communication, but may be used also for
+synchronous IO (see secop.stringio.StringIO)
 """
 
 import socket
@@ -34,7 +35,7 @@ import ast
 from serial import Serial
 
 from secop.lib import parseHostPort, tcpSocket, closeSocket
-from secop.errors import ConfigError
+from secop.errors import ConfigError, CommunicationFailedError
 
 
 class ConnectionClosed(ConnectionError):
@@ -133,7 +134,11 @@ class AsynTcp(AsynConn):
         if uri.startswith('tcp://'):
             # should be the case always
             uri = uri[6:]
-        self.connection = tcpSocket(uri, self.defaultport, self.timeout)
+        try:
+            self.connection = tcpSocket(uri, self.defaultport, self.timeout)
+        except (ConnectionRefusedError, socket.gaierror) as e:
+            # indicate that retrying might make sense
+            raise CommunicationFailedError(str(e))
 
     def disconnect(self):
         if self.connection:
@@ -215,6 +220,7 @@ class AsynSerial(AsynConn):
             self.connection = Serial(dev, **options)
         except ValueError as e:
             raise ConfigError(e)
+        # TODO: turn exceptions into ConnectionFailedError, where a retry makes sense
 
     def disconnect(self):
         if self.connection:
diff --git a/secop/stringio.py b/secop/stringio.py
index adcc67b..cb6bd9f 100644
--- a/secop/stringio.py
+++ b/secop/stringio.py
@@ -74,19 +74,13 @@ class StringIO(Communicator):
         self._conn = None
         self._lock = threading.RLock()
         self._end_of_line = self.end_of_line.encode(self.encoding)
-        self._connect_error = None
         self._last_error = None
 
     def connectStart(self):
         if not self.is_connected:
             uri = self.uri
-            try:
-                self._conn = AsynConn(uri, self._end_of_line)
-                self.is_connected = True
-            except Exception as e:
-                # this is really bad, do not try again
-                self._connect_error = e
-                raise
+            self._conn = AsynConn(uri, self._end_of_line)
+            self.is_connected = True
             for command, regexp in self.identification:
                 reply = self.do_communicate(command)
                 if not re.match(regexp, reply):
@@ -157,7 +151,7 @@ class StringIO(Communicator):
         """send a command and receive a reply
 
         using end_of_line, encoding and self._lock
-        for commands without reply, join it with a query command,
+        for commands without reply, the command must be joined with a query command,
         wait_before is respected for end_of_lines within a command.
         """
         if not self.is_connected:
@@ -170,17 +164,18 @@ class StringIO(Communicator):
                 else:
                     cmds = [command]
                 garbage = None
-                for cmd in cmds:
-                    if self.wait_before:
-                        time.sleep(self.wait_before)
-                    if garbage is None:  # read garbage only once
-                        garbage = self._conn.flush_recv()
-                        if garbage:
-                            self.log.debug('garbage: %s', garbage.decode(self.encoding))
-                    self._conn.send((cmd + self.end_of_line).encode(self.encoding))
                 try:
+                    for cmd in cmds:
+                        if self.wait_before:
+                            time.sleep(self.wait_before)
+                        if garbage is None:  # read garbage only once
+                            garbage = self._conn.flush_recv()
+                            if garbage:
+                                self.log.debug('garbage: %s', garbage.decode(self.encoding))
+                        self._conn.send((cmd + self.end_of_line).encode(self.encoding))
                     reply = self._conn.readline(self.timeout)
                 except ConnectionClosed:
+                    self.closeConnection()
                     raise CommunicationFailedError('disconnected')
                 reply = reply.decode(self.encoding)
                 self.log.debug('recv: %s', reply)
@@ -221,6 +216,7 @@ class HasIodev(Module):
         try:
             self._iodev.read_is_connected()
         except (CommunicationFailedError, AttributeError):
+            # AttributeError: for missing _iodev?
             pass
         super().initModule()