diff --git a/raspberry/roberto/Serial.py b/raspberry/roberto/Serial.py
index 04baecd..b7124bb 100644
--- a/raspberry/roberto/Serial.py
+++ b/raspberry/roberto/Serial.py
@@ -54,8 +54,8 @@ class Serial(object):
                     command = data[0]
                     argument = data[2:].strip()
                     if command and command in self._callbacks:
-                        cb = self._callbacks[command]
-                        if cb:
+                        callbacks = self._callbacks[command]
+                        for cb in callbacks:
                             cb(argument)
                     else:
                         print("no callback found for command '%s'" % command)
@@ -66,8 +66,10 @@ class Serial(object):
             #raise
 
     def add_callback(self, cmd, callback):
-        self._callbacks[cmd] = callback
+        if not cmd in self._callbacks:
+            self._callbacks[cmd] = []
+        self._callbacks[cmd].append(callback)
 
     def del_callback(self, cmd, callback):
         if cmd in self._callbacks:
-            del self._callbacks[cmd]
+            self._callbacks[cmd].remove(callback)