Harneel - rather than pasting the m file in your question, just attach it to your question using the paperclip button. Or just copy and paste the verbinden_Callback callback. There is nothing wrong with the callback but there is a problem with the serial port as it seems to still be available even though you have closed it. You may have to delete it as described at a similar question http://www.mathworks.com/matlabcentral/answers/9509-serial-port-is-not-available-in-gui-but-it-actually-available-in-my-pc. Try changing your code to the following
function verbinden_Callback(hObject, eventdata, handles)
% Kraftmesser initialisieren
kraftmesser = instrfind('Type','serial','Port','COM1','Tag','');
if ~isempty(kraftmesser)
fclose(kraftmesser);
delete(kraftmesser);
end
kraftmesser = serial('COM1');
% Steuerung initialisieren
steuerung = instrfind('Type','serial','Port','COM2','Tag','');
if ~isempty(steuerung)
fclose(steuerung);
delete(steuerung);
end
steuerung = serial('COM2');
fopen(steuerung);
fwrite(steuerung,'#1g=+1/r');
fclose(steuerung);
What happens when you now run your GUI? Also review your code for the COM1 serial port. It isn't clear why you need it since you never actually try to write anything to it.
