Corne - in the line of code where you assign the callback, you need to make clear that another parameter is to be pased in. This other parameter can be whatever you want, but usually for GUIs, I pass in the handle to the figure/GUI. So your callback assignment would become
handles.s.BytesAvailableFcn = {@Serial_int_Fcn, handles.figure1};
and your callback function signature would become
function Serial_int_Fcn (hObject, eventdata, hGui)
handles = guidata(hGui); % get the updated handles structure
% your code
end
The handles structure doesn't automatically get passed in as the third parameter to the callback...you need to pass this in. While we could pass in handles like
handles.s.BytesAvailableFcn = {@Serial_int_Fcn, handles;
the problem is that handles is a copy at the time you do this callback assignment and so will not have the updates that any other functions/callbacks might do against it (in case you are using it to store local data).
