user31105: Last week I retried some of my Arduinos and one Mega was working without a problem, but two Uno's didn't work anymore. The error I got was something like:
avrdude: stk500_getsync(): not in sync: resp=0xyy 

Where yy where all different kind of values (I cannot copy/paste it as I'm not near the Arduino's).
Does anybody know where to find a document what the response values mean?

user4143: They don't mean anything.
AVRDude sends a packet, and expects a certain response. When it gets something that it's not expecting it tells you it's "out of sync", and tells you what the actual response was it got. That is, what byte it received over the serial port.  That could be something that your sketch is sending, or complete garbage if you're communicating with a floating serial input, or anything at all.

31105: <p>Last week I retried some of my Arduinos and one Mega was working without a problem, but two Uno's didn't work anymore. The error I got was something like:</p>

<pre><code>avrdude: stk500_getsync(): not in sync: resp=0xyy 
</code></pre>

<p>Where yy where all different kind of values (I cannot copy/paste it as I'm not near the Arduino's).</p>

<p>Does anybody know where to find a document what the response values mean?</p>
 4143: <p>They don't mean anything.</p>

<p>AVRDude sends a packet, and expects a certain response. When it gets something that it's not expecting it tells you it's "out of sync", and tells you what the actual response was it got. That is, what byte it received over the serial port.  That could be something that your sketch is sending, or complete garbage if you're communicating with a floating serial input, or anything at all.</p>
