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copied!<p>Deleted my other "answer" about the CRC because I don't think that is the problem. Dan mentioned in a comment that he got the code from <a href="http://trackday.cc/b2evo/blog2.php/2007/08/02/net-xmodem" rel="nofollow">http://trackday.cc/b2evo/blog2.php/2007/08/02/net-xmodem</a>. The code pasted in the original question comes directly from that source.</p> <p>I downloaded it and started looking it over. It has a lot of flaws(*), mostly non-fatal. However, this part jumped out at me in the XModemReceive method:</p> <pre><code>#region fill packet with datastream xbuff[0] = (byte)c; for (i = 0; i < (bufsz + (useCRC ? 1 : 0) + 3); i++) { xbuff[i + 1] = (byte)this.port.ReadByte(); } #endregion </code></pre> <p>If SerialPort.ReadByte encounters End Of Stream it will return -1 (i.e. 255 in byte form). If ReadByte had timed out, a TimeoutException would occur. If EOS is encountered, the correct course of action might be to abort the current packet. I have not spent any time thinking that through.</p> <p>First I would alter that code just to test if this is in fact the problem you're seeing:</p> <pre><code>xbuff[0] = (byte)c; for (i = 0; i < (bufsz + (useCRC ? 1 : 0) + 3); i++) { int byteValue = this.port.ReadByte(); Debug.Assert(byteValue >= 0, "byteValue >= 0"); xbuff[i + 1] = (byte)byteValue; } </code></pre> <p>That is not a fix, just a test. If it is asserting (in a Debug build), than we can think about fixing it.</p> <p>(*) I <a href="http://tergiver.wordpress.com/2011/03/13/whats-wrong-with-this-code/" rel="nofollow">blogged</a> about the first few 'flaws' if you're interested in reading about them.</p>

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