--- protocol/calc.pl	2007/04/22 22:50:42	12
+++ protocol/calc.pl	2007/04/22 22:56:35	13
@@ -25,4 +25,7 @@
 	printf("# %-2d '%s' %2d %02x -- %2d %02d\n", $i,$c,$o,$o,$sum,$sum);
 }
 
-printf("checksum: %s == %02x\n", $orig, $sum);
+my $xor = 0;
+map { $xor ^= ord($_) } split(//,$p);
+
+printf("checksum: %s == %02x [xor=%02x]\n", $orig, $sum, $xor);