> $?? $A1 $A1 CRC CRC $F7(!) Wasn't the CRC initialized with a special value? AFAIR it was $CD $B4. By the way, the byte $xx after the CRC ($F7 $xx) is written in any case (e.g. $F5, $F6 are not changed to sync bytes ($A1, $C1)), I forgot to mention that.