xmodem problems

OK, so I hacked a quick 'n dirty xmodem implementation together in RSI BASIC on the QX10 so that I could transfer a few essentials over via the serial link. It worked on a few files, but I've found a couple of problem files where it will consistently block when trying to send an ACK back to the sender following a packet receive - in one file it's on packet 76 every single time, while in the other it's always packet 161 (it receives the complete packet OK, checksum is valid, packet sequence numbers are all good, but it stalls writing to the serial port). Any thoughts? I can't think of a reason why it should work fine on some files, but barf consistently - and at different points - with these other two. I did wonder if there was a particular byte encountered in these packets which hadn't been seen previously in these files (or any of the others I've sent successfully) which was somehow tripping up the underlying serial code on the QX-10, but that's not the case. I suppose there might be a particular magic *sequence* of bytes which is received and causing problems, but that's harder[1] to check for. Oh, I've disabled the disk-write code on the QX-10 just to rule that out, too; it still fails even when it's just tossing received data away. [1] hmm, although I could just zero out the problem packets and see if that stops it happening, I suppose. Will report back. cheers Jules