If I only want to support the CRC version of XMODEM, the receiver begins by sending an ASCII "C" instead of NAK, and then switches over to using NAK once underway if there is a failed packet.
The question is when *exactly* does this switchover happen?
If the very FIRST Packet fails its crc, does it get a NAK? or does the receiver respond with a "C" until it gets its first good packet? I ask because if the first packet (or something) was received, but it wasn't good, will NAK confuse the sender into thinking it should do checksum instead of CRC?