XMODEM

X-Modem:

Offset            Contents
0                    SOH = Ascii 01
1                    Block #, start with 1, wrap to 0 after FF
2                    Ones complement (XOR with 0xFF) of block #
3-130            128 bytes of data
131                 CRC Checksum of bytes  (carry ignored)

• Delphi (Pascal) code for XMODEM version 16 BIT CRC

Remember also that the 'window size' is 1. An ASCII ACK must be sent to the sender before the next packet will be sent. If the checksum doesnt match, a NAK must be sent. After 10 NAKs, the transfer should abort.

Archive:

• PICList post "X-modem protocol"

See also:

• http://www.geocities.com/westfw/ymodem.txt X / Y MODEM protocol references.