Xmodem protocol in MacTerminal supports checksum error checking, and so
you should set Xon/Xoff to OFF if you're using Xmodem. If an Xon/Xoff
handshake occurs while if Xon/Xoff is on, the checksum received at the
end of the block will not match the calculated one; this situation
generates a request to resend the block, which causes the receiver
to receive duplicate text.