Help for Ymodem

This protocol is a variation on Xmodem.  It transmits data 1n 1k
(1024) or 128 byte blocks, transfers the filename and date and time,
and allows multiple files to be sent per transfer.

Because of the need to wait for acknowledgement after each block,
Ymodem is capable of approximately 90-95% the maximum theoretical
transfer rate when using 1k blocks.