Xmodem is the simplist of the hyperterm-compatible file upload protocol.
For Xmodem, divide your data into block of 128 bytes each.  For each block
send SOH <blknum> <NOT blknum> <checksum>  Wait for an ACK to be received
before sending the next block, resend the block if you get a NAK.  Terminate
the file by sending just an EOF instead of a new block.

Ymodem adds some filename control and upgrades the checksum to a CRC, and
some versions allow for a larger blocksize.

Zmodem adds sliding windows for full bandwidth utilization.

Basic source code implementing Xmodem sent under separate cover...

BillW