0. Description

This file describes the protocol of the HP 49G flash upgrade system.
It *IS* not complete so it may be that the upgrade utility does not
work with future ROM versions.

1. What's going on

After setting the calc to "Terminal mode" there must be send a sequence
of bytes to the calc to switch to the "Download menu".

When pressed 1 for "Download System" the calc first erases the old system
and then starts the download by requesting the system files.

2. Setting the calc to "Download menu"

Simply send the bytes

02h B8h 03h 00h

to the calc and it will show the "Download menu".

3. System file request

The calc sends

"G" + 00h + name length + requested file name

to get a file. The "remote side", which is the PC, now must send a

06h

to acknowlegde the request and the existence of that file.

4. The Download

The calc then starts a xmodem request with "HP CRC" which I do not
know. After four tries it falls back to "xmodem-1K" which is 
implemented in the GNU sx-program which I used for this project
and the download starts. I hope that HP will not remove the
standard xmodem protocol for ROM uploads - otherwise this will
not work anymore.

That is it! Not that magically, isn't it? ;-)
