zmodem sendcmd

[Vance Shipley]

How do you send a file when using screen as a zmodem endpoint?

I want to download a new BIOS to my net4801 embedded computer.
My workstation is a MacBook and I have a USB serial adaptor.
To talk to the console port of the net4801 I use screen inside
a Terminal window.

        $ screen /dev/tty.PL2303-0000101D

As I understand it if I wanted to receive a file from the net4801
I would enable zmodem catch mode and initiate the file transfer 
from the other end (e.g. 'sz -b foo.bin).  Screen would then 
recognize the zmodem protocol and use the command defined by
'zmodem recvcmd' to receive the file locally.

I see zmodem recvcmd set to: !!! rz -vv -b -E

This syntax matches that of the exec command in screen.  I then
assumed that I could run it the other way and send a file by
using 'exec !!! sz -vv -b -X foo.bin' however I have not been
able to get that to work.

I see zmodem sendcmd is set to: !!! sz -vv -b

I don't understand the intention of this.  Would I start the
transfer at the other end with 'rz foo.bin' and screen would
recognize the zmodem protocol and use the command defined by
'zmodem sendcmd' to send the local file foo.bin?  That is about
the only way I could understand it working.

To further complicate things I'm actually trying to use XMODEM
as that is what the net4801 requires.  Maybe some of my problems
are because the zmodem handling in screen doesn't understand 
XMODEM mode.  In any event I don't understand the intention of
zmodem sendcmd.

Thanks,

        -Vance