The full reference manual is available online

Anyhow, I had trouble getting anything out of the 232 port, it gets stuck in a boot loop and never makes any sign of life on serial. This needs hardware handshake and I even probed RTS and CTS with the scope to make sure my USB converter does support hardware handshakes

Whatever boots on there just doesn't chatter on serial. However, these things have quite the capable debug console that you can activate by triggering reset while asserting abort

Success!

And that's where I got stuck for today: trying to get atftpd running on modern Linux. Something is broken.

Now, I do have a box full of VME goodies I never did anything with because I lacked a backplane and power supply. Now that I do have those... In that box is a pile of OS9 reference manuals and a couple of GPUs for the VME bus. Are you thinking what I am thinking? 🐧

Wow, it's surprisingly difficult to find a TFTP server that actually works.
atftpd seems to be completely broken under current *buntu, as is tftpd-hpa. I tried https://pypi.org/project/ptftpd/, which does not handle write requests properly. Reading the code, it does not keep track if the client is currently writing to a file and assumes the client wants to overwrite the just created file after the first packet. Seems like the author never even tested write requests.

This one https://github.com/sirMackk/py3tftp works, but only for a limited number of packets, after which it just gives up. I did not read the source here. Limiting myself to 512k chunks at a time did the trick.

Dumping the boot ROM, 512k at a time.

It does get a lil' tedious

If anyone wants to have a look, here are the dumps. Including the floppy image and the single file on the floppy.