I really, really like the direction you're proposing to go in here. Am with you all the way.

Everything worked out great. I haven't even hooked up the blanking signal, but the picture is already fantastic. It'd probably look even better on an old analogue VGA monitor, but I don't have one to test it.

I've used Kicad for all my projects, and either Seeed Studio or Itead for PCBs... https://pcbshopper.com/ tells you who's the cheapest including shipping.

After fixing a few bugs and implementing some more ACSI commands (such as MODE SENSE page code 0) I can get it to recognise the partitions are there and even assign drive letters to them. I can't use the drives yet because there's a problem in my SD card handling routines. So I have 2 tasks to do no...

Note that on TOS 1.0x you have to first select an existing drive icon (e.g. floppy drive) by clicking on it before the menu option becomes active. ...well, that's clearly a ridiculous UI decision! Unfortunately I wasn't able to test this again tonight because I've done tons of refactoring in the me...

Do you say that microcontrollers interrupts are not quick enough to handle this protocol? Yup, at least for an ATMega328. For example, the commands are sent using a series of short pulses 250nS in length. It's necessary to toggle an output pin within 30nS of the beginning of that pulse, then read 9...

At first glance I'd agree that this looks like a proper root sector. So, just to avoid any misunderstanding: If you reboot with this harddisk, then load AHDI (from floppy), you don't get any hard disk drive letters C: etc.? (Note that with some TOS versions they might not appear automatically as ic...

Yeah, it's not the ACSI FORMAT command I'm really talking about here (which does nothing in my implementation) but the bad blocks check, which writes and then reads every sector of the drive. I checked the first 8 sectors and they have stuff in them that looks like a TOS partition table in a hex edi...

Thanks @czietz. I guess that means there's something wrong with the data on the disk itself.

So, I think I got this all pretty close to working... I still need to speed up the SD card transfers because there's a 3-second timeout on ACSI transfers, after which the operation gets aborted. And of course the AHDI drivers insist on doing 128-sector bursts which is too many for most of my SD card...