"Worse, I'm already kind of dissatisfied with the iMate - it doesn't distinguish left and right for Alt/Shift/etc."<p>This may be a limitation of the keyboard itself. I remember looking into low level keyboard remapping over a decade ago in OS X (via ~/Library/Keyboards), and I have a vague memory of reading what was at the time an ancient piece of Apple reference material that some keyboards do not distinguish between left and right modifier keys.
I use an iMate to connect my Extended II to my Powermac G4. I recently replaced the battery to be able to turn on the computer. I appreciate the traces, but I actually thought this was all well known. I use the G4 every day to make music and for recreational programming. I've also got an older Powermac which I enjoyed more precisely because of the Extended II[0], but the G4 is much faster so getting the iMate working was great.<p>Also, on Mac OS 9, the iMate doesn't just allow the use of ADB keyboards, but as far as I know actually acts as a proper ADB bridge, so dongles, modems[1] and other ADB peripherals work as well.<p>[0] It's like having sunshine in your fingers!
[1] Yes, there were a few ADB modems, and yes, they were slow.
I have an iMate (admittedly I haven't hooked up my Apple Extended II in a few years), but I think the writing is on the wall. I ended up buying a modern adapter from some site called tinkerBOY but I just realized I never even tried it yet.<p>Personally, for me, the primary attraction is the feel of the old Alps switches. I don't want to dedicate my desk to some battleship (ok nerds, I know that "battleship" keyboards are even bigger). And I can get that from modern switches in a more manageable chassis. I currently have the "Zilent" switches from Zelio which give me good tactility without too much clickiness.
Regarding the astonishing flexibility of USB -- I've got a 1994 vintage keyboard / mouse combo (lexmark/IBM M4-1, compact keyboard + trackpoint) -- I use it regularly using a USB to PS/2 adapter connected to a kvm switch built into a 2010 vintage monitor (usb 2) that's connected to a USB-C hub attached to a 2020 macbook's USB-C and a surface pro's port replicator. Once a week when switching inputs or waking up something gets in a state where I have to unplug the keyboard and plug it back in (IBM's typical over-engineering means there's a wacky plug on the keyboard so it's easy to get to). But, "1994 -> 2000 (ps2->usb) -> 2010 (monitor) -> 2020 (macbook)" is a pretty broad stack and it works almost perfectly.<p>I think these are the wacky keyboards where they had the "D" and "K" keys marked with a nub to indicate home keys. Drove me crazy enough that I'd deface them by shaving off the original nub and put a "proper" ridge into the f and j keys.
I tend to switch every few weeks between Apple's current wireless keyboard and the Extended Keyboard II with an iMate attached. I've had the iMate for about 25 years, and I've also been wondering about the battery. Thanks to the author for finding out what it's for! I do have the same gripe as the author regarding the iMate not distinguishing between left and right modifier keys, though.
Anyhow, after all those years, it's indeed amazing how the whole ancient setup still works flawlessly with my current ARM Mac.
Just get yourself a teensy and use this thing: <a href="https://github.com/tmk/tmk_keyboard/tree/master/converter/adb_usb">https://github.com/tmk/tmk_keyboard/tree/master/converter/ad...</a>
I seem to remember depletion-node FETs were available in the late 90s, which could have solved this problem in a more elegant way than a "lifetime" battery.
I've had an excellent experience with <a href="https://www.drakware.com/shop/p/adb2usb" rel="nofollow">https://www.drakware.com/shop/p/adb2usb</a> for a while now, I daily an M5301 at home and an M0116 in office.