Backstory
When I was very young my mum had a Power Macintosh 8500, which she used for her desktop publishing work. Eventually, as with the, I believe, Quadra that preceded it, the time came that the Power Macintosh was replaced and she switched to a Power Mac G4. That was just around the time that I was old enough to start being interested in computers. My parents would sit me in front of that Power Macintosh that we still had, and let me play various games. I remember with fondness the ‘Lemmings’, ‘Fury of the Furries’, a bunch of what I now believe was Dutch educational games, and the beginnings of ‘online gaming’, which then was browser games running on Internet Explorer 5.
However, this is not the point of this story. The point is that the Power Macintosh came with the Apple Extended Keyboard II, which, only years later, I learnt, is still regarded as one of the best mechanical keyboards ever made. It was the Apple equivalent of IBM Model M. This keyboard has been at my parents’ all these years, and even though the plastic yellowed with age, it still works with that old Power Macintosh and other old Macs.
I decided it would be nice to make it useful, and get it running again, this time with modern hardware. Once I gave it a retrobrite treatment (in my case just some hydrogen peroxide in the form of a hair bleach cream) I decided to look into my options of transforming it to USB: an obvious choice was one of the ADB to USB adapters, but as they can get quite pricey, I decided to make one myself, and maybe learn a thing or two in the process.
It is not something that hasn’t been done before either. There is a variety of bigger and smaller projects accomplishing the same thing, and more, with Arduinos, Teensies, etc. I the end I decided I wanted to go the hardcore route and write the code myself. For the development platform I selected the STM32 board called the ‘Blue Pill’, as it was very cheap, and had many GPIO pins (which is something I am planning to use in an upcoming project).
In this post however, I want to solely focus on giving the specification of the ADB protocol. The reason for this is that a lot of the information for it I found scattered around the web, and I would have loved to find one comprehensive, but not too waffly summary of it. In the end, most of the information that was useful to me came from Apple’s Guide to the Macintosh© Family Hardware, hereafter referred to as Apple’s Guide. However even in there, there is some lack of clarity regarding how service requests are issued, for example.
For the code I refer to my GitHub project, stm32-adb2usb.
ADB: An Introduction
Apple Desktop Bus is a serial connection that allows chaining multiple devices, utilising a single bus, not unlike I²C. It is uniquely suited towards human input devices such as mice and keyboards, as well as tablets and trackballs, where already the design considerations behind it are such that user input is responsive and snappy. Capable of transmission speed of 10 kb/s, although in reality slower because of a shared bus and polling frequency of the host.
There is one host which drives the bus and issues commands to the devices. Besides one special case, the short period when a device can assert a request signal, the devices are not allowed to use the bus unprompted. The bus when unused is pulled up to 5V high, and each of the devices has the ability to pull it low.
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