Okay. Please excuse the mediocre photos, but the hardware promises to just get less accessible for a while and there isn't much time for retouching.
All the converters in the OP are using an ATmega32U4 as the MCU. Most use LUFA to simplify programming its USB functionality.
The earlier version of this effort, ten years ago, used one of the first PICs to have a USB stack on the chip. But without even in-circuit programming. Here is that board on the left and the newer dev board on the right, using an Arduino Micro. As you can see, without the optional LEDs and switches to select keyboard model, there is nothing other than connectors.
- LMKBD & LMKBD2 dev boards
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Most are in old reliable Hammond black ABS boxes, rather than anything fancy with milling or 3D printing.
- Keyboard converters
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- Keyboard connectors
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The Model M is Soarer's Converter on a board with the various different connectors IBM used over the years for essentially the same protocol.
The ADB interface is just TMK's. If I had a
MacIvory keyboard template, which put bucky bits on the arrow keys, I might need more than it. But I don't.
The Sun Type 4 is a custom interface designed for simplicity. There are several others out there, but they either don't use a high-level USB library like LUFA or bit-bang the serial interface. By just adding an inverter, the MCU's UART can do it, since it's chip levels, only inverted like in RS-232.
The 5251 interface can be worked out entirely from the service manual, which is in Bitsavers. The hard part of the design is the voltages. The signals are +5V, but the MOS controller itself needs -5V. I added an isolating DC-DC converter (with a filter cap for good measure). The solenoid (!) needs +8.5V at higher current than USB is gonna give. I just added an external power adapter (from a game controller?).
The Symbolics is straightforward, fitting the RJ jack into the case being about the hardest part.
The Space Cadet required getting the 8748 back in working order, since they keep a charge for years, not decades. The microcode, source and binary, are online. Back in '95, I used a parallel port programmer (from a swap-meet or someplace like that) with a Pentium that was already too fast to be entirely reliable.
- Ribbon cable to converter
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- Converter placement
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The jumpers out the back are reset and ground, so I can enter the bootloader without opening the case.
I wasn't able to repeat the 8748 feat more recently when the other Space Cadet here, without a case, had lost its program (due to getting more light in, I assume). So I had to resort to a direct interface to it, with the AVR doing the scanning and USB. Cheating, I know, but it's going to need a new case anyway, so it's slipping away from original.
- Direct connection
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The Knight keyboard is all 74-series TTL, so it would be possible to buzz out the circuit. But, fortunately, the Space Cadet protocol is actually backwards compatible with it. So it was just getting the timing of the shift register right. The remaining hard part was tracking down an Amphenol mini-hex connector, which wasn't possible after it was obsolete and before eBay had taken off.
- TK converter
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- Cooper Interconnect 126-221
- Mini-hex.JPG (132.79 KiB) Viewed 12219 times
CONVERTED...?!??!? 
