Cameron Kaiser's Sidecar Build Gives the MOS Technology KIM-1 the Character Display It Never Had

Vintage computing enthusiast Cameron Kaiser has built a little upgrade to MOS Technology's KIM-1 microcomputer — giving the classic machine a "sidecar screen" as a display device somewhat better than its stock six-digit numeric LED display.

"[This is] a sidecar screen for my KIM-1 that's large enough to be useful, small enough to be portable, and efficient enough to be powered by my unit's built-in power supply," Kaiser writes of his creation. "Sure, you could hook up a serial terminal like the Silent 700 [but] that means lugging around two units — which seems like a lost opportunity because this particular KIM-1 is completely self-contained with a power supply and I/O [Input/Output] card in a hard plastic briefcase I can simply pick up and take with me."

Released in 1976, the MOS Technology Keyboard Input Monitor 1 (KIM-1) served as a starter kit for the company's 6502 microprocessor — and while originally intended as little more than a quickstart platform for engineers rapidly found a niche with hobbyists eager for a taste of personal computing. The stock machine, though, lacked a little in the niceties we've come to take for granted: its input device was a hexadecimal keypad, and its display six glowing LED digits.

To improve on that latter feature, Kaiser picked up a relatively low-cost 20×4 character-based liquid-crystal display (LCD): the Matrix Orbital LK204-25. "I'm chagrined to admit," Kaiser notes, "as with so many other present-day retrocomputing peripherals, that the LK204-25's onboard microcontroller is substantially more powerful than the KIM-1's 6502."

This overpowered display device came with an RS232 connection, which was then level-shifted to the TTL level required of the KIM-1's general-purpose input/output (GPIO) pins. Kaiser then set about creating a second serial port in software, which could feed data to the display without tying up the machine's built-in serial port.

"I think the LCD adds particular value to the KIM's handful of word and text games, neither of which is the basic unit's strong suit," Kaiser writes. "I might do some other game ports to the LCD; Hunt the Wumpus does a lot of text display on the LEDs and would be a fairly well-suited, if large, undertaking. However, I'm already thinking of upgrading the panel to one of the new LK204-25s.

"This would be more expensive than what I have here, but I'd get the features of the 1.3 firmware, and with a TTL-specific unit I can get rid of the level shifter and have a free 5V and ground line for some other interesting peripheral — and a free LK204-25 with a built-in serial port I can give to something else."

The full project write-up is available on Kaiser's blog, while the source code is published to GitHub under the permissive two-clause BSD license — along with an adaption of Jim Butterfield's Re-Verse game, tailored for the display.