Dr. Scott M. Baker's Heathkit H8 Gets Another Upgrade — to a Kinda-Sorta IBM Clone Running MS-DOS

Dr. Scott M. Baker's vintage Heathkit H8 kit-computer has undergone another transformation, receiving an upgrade which boosts the device's capabilities considerably — delivering a semi-IBM-compatible machine running on Intel's venerable 80168 processor capable of running Microsoft MS-DOS.

"While the board presented here is in the context of being used with the H8 computer and it’s onboard peripherals (H8-4 serial board, front panel, speech synthesizer, etc)," Baker explains of his latest creation, "the board is also a fully capable single board computer in and of itself with onboard serial I/O and even a 5V barrel jack to power it."

The Heathkit H8 launched in 1977 as a kit-computer and trainer for the Intel 8080A microprocessor, offering an affordable and reasonably-expandable system capable of running Digital's popular CP/M operating system. Baker's particular Heathkit H8, though, is rapidly becoming the most-tinkered-with model in history, following a range of upgrade and downgrade projects — including a Raspberry Pi-powered interface board, a custom speech-synthesis board, a bubble-memory storage system, a 16MB RAM board, and an Intel 8008 CPU board which considerably degraded the machine's capabilities in the spirit of "why not?"

His latest creation goes in the opposite direction of the downgrade board, replacing the stock Intel 8080A with an Intel 80186 — using his experience with building a Retrobrew SBC-188 single-board computer, based on the Intel 80188 with its eight-bit data bus, as a basis. "Adapting the SBC-188 design to an 80186 seemed like a conceptually simple task," he explains. "The SBC-188 already had a BIOS, and that BIOS featured serial port emulation of the video display. Perfect for an H8 build."

First, Baker redesigned the SBC-188 to support the 80186's 16-bit data bus — twice the width of the 80188 for which it was designed — through the addition of extra RAM and ROM chips and two bus transceivers. A new interface connects the device to the H8's bus, while a handful of on-board peripherals — including a Dallas 1302 real-time clock (RTC) with storage for BIOS settings, a WD37C65 floppy drive controller, and an IDE bus with CompactFlash card reader for storage.

"The H8’s Panel Monitor (PAM) is an iconic part of the computer," Baker notes of the machine's glowing display and hexadecimal keypad. "The 8080 PAM won’t work on this 80186 board, but fortunately I’m quite adept at writing front panel monitors, having already written them for the 8008 and the Z8000. In this case, I quickly ported the Z8000 assembly over to x86 assembly and had a front panel monitor working in a few hours."

The result is, to our knowledge, the first H8 capable of running MS-DOS — though things are understandably buggy. Microsoft's GW-BASIC, Baker notes, seems to run fine at the command line but won't run in interactive mode; Infocom's Zork series of text adventures work so long as you disable the status bar; but "any DOS game that uses graphics" stands "not a chance" of running.

More information on the project is available on Baker's blog, while the project source code and hardware design files are to be published on GitHub in the near future.