Dr. Scott M. Baker's Raspberry Pi-Powered IOC Board Upgrades a Whizz-Bang ISIS-II Multibus System
Intel's vintage 80/24A single-board computer gets a companion board delivering scads of RAM, ROM, and floppy disk emulation to boot.
Engineer and vintage computing enthusiast Dr. Scott M. Baker has been hard at work turning an Intel 80/24A single-board computer into an MDS-80 Series II compatible development system dedicated to the ISIS-II operating system — complete with a Raspberry Pi-powered upgrade board.
"I’ve had a fascination with ISIS-II and Multibus ever since a year or so back when I got my hands on an iPDS Personal Development System," Baker explains. "ISIS-II was a fun OS to play with, and I spent some time making speech synthesis and music multimodules and writing software to use them on the iPDS. However, the expandability of the iPDS is somewhat limited, and it’s cumbersome to pull boards in and out of the iPDS when experimenting. I sought out a platform that would give me more versatility and expandability while still letting me run either ISIS-II or CP/M. That platform the is the MDS-80 series of development computers, and those computers used Multibus."
Targeting Intel's 8080-family CPU, Baker set about tracking down an Intel 8085-powered 80/24A single-board computer — and while, as the name implies, that would be enough to get started, more memory was required to run either ISIS-II or the better-known CP/M operating systems. Picking up an original 128kB board, Baker then turned his attention to an input/output controller (IOC) board — opting to build his own feature-packed version rather than using original hardware.
"Rather than using an [Intel] 8080 CPU for the IOC, I used a Raspberry Pi [Zero], and rather than using a physical floppy drive, I served floppy images from the Raspberry Pi's internal SD Card," Baker explains. "I want to stress that the primary CPU in the vintage computer is still the 8085 on the 80/24A board, and all of the original peripherals (serial port, parallel ports, counter/timers, interrupt controller, RAM/ROM, etc) are still there. It’s only the IO controller that uses a Raspberry Pi, and it only does this to provide a disk interface."
The resulting board not only acts as a floppy disk emulator, but also delivers an eight-bit parallel interface, up to 512kB of system RAM, up to 512kB of flash memory selectable in 64kB chunks via on-board DIP switches, and two "multimodule slots" for hardware expansion.
The project is documented in full on Baker's website; design files for the custom IOC board and the Python software to run it are available on GitHub under an unspecified open source license.