Dr. Scott M. Baker Builds a New Storage Card for the Heathkit H8 — Using 1970s Bubble Memory

Engineer Dr. Scott M. Baker is once again upgrading his classic Heathkit H8 microcomputer, but this time it's really more of a side-grade — adding vintage bubble memory for non-volatile storage, in place of a modern CompactFlash card interface.

"Bubble memory was a technology of the 1970s, perhaps enduring a slightly longer life cycle in the industrial machinery space, where machinery vibration was particularly problematic for storage devices like floppy disks or hard disks," Baker explains.

"Bubble memory makes use of special magnetic patterns deposited on a substrate such as garnet, to be able to migrate magnetic 'bubbles' in a loop configuration. You can rotate the bubble you’re interested around the loop and the duplicate it for reading, or you can swap in a different bubble for writing. Magnetic bubble memory is a non-volatile memory and will preserve its contents on power-off."

To make use of bubble memory technology, Baker designed a new add-on card for his kit-form Heathkit H8 microcomputer — a device launched back in 1977 and designed to run Digital's CP/M operating system on an Intel 8080A processor with a static RAM (SRAM) card offering a meager by modern standards 4kB of memory. Baker's newly-designed bubble memory board, though, isn't designed to replace the system's RAM but instead offer non-volatile storage so programs and data aren't lost on power off — replacing the paper tape or cassette storage an original H8 was likely to use.

"Craig’s SBC-85 design provided me with almost everything I needed to start with," Baker explains, referring to an earlier deep-dive bubble memory project by Craig Andrews which served as inspiration for Baker's own design. "There’s the schematic of his SBC-85 board itself, as well as 8085 assembly code for reading and writing sectors and exercising other commands on the bubble memory controller. My tasks were the following: interface the bubble memory controller to the H8 Bus; write a storage driver for HDOS, so the bubble memory could be used as a disk; add support to Douglas Miller’s Newmon monitor for the H8, so that bubble memory could be booted directly. That's about a weekend’s worth of tinkering…"

Baker's Heathkit has been enjoying something of an overhaul over the past year, starting with the creation of a custom add-on board granting the microcomputer the power of speech. This was followed by a means of interfacing a modern Raspberry Pi with the machine, then a custom memory board design — which crammed 16MB, and potentially up to 32MB, of RAM into a design built for a 4kB board. And now, it boasts 1Mb (128kB) of bubble memory storage — using period-appropriate hardware, unlike modern CompactFlash-based alternative storage boards.

More information on the project is available on Baker's website, though the schematics, hardware design files, and software have not been made available publicly while the most recent board revision undergoes final verification.