Craig Andrews Brings Back Magnetic Bubble Memory with a 1Mb SBC-85 Add-On Memory Module

UPDATE2020-04-20:

Andrews has successfully debugged the initial issues with the bubble memory expansion board, posting in a project update that "the problem was not hardware related but there was simply an issue in my initialisation routine."

Original story continues below.

Craig Andrews, creator of the SBC-85 Intel 8085-based 8-bit single-board computer, has dug deep for his latest add-on module: a 1Mb memory expansion based on magnetic bubble memory technology.

"Anyone involved in computers in the early 1980s could not have avoided hearing about magnetic bubble memory. This non-volatile storage promised very high storage density but it's robustness and essentially zero error rate was its real claim to fame," Andrews explains of the technology's attraction. "By 1980 pretty much everyone but Intel had given up on the hope for low cost, large scale, non-volatile magnetic bubble memory systems. By the mid 1980s interest was waning and by 1990 bubble memory had completely become a thing of the past. Of course 2D magnetics & skyrmions are all the rage again."

Unlike traditional dynamic RAM (DRAM), which holds memory contents as electrical charges needing constant refreshing, bubble memory works more like a traditional hard drive by storing data as magnetic charges. Initially developed by Andrew Bobeck and Paul Charles Michaelis in the 1960s, bubble memory was seen as a far more reliable replacement for core-rope memory — but Intel's 7110, a 1Mb implementation which forms the heart of Andrews' add-on board, was released in the late 1970s as a commercial flop.

"The SBC-85 1Mbit bubble memory board requires buffering, address decoding, a very accurate 4MHz clock, and some power fail circuitry. These components are located along the bottom portion of the PCB," Andrews notes. "With the decoding and support provided, the remaining design of the SBC-85 1Mbit bubble memory board is based on the BPK-72 bubble memory developer's kit.

"Center stage on the board is the 1Mbit bubble memory device itself which is the Intel 7110 and where the actual magnetic bubbles are found. On the SBC-85 Bubble board this is the largest component in the center top. The Intel 7220 Bubble Memory Controller is the face of the bubble system seen by the outside world. This chip coordinates all actions in side the 7110 bubble memory as well as being the communications device to the system. When writing to memory, the data is written to the 7220 which then stores it in the bubble chip and when reading data the request is made to the 7220. All internal workings of the bubble system are hidden from the user and the interface comes down to communicating with this single chip."

Other support components include an Intel 7250 coil pre-driver, Intel 7254 drivers for the X and Y coils, an Intel 7242 formatter/sense-amplifier, and an Intel 7230 current pulse generator. "The SBC-85 1MBit bubble board is a 2-layer 100mm x 125mm board with edge connectors for the SBC-85 backplane. There are no other I/O on the board other than through this backplane connector. All communication with the board is via a two I/O ports defined by the onboard SIP switch."

Though the hardware design is finalised, Andrews is currently working on getting the hardware running reliably - which is proving tricker than anticipated. In particular Andrews is hitting underflow errors when writing to the bubble memory, and has asked for assistance: "Anyone familiar enough to with the intel 7110 and 7220 bubble system to be able to give advice?"

More information on the board can be found on Andrews' project page, while details on the SBC-85 itself can be found on its dedicated website.