Core Memory Display


My brother is a computer programmer, and for his birthday I wanted to get him a piece of computer history. We both really enjoyed the Computer History Museum in California with their many displays of esoteric data storage devices. While core memory is not strange or complex, it looks cool, and was much easier to find than mercury delay line memory (not to mention safer).

For this project, I created a museum display for both its presentation and protection.

The particular core pictured here was sold as a Soviet memory plate originally from a Saratov2 (PDP8 clone I read). It contains 4096 bits. Naturally there were not any off-the-shelf parts that fit this perfectly, so I designed and fabricated the display from scratch.

Core memory works by storing the data in the magnetic field of small toroids of ferrite material. Each core stores one bit by flipping its magnetic field polarity. A series of wires woven through a grid of these cores act to read and write these magnetic fields. Reading is done by changing the field to a known state, and using a sense wire to see if the field flipped or stayed the same. This, however, also sets the bit back to zero, so it has to be immediately re-written. To write, it is assumed the bit is at zero, and a reverse current is used to set the bit to 1. An inhibit line is used (sometimes the same wire as the sense line) to help write a zero.

Design and Fabrication

It was important that the display setup be unobtrusive, while also protecting it enough to be handled. Getting up close and inspecting the details is part of what makes this so interesting. There was also the thought that perhaps one day someone might want to power this up, and it would be nice to not remove it from the display. Since the frame already had mounting holes, it seemed logical to re-use those.

The whole thing was designed in CAD. While it’s very simple, I was planning to use several computer controlled machines which would benefit from having the model geometry in the computer. I also made use of 3D renderings to judge materials and proportions before building.

The clear panels were laser cut from cast acrylic. I would have preferred tempered glass, but was limited in the tools I had to properly cut that. I spent some time looking for commercial threaded standoffs, but never found exactly what was needed. I decided making them would take less time, and it did. It was important to be precise with the lathe work, especially for the decorative screws. I had to grind my own flat-bottom drill bit for the decorative screw caps so the hole would be deep enough for the threads.

The base was CNC cut from a solid piece of maple. Everything was then sanded and polished to a shine. I also built a custom shipping/storage box, so it had a place to live when not on display.