If you want read-only memory today, you might be tempted to use flash memory or, if you want old-school, maybe an EPROM. But there was a time when that wasn’t feasible. [Igor Brichkov] shows us how to make a core rope memory using a set of ferrite cores and wire. This was famously used in early UNIVAC computers and the Apollo guidance computer. You can see how it works in the video below.
While rope memory superficially resembles core memory, the principle of operation is different. In core memory, the core’s magnetization is what determines any given bit. For rope memory, the cores are more like a sensing element. A set wire tries to flip the polarity of all cores. An inhibit signal stops that from happening except on the cores you want to read. Finally, a sense wire weaves through the cores and detects a blip when a core changes polarity. The second video, below, is an old MIT video that explains how it works (about 20 minutes in).
Why not just use core memory? Density. These memories could store much more data than a core memory system in the same volume. Of course, you could write to core memory, too, but that’s not always a requirement.
We’ve seen a resurgence of core rope projects lately. Regular old core is fun, too.
https://hackaday.com/2022/01/13/soviet-era-auto-dialler-uses-magnetic-rope-core-memory/
Probably the best use that existed in civil applications you could actually buy.
Core rope memory strikes me as the most absurd ideas for how to make memory as well as the most brilliant. I say this because of the shear simplicity of it. All you really need is some common materials and you suddenly have the ability to store data. Sure, it’s not power efficient or even compact but it works.
Mercury delay line memory is crazier to me because it’s literally just a tube full of mercury that looks like a mini nuclear reactor that somehow stores bits
Not magic but ripples
To store information, you can also use self-holding relays instead.
If you have three relays, a simple pulse toggle flip-flop can be built.
Not efficient, but useful in a post-apocalyptic world. :)
Primitive example: https://www.youtube.com/watch?v=D5k9SVVx2wc
ROM memory can be made using diode matrices, as well, not just core memory.
A diode matrix also keeps information when there’s high radiation or strong energy fields.
Boards with diode matrices are large, but if you have space it’s not a problem.
“Sure, it’s not power efficient or even compact but it works.”
The cores you see here are fairly large but from the old nasa videos they used a much smaller core. Not as small as core memory but closer to that than used here.
It isn’t compact compared to the not yet invented ICs, it is far more compact than using transistors. Even todays TO-92 packages are larger.
Also don’t forget that the “rope” part of rope memory is due to not requiring any pcb or 2d mounting, the cores end up spaced out over the wire bundles, and can be wrapped or bunched up to fit in a smaller 3d space.
I’d be interested to know just how compact it could be made; were someone to put modern techniques to work.
The requirements of hard drives and tape mean that there’s a lot of expertise in thin films with tailored magnetic properties; and the IC people know how to put down very, very, detailed metal and insulator layers. I suspect that mixing those two processes, at very least, wouldn’t be cost effective vs. mask ROM or flash; and it would quite possibly be less dense or more power hungry even in principle; but it seems like you would definitely be able to get a massive increase in density vs. conventionally assembled core rope memory.
Love the background ambience of monitoring HAM
When I was a teenager in Minneapolis, Control Data was paying people (often mothers looking for extra income) to thread wires through core memory in their own garages. I eventually ended up working as an component engineer for Collins Radio, and one of the components I was responsible for was the very tiny cores Collins was putting into thin film hybrid circuits for Art Collins’ attempt at making computers (an obsession that eventually bankrupted the company). I still have a few of those cores after 55 years.
I picked up an unusual RTTY(?) Morse code converter console at a hamfest a couple of years ago. It has an 8 bit wide core rope memory in it. I posted some pictures of it on VCfed at the time:
https://forum.vcfed.org/index.php?threads/what-is-this-post-photos-of-mystery-items-here-vintage-computers-only.58631/post-1285480
Stranger even that core ROM, I think, is printed circuit board ROM, made famous in the HP 9100 series, http://www.hp9825.com/html/the_hp_9100_rom.html
A ROM made entirely of printed copper traces, no cores or diodes involved.
thanks for posting this. I have been looking for something similar for months in order to play with making a cheaper mechanical keyboard without diodes but without sacrificing performance.