Learning the ropes…

July 24, 2014 | Retrocomputing | By: Mark VandeWettering

Over the past few years, I’ve expressed an interest in the AGC, or Apollo Guidance Computer. If you haven’t had the time to look at it, the Wikipedia page is good enough to get a brief overview, but if you want to dig deep, you can find all sorts of information and simulators.

I found myself looking back into it this week for two related reasons: one, the anniversary of the Apollo 11 landing, which I still remember, and because of a new argument that I’ve read (but won’t dignify with a link) that claims the moon landings were fake because the AGC could not have worked. But I must admit, he pointed at one bit of the AGC, its core rope memory which he claimed couldn’t work. I think the safer claim would be that he didn’t understand how it worked, but when I thought about it, I realized that I didn’t really know how it worked either. And that bothered me, so I thought I’d dig into a bit more.

Here’s a brief, high level introduction:

The basic idea is pretty simple, and relies on using ferrite toroids as transformers. Imagine that you have two wires going through a ferrite core. If you send a pulse in one wire, it will generate pulse on the other wire. This principle is used in all transformers, which may vary in the number of turns to step the voltage up and/or down by varying the number of turns through the toroid. You can generate a simple memory using this principle. This kind of memory is demonstrated admirably by SV3ORA who created a 70 bit ROM that serves as a decoder for 7 segment LEDS A pulse (or pulse stream, even better) on one of the ten input lines generates the appropriate set of output voltages to display the corresponding numeral on a 7 segment LED display. His webpage has some nice, easy to follow circuits, and a cute little video of it working.

But if you look at the diagram for the Apollo Guidance Computer, it looks a little different. It has a series of “inhibit” lines that weave in and out of the cores, in addition to some sense lines.

Screen Shot 2014-07-24 at 8.35.03 PM

The first description I found was around page 90 of this report, MIT’s Role in Project Apollo, Volume 3. But to be honest, I didn’t really understand it. Luckily, I found what appears to be a better description: P. Kuttner’s 1963 paper, The Rope Memory — A Permanent Storage Device. I still need to work through the details, but it makes a lot more sense to me. I begin to see how the address decoding works. I’ll ponder it a bit more, but it is beginning to make sense, and as it makes more sense, I see it for the clever bit of engineering it is. It was a remarkable bit of engineering in its day, and allowed densities of 1500 bits per cubic inch, including all the address decoding. Very cool.

Addendum: Hacker friend Jeff Kellem was unable to post a comment to this blog (got trapped by the spam filter, no doubt because of the high number of links, which normally indicate spam, but in this case indicates SCIENCE!) but he was kind enough to drop me an email with additional reading. I’ll reproduce it here:

You might find this July 1976 issue of BYTE magazine interesting:

Coincident Current Ferrite Core Memories

Also, maybe check out:

Magnetic Core Memory Systems

Ferrite CorePlanes and Arrays: IBM’s Manufacturing Evolution

And start with Volume 1, Issue 2 (May 1973) of Amateur Computer Club Newsletter, there’s a several part series titled “Core for Stores” in there:


Look forward to reading more about your exploration into core memory.


All very cool resources. All us old-timers probably remember Byte magazine (but to be honest, I didn’t recall that they had ever had an article addressing core memory) but I had never actually heard of the Amateur Computer Club newsletter. It’s deliciously old and homebrew. The description of core memories is great, it includes some of the drive circuitry that one would have built back in 1973. I’ll have to check it out further.

Addendum2: If you want to go to a lot of trouble (and per bit, a huge expense) to make a core memory that can be read by your Arduino, Wayne has a lot of advice and detail on his page.


Comment from Dan – WA6PZB
Time 7/30/2014 at 3:21 pm

Great post, got inspired and ordered some ferrite cores from ebay just to try maybe a single bit or 4 bit setup. Let me know if you want a few and I can send you some.

Comment from Jim Gage
Time 8/31/2014 at 11:52 pm

Hi Mark! Came across your blog and found the morse beacon for Arduino sketch. Perfect for my use. I have been browsing your articles, and wanted to say how much I appreciate your continued interest in things other folks have swept under the carpet. The 2600, rope memory, CW, yeah, I love it all. I guess we are about the same age, since my first comp was a brand new Sinclair 1000. Great experience, thise old electronics. I was a ham as well, but never proceeded past the novice class. CW was my love. Now it is astronomy! I see the tags over there on the left side of this page, and have to go soon… lots of stuff to read here 😛 I will use that CW sketch in a little project I am building for a friend, when it is done I will try and present it for you here. Thanks again!
Jim in Sweden