In their own words: Ramon Alonso on Core Rope Memory
Ramon Alonso, designer of the DSKY, remembers how the Engineers at the MIT Instrumentation Lab ended up with Core Rope Memory in the Apollo Guidance Computer. Alonso worked in computer science and engineering at the lab (now Draper).
"Before Apollo, MIT’s Instrumentation Lab was engaged in a project to send a probe to Mars, courtesy of the Air Force ca. 1962. Hal Laning and I thought what was needed was a computer that consumed virtually no power for the months it took for a craft to get there, and then wake up and do its job. I read, in something like the old Electronics Design magazine, how someone in Australia had proposed a “rope” memory. It worked by passing the Ones through the core, the Zeroes around it. The cores used a magnetic material called Molly Permalloy wound around a ceramic bobbin core. The Molly Permalloy’s magic property was it has fairly large hysteresis. You could pass a current through it and it would be biased and switch weakly. The selection scheme was to bias off all but a selected core, and try to switch them all. Then, the unbiased single core switches, would put out signals on all the “one” wires going through it. An early ROM."
"When Apollo came, we could not think of an alternative. The Rope memory had a density many times that of ferrite coincident core memories, the stuff the Erasable Memory was made of. When the decision to go Rope was made, there were some dismayed comments, such as “you mean we can’t change the code on the launch pad?” The discipline this imposed on the software had something to do, I think, with the astounding quality of that effort."
"Making the Rope was a bear, until a small company (Sippican), and its chief Sam Francis, had the brilliant idea of putting the cores on their side, rather than try to string them like beads. Then Raytheon created an X-Y table, driven by a punched paper tape, that positioned the core to be threaded. This is what you see in the photo where “girls,” grandmothers mostly, are shown making a Rope."