Random Access Memory From A Rotating Drum In A Bendix G15

When it’s the 1950s and you are tasked to design a computer system that features not only CPU registers but also a certain amount of RAM, you do not have a lot of options. At this point in time, discrete logic was the rule, and magnetic core memory still fairly new and rather expensive. This is where the rotating drum comes in, which is somewhat like a cross between an old-style cylinder record and a hard drive. In a recent [Usagi Electric] video, a 1950s Bendix G15 system is demonstrated, which features such a rotating drum device, alongside both tube-based circuits and newfangled diode-based circuitry.

Simplified diagram of a rotating drum random access memory unit, showing the read-erase-write process as the drum spins.
Simplified diagram of a rotating drum random access memory unit, showing the read-erase-write process as the drum spins.

This particular unit was borrowed from the System Source museum, with the intent to restore it to a working condition. Part of this process involved figuring out the circuitry, which was made easy by the circuit schematic drawings that came with the original machine. According to the official brochure by the manufacturer, the ‘short lines’ that are intended for the CPU registers, the access time was less than 1 millisecond, which is pretty darn fast considering the era and the discrete CPU’s clock speed.

For the drum itself, however, popping the cover off the unit showed that it had suffered some damage that had resulted in the multiple heads contacting the surface. Despite this disappointment, it’s not the end of the restoration, however. The museum has one more Bendix G15 standing around, with a rotating drum unit that looks to be in mint condition. The damaged magnetic coating on the other rotating drum may conceivably be resurfaced, which if successful could provide new hope to a lot of retro systems out there that also use magnetic media, whether in drum or disk format.

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How To Debug A Faulty Memory Board


While this is not exactly a hack or a fail, it definitely is an inspiring example on how to debug a faulty card.

[Quinn] is one of the very few hobbyists out there that designed her own 6502 based computer. For the young readers of Hackaday, the MOS 6502 was introduced in 1975 and has been used in the Aple // line, the Commodore 64, the Vic-20, the Atari computers, the Nintendo Enterntainment System and others.

[Quinn]’s homemade new RAM board had been working for many weeks until it started to show some weaknesses by only sporadically passing the boot RAM test. Assuming the RAM was the problem, she started by making a more advanced memory test, which showed errors at random addresses.

She didn’t have any more of the same memory chips on hand which could be used with a fresh PCB. Determined to power through the issue, she etched a new board with a new memory design. Unfortunately it also gave memory errors at boot. Only one culprit was left, which is shown in the picture above. It’s a small sizing error in the board artwork which was just enough to cause a misalignment on the connector.

The article contains many details about her debugging process, so it definitely is worth the read.