[Ken Shirriff] Becomes A Core Memory Repairman (Again)

December 29, 2017 by Mike Szczys 5 Comments

Lately, [Ken Shirriff] has been on some of the most incredible hardware adventures. In his most recent undertaking we find [Ken] elbow-deep in the core memory of a 50-year-old machine, the IBM 1401. The computer wasn’t shut down before mains power was cut, and it has refused to boot ever since. The culprit is in the core memory support circuitry, and thanks to [Ken’s] wonderful storytelling we can travel along with him to repair an IBM 1401.

From a hardware standpoint core memory makes us giddy. It’s a grid of wires with ferrite toroids at every intersection. Bits can be set or cleared based on how electricity is applied to the intersecting wires. [Al Williams] walked through some of the core memory history last year and we enjoyed hearing [Pamela Liou] recount the story of how textile workers consulted on the fabrication of core memory for the Apollo missions during her OHWS Talk in October. But giddiness aside, core memory has pretty much gone the way of the dodo having been displaced by technologies that take up exponentially less space.

Bad inductor (green housing has been dissolved away)

We chuckle at [Ken’s] mention of the core memory capacity for the IBM 1401. It has 4000 characters of memory built-in (with another 12,000 in an expansion box) and he goes on to detail that these are 6-bit characters on a machine that operates in decimal and not binary (hence 4k instead of the base-2 friendly 4096).

You may remember his work a few years back to repair core memory on the same model. The Museum has two 1401’s, which turned out to be a huge help in trouble-shooting this. After tracing out the control lines, the repair team began swapping cards between the working and non-working machines. They were able to bring it back online — establishing one of the green inductors was bad — only to be struck with a second fault in the power supply.

Get this, [Ken] comments that “the whole computer is pre-silicon”. When working through the PSU, some suspect transistors were replaced with germanium power transistors. Those may have been a red-herring, as a penciled-in fuse on the original schematics turned out to be the linchpin of the PSU repair. Buried deep in the assembly, replacing the designed-to-fail part let the ancient beast awake once more.

Machines of this quality were heavily documented, and the schematics make this type of trouble-shooting a lot more manageable. But it’s still as much an art as it is skill. Make sure to give [Ken’s] article a read, and look around at the other repair jobs he’s documented — keeping these machines in service is becoming wizard-level work and we love being able to follow along.

Decoding Rediscovered Rope Memory From The Apollo Guidance Computer

September 2, 2016 by Brian Benchoff 43 Comments

On August 25th, 1966, an Apollo Command Module was launched aboard a Saturn IB rocket in mission AS-202. This mission was intended to immediately precede the ill-fated Apollo 1 mission, the AS-202 was unmanned, serving as a test of flight hardware, fuel cells, and the guidance and navigation control systems. This mission used the first Apollo Guidance Computer ever flown, and this mission was vital to testing the computer that would take men to the moon.

While the software from the later missions exists and is available on Github, the earlier Block I spacecraft, including the unmanned Apollo 4 and Apollo 6 missions, are poorly documented. [Francois Rautenbach] was lucky enough to get his hands on the rope memory modules from the AS-202 mission. Now he’s investigating these modules with oscilloscopes and x-rays to recreate some of the first software that was flown in space.

The procedure to extract the data from these rope memory modules is a bit harder than reading a bit of Flash off a chip. Rope memory is weird, but with a contraption made out of a lot of relays and an oscilloscope, [Francois] was able to capture data from these memory modules.

Of course, [Francois] first needed to figure out the pinout for the gigantic backplane connector on each of these memory modules. To do that, he checked out a Block II AGC, read the schematics very carefully, and reverse engineered a connector that isn’t made anymore. The next step was x-raying the rope memory modules to see how they were assembled. Even though these memory modules contain the only extant copy of the Block I AGC software, even reading one bit off of these modules is an amazing case of technological archeology.

The answer to the obvious question — where did these modules come from — is exactly what you would expect. These memory modules were picked up off a scrap heap forty years ago. The gentleman who found these modules was kind enough to give them to [Francois]. Check out the videos below for [Francois]’ video logs. If you’re into slightly more destructive testing of forgotten Apollo flight hardware, [Fran Blanche] tore down a few modules from the Apollo Launch Vehicle Digital Computer a few years ago.

Thanks to [Vincent], [Danie], and [Kent] for jumping on this one and sending it into the tip line.
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Thanks For The Memories: Touring The Awesome Random Access Of Old

March 8, 2016 by Al Williams 41 Comments

I was buying a new laptop the other day and had to make a choice between 4GB of memory and 8. I can remember how big a deal it was when a TRS-80 went from 4K (that’s .000004 GB, if you are counting) to 48K. Today just about all RAM (at least in PCs) is dynamic–it relies on tiny capacitors to hold a charge. The downside to that is that the RAM is unavailable sometimes while the capacitors get refreshed. The upside is you can inexpensively pack lots of bits into a small area. All of the common memory you plug into a PC motherboard–DDR, DDR2, SDRAM, RDRAM, and so on–are types of dynamic memory.

The other kind of common RAM you see is static. This is more or less an array of flip flops. They don’t require refreshing, but a static RAM cell is much larger than an equivalent bit of dynamic memory, so static memory is much less dense than dynamic. Static RAM lives in your PC, too, as cache memory where speed is important.

For now, at least, these two types of RAM technology dominate the market for fast random access read/write memory. Sure, there are a few new technologies that could gain wider usage. There’s also things like flash memory that are useful, but can’t displace regular RAM because of speed, durability, or complex write cycles. However, computers didn’t always use static and dynamic RAM. In fact, they are relatively newcomers to the scene. What did early computers use for fast read/write storage?

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