Reverse Engineering A 1960s Diode-Transistor Logic IC

November 9, 2026 by Maya Posch 6 Comments

Before transistor-transistor logic (TTL) took over, diode-transistor logic (DTL) was a common sight especially in pre-IC circuits where it was a major improvement over resistor-transistor logic (RTL). That said, it’s rare to see DTL used in ICs considering how e.g. TI’s 5400 and 7400 series set the trend in the 1960s. This makes the Signetics ST680a quad-NAND gate DTL DIP IC that [Ken Shirriff] ogled recently a bit of a treat.

This was also one of the earliest ICs to use the then brand new plastic dual in-line package (PDIP) for modular convenience, even if [Ken] was none too happy about as unlike ceramic DIP, PDIP requires more extreme methods to decap to access the die for reverse-engineering.

Overall, TTL is faster and less power-hungry than DTL, but it took a while to work out its issues, including noise immunity and fan-in/out issues. Until then alternatives like DTL still made sense as a well-understood and mature technology. In the article an example is provided of how a DTL NAND gate as used in this IC works, which is really quite simple in how it uses multiple diodes to sum the inputs, which subsequently drives the transistors.

We’d like to thank [CuriousMarc] for finding this IC in his strategic reserve of unknown ICs and for doing the initial decapping using hot air, as well as [Ken] for doing the analysis and documenting of this fascinating glimpse into early semiconductor logic history.

Inside A Mystery Aerospace Computer With [Ken Shirriff]

June 1, 2024 by Dan Maloney 10 Comments

When life hands you a mysterious bit of vintage avionics, your best bet to identifying it might just be to get it in front of the biggest bunch of hardware hounds on the planet. After doing a teardown and some of your own investigation first, of course.

The literal black box in question came into [Ken Shirriff]’s custody courtesy of [David] from Usagi Electric, better known for his vacuum tube computer builds and his loving restoration of a Centurion minicomputer. The unit bears little in the way of identifying markings, but [Ken] was able to glean a little by inspecting the exterior. The keypad is a big giveaway; its chunky buttons seem optimized for use with the gloved hands of a pressure suit, and the ordinal compass points hint at a navigational function. The layout of the keypad is similar to the Apollo DSKY, which might make it a NASA artifact. Possibly contradicting all of that is the oddball but very cool electromechanical display, which uses reels of digits and a stepper-like motor to drive them.

Inside, more mysteries — and more clues — await. Unlike a recent flight computer [Ken] looked at, most of the guts are strictly electronic. The instrument is absolutely stuffed with PCBs, most of which are four-layer boards. Date codes on the hundreds of chips all seem to be in the 1967 range, dating the unit to the late 60s or early 70s. The weirdest bit is the core memory buried deep inside the stacks of logic and analog boards. [Ken] found 20 planes with the core, hinting at a 20-bit processor.

In the end, [Ken] was unable to come to any firm conclusion as to what this thing is, who made it, or what its purpose was. We doubt that his analysis will end there, though, and we look forward to the reverse engineering effort on this piece of retro magic.

The 555 And How It Got That Way

October 10, 2018 by Dan Maloney 117 Comments

There’s a certain minimum set of stuff the typical Hackaday reader is likely to have within arm’s reach any time he or she is in the shop. Soldering station? Probably. Oscilloscope? Maybe. Multimeter? Quite likely. But there’s one thing so basic, something without which countless numbers of projects would be much more difficult to complete, that a shop without one or a dozen copies is almost unthinkable. It’s the humble 555 timer chip, a tiny chunk of black plastic with eight leads that in concert with just a few extra components can do everything from flashing an LED a couple of times a second to creating music and sound effects.

We’ve taken a look under the hood of the 555 before and featured many, many projects that show off the venerable chip’s multiple personalities quite well. But we haven’t looked at how Everyone’s First Chip came into being, and what inspired its design. Here’s the story of the 555 and how it got that way.

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