Ken Shirriff
NASA used this "Up-Data Link Test Set" to test one of the electronic systems onboard the Apollo spacecraft. I've been reverse-engineering this box and I found that it uses LED-based opto-isolators. I didn't realize that they had LEDs and optoelectronics in the mid-1960s, so I investigated... It turns out that opto-isolators existed back then but were extraordinarily expensive. 1/N
Jul 12, 2025 at 2:15amWeb
Show threadKen ShirriffJul 12, 2025
Texas Instruments introduced LED-based opto-isolators in 1964. They used an infrared GaAs LED shining onto a silicon photo-transistor. This device had the curious name of "molecular multiplex switch/chopper"; making small devices through "molecular electronics" was the hot thing back then. The device cost $275 (almost $3000 in current dollars; nowadays, an opto-isolator costs a few pennies. 2/3
Show threadKen ShirriffJul 12, 2025
Sources: Texas Instruments ad from Electronics, 1964: https://www.worldradiohistory.com/Archive-Electronics/60s/64/Elelctronics-1964-05-18.pdf#page=15
Price from "The Optical Link: A New Circuit Tool", Electronics World, 1965: https://www.worldradiohistory.com/Archive-Electronics-World/60s/1965/Electronics-World-1965-09.pdf#page=34
Show threadrob pikeJul 12, 2025
@kenshirriff I bought my first LED from an HP ad on the back cover of Scientific American in 1970. Sent them a dollar and a self-addressed stamped envelope, and a week or two later received a now-canonical red LED in the post. Those were the days.
Show thread1000millimeterJul 12, 2025
@robpike @kenshirriff Do you still have it?
Show threadGiles GoatJul 12, 2025
@kenshirriff "I'll just have a look to see what he's talking about" .. -> me about to loose all day reading 1964 magazines ! .. ๐Ÿ˜‚
Show threadBrialaJul 12, 2025
@kenshirriff One of the best decisions Roland et al did was specify opto-isolators in the MIDI spec almost 20 years later. No ground loops!
Show threadfruchtiJul 12, 2025
@kenshirriff TIL about back-to-back BJT choppers. I would not have guessed that inverted operation works out to be better for choppers!
Show threadpenguin42Jul 12, 2025
@kenshirriff It seems an odd choice to use the expensive components on a ground test set like that.
Show threadJyrgen NJul 12, 2025
@kenshirriff I didnโ€™t know they are called opto-isolators โ€” in German, they are called Optokoppler (opto-couplers). But in they end both names seem apt.
Show threadGokul DasOct 24, 2025

@jyrgenn We also call them optocouplers in English, here in India. Very useful when you have a dozen different 'grounds' sprawling all over a structure the size of a 15 story building. 'Ground loops' are a massive headache with hilarious outcomes at times.

@kenshirriff

Show thread๐Ÿ‘ฝ๐Ÿฆ๐Ÿฆ‡๐Ÿ‰๐Ÿ’ปJul 13, 2025
@kenshirriff "Get to the chopper!"
...
"Wrong chopper!"
Show threadAndrew ZonenbergJul 12, 2025
@kenshirriff what's updata?
Show threadKen ShirriffJul 12, 2025

@azonenberg Not much. What's up with you?

But seriously, the Up-Data Link (UDL) was a box on the spacecraft that allowed ground control to send data commands up to the spacecraft. These commands could activate relays, control the Apollo Guidance Computer, or set the spacecraft's clock. This Test Set read messages from paper tape, sent the messages to the UDL box , and verified that the UDL box produced the correct response.

Show threadCsepp ๐ŸŒขJul 12, 2025
@kenshirriff @azonenberg Did these have some sort of encryption, or did they just not bother with that back in the day? 
Show threadKen ShirriffJul 12, 2025
@csepp @azonenberg There was no encryption on the up-data link. The only part of Apollo that used encryption was the Saturn booster destruct system, which allowed the Range Safety Officer to blow up the rocket if something went wrong. The escape tower on the Apollo capsule would pull the astronauts to safety, hopefully.
Show threadCsepp ๐ŸŒขJul 12, 2025
@kenshirriff @azonenberg Huh, interesting. Were they not worried about sabotage?
Show threadAydanJul 12, 2025
@csepp @kenshirriff @azonenberg clearly at least a little. imagine if they didn't encrypt the self destruct 
Show threadtntJul 12, 2025
@csepp @kenshirriff @azonenberg Note that AFAIU the crew has to specifically allow ground to send commands. By default the switch if turned off and ground would request the astronaut to turn it on to allow them to send commands when needed.
Show threadAndrew ZonenbergJul 12, 2025
@tnt @csepp @kenshirriff That would make sense just to reduce opportunities for rf noise or system malfunctions to do things they shouldn't
Show threadKen ShirriffJul 15, 2025
@tnt @csepp @azonenberg There was a switch to enable or block commands from ground to the Apollo Guidance Computer. But the astronauts couldn't block commands from ground that controlled relays. (The astronauts could manually reset most, but not all, of these relays.)
Show threadHenryk PlรถtzJul 12, 2025

@csepp @kenshirriff @azonenberg Back in the day everything was already very hard as it is. Nobody had a GNURadio yet.

See for example https://en.wikipedia.org/wiki/Max_Headroom_signal_hijacking
The only, but primary, thing that prevented just anyone from sending signals was that you needed access to the right equipment.

Encryption on the internet, arguably much easier to intercept, didn't arrive until the late 90s. And only then in very limited doses for parts of(!) banking websites.

Max Headroom signal hijacking - Wikipedia

Show threadMike KnellJul 12, 2025
@henryk @csepp @kenshirriff @azonenberg People get very excited about broadcast TV hijacking incidents but what's far more interesting is how few of them they were. You didn't need GNU Radio to put together a basic TV transmitter, and at least on the (UK) transmission networks I'm familiar with there was basically nothing stopping someone from, say, transmitting on the frequency of a major transmitter when it was off air at night and getting that picked up and relayed by dozens of transmitters.
Show threadMike KnellJul 12, 2025
@henryk @csepp @kenshirriff @azonenberg What was necessary (and remains necessary) is the part which SDR can't fix: you need substantially more output power than you get from a plug-and-play SDR. It's easy enough to get hold of a power amp but both analogue and digital TV signals are wideband and need a decently linear amplifier and minimal distortion to work. Not so much security through obscurity as security through complexity. :)
Show threaddraeathJul 12, 2025
@kenshirriff @csepp @azonenberg I assume the abort system (not the escape tower part) was based on other ballistic missile abort systems, right? Or do you think they ciphered it specifically instead of it being an already working control system?
Show threadKen ShirriffJul 12, 2025

@draeath @csepp @azonenberg It's complicated. Unmanned launch vehicles used the AN/DRW-13 for destruction; I think that's a military system. Mercury used a different system. They moved to the Digital Range Safety Command System (DRSCS) for Saturn and Skylab. Space Shuttle used the Command Destruct System (CDS).

Some info: https://ntrs.nasa.gov/api/citations/19740014779/downloads/19740014779.pdf#page=19

Show thread๏ฟฝJul 12, 2025
@csepp @kenshirriff @azonenberg i wonder, if old satellites uses encryption or at least some out of date or "proven broken" encryption. like dunno, 20bit symmetric key sizes etc?
Show threadLocalJul 12, 2025
@kenshirriff @azonenberg That's so cool ๐Ÿ˜Ž Great gem of science history.
Show threadChris MillsJul 12, 2025
@kenshirriff Neat! I feel like I may have a seen a counter like this in my father's laboratory at #BrandeisUniversity back in 1973-74, but loaded with black punch paper and being used to count decay events as part of his #physics experiments.
Show threadEndyJul 12, 2025

@kenshirriff Wow. Thank you for sharing this, I didn't realize how far back LEDs went either, they've always "just been there".

Of course, any diode can become light-emitting - once! (then it probably becomes smoke-emitting)

Show threadSpace HoboJul 12, 2025

@egoldblatt @kenshirriff Light-emitting Resistors: incandescent lamps
Light-emitting Capacitors: carbon-arc lamps
Light-emitting Transformers: fluorescent lamps

Anyone interested in designing a light-emitting memristor?

Show threadzl2todJul 12, 2025

@kenshirriff

The first LED I saw was in about 1970 at an open day for a scientific research institute.

Show threadJyrgen NJul 12, 2025
@kenshirriff ๐Ÿ˜๐Ÿ˜ ยกNixie tubes! ๐Ÿ˜๐Ÿ˜
Show threadKen ShirriffJul 12, 2025
@jyrgenn The displays look like Nixie tubes, but they are something different: edge-lit lightguide displays. There are 10 plastic sheets with etched dots for each digit, illuminated by one of 10 bulbs. When a bulb is illuminated, you see the dots in the corresponding plastic layer.
Show threadJyrgen NJul 12, 2025
@kenshirriff Oh, wow, that is nice, too. I didnโ€™t know this technology existed. Thanks!
Show threadGokul DasOct 24, 2025

@kenshirriff For some strange reason, these displays that I didn't know existed till now, remind me of epaper ebook reader displays (kobo, kindle type). More specifically, their front-lit night light system. Do they have anything in common?

@jyrgenn

Show threadmasukomiJul 12, 2025
@kenshirriff TIL what an opto-isolator is. Cool