{"id":100317,"date":"2025-10-08T13:00:34","date_gmt":"2025-10-08T17:00:34","guid":{"rendered":"http:\/\/pappp.net\/?guid=a0665cab23644a1c3223b330f257d676"},"modified":"2025-10-08T13:00:34","modified_gmt":"2025-10-08T17:00:34","slug":"ask-hackaday-why-is-ttl-5-volts","status":"publish","type":"post","link":"https:\/\/pappp.net\/?p=100317","title":{"rendered":"Ask Hackaday: Why is TTL 5 Volts?"},"content":{"rendered":"

Source: Hack a Day<\/a><\/p>\n

Article note: I hope they get a real sourced answer, because every time I do logic voltage lectures I wonder how 5V was actually chosen, and I'm not good enough at analog electronics to be sure of my own reasoning.\n \nI commented my best shot: \n\nThe earlier (1963) Sylvania SUHL family of TTL IC parts was also Vcc=5V, and it appears to have been selected to get +1\/-1.5V of noise immunity and swing of >3V as a \u201cnice round numbers\u201d thing with the particular topology and transistors they were using.\nThat\u2019s as close to an answer as I\u2019ve found when I\u2019ve gone looking in the past.\nWhy 5 and not 4.7 (E series preferred number ) or the like I\u2019ve never found an answer with a period source for.\n\nThere\u2019s an interview with Tom Longo who did the original TTL IC designs ( https:\/\/archive.computerhistory.org\/resources\/access\/text\/2016\/12\/102762590-05-01-acc.pdf ) that refers to the talk his group gave describing those first designs ( https:\/\/ieeexplore.ieee.org\/abstract\/document\/1473619 ) but\u2026 AFIK full text doesn\u2019t exist, just the abstract for the talk, and it\u2019s the most likely place for a true traceable first-cause to be, if indeed there is a surviving record.<\/div>
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The familiar five volts standard from back in the TTL days always struck me as odd. Back when I was just a poor kid trying to cobble together my first circuits from the Forrest Mims Engineer’s Notebook<\/em><\/a>, TTL was always a problem. That narrow 4.75 V to 5.25 V spec for Vcc was hard to hit, thanks to being too poor to buy or build a dedicated 5 V power supply. Yes, I could have wired up four 1.5 V dry cells and used a series diode to drop it down into range, but that was awkward and went through batteries pretty fast once you got past more than a few chips.<\/p>\n

As a hobbyist, the five volt TTL standard always seemed a little capricious, but I strongly suspected there had to be a solid reason behind it. To get some insights into the engineering rationale, I did what anyone living in the future would do: I asked ChatGPT. My question was simple: “How did five volts become the standard voltage for TTL logic chips?” And while overall the answers were plausible, like every other time I use the chatbot, they left me wanting more.<\/p>\n

<\/span><\/p>\n

Circular Logic<\/h2>\n
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TTL, 5 volts and going strong since 1976 (at least). Source: Audrius Meskauskas<\/a>, CC BY-SA 3.0.<\/figcaption><\/figure>\n

The least satisfying of ChatGPT’s answers all had a tinge of circular reasoning to them: “IBM and other big computer makers adopted 5 V logic in their designs,” and thanks to their market power, everyone else fell in line with the five volt standard. ChatGPT also blamed “The Cascade Effect” of Texas Instruments’ standardization of five volts for their TTL chips in 1964, which “set the tone for decades” and forced designers to expect chips and power supplies to provide five volt rails. ChatGPT also cited “Compatibility with Existing Power Supplies” as a driver, and that regulated five volt supplies were common in computers and military electronics in the 1960s. It also cited the development of the 7805 linear regulator in the late 1960s as a driver.<\/p>\n

All of this seems like nonsense, the equivalent of saying, “Five volts became the standard because the standard was five volts.” What I was after was an engineering reason for five volts, and luckily, an intriguing clue was buried in ChatGPT’s responses along with the drivel: the characteristics of BJT transistors, and the tradeoffs between power dissipation and speed.<\/p>\n

The TTL family has been around for a surprisingly long time. Invented in 1961, TTL integrated circuits have been used commercially since 1963, with the popular 7400-series of logic chips being introduced in 1964. All this development occurred long before MOS technology, with its wider supply range, came into broad commercial use, so TTL — as well as all the precursor logic families, like diode-transistor logic (DTL) and resistor-transistor logic (RTL) — used BJTs in all their circuits. Logic circuits need to distinguish between a logical 1 and a logical 0, and using BJTs with a typical base-emitter voltage drop of 0.7 V or so meant that the supply voltage couldn’t be too low, with a five volt supply giving enough space between the high and low levels without being too susceptible to noise.<\/p>\n

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The 1961 patent for TTL never mentions 5 volts; it only specifies a “B+”, which seems like a term held over from the vacuum tube days. Source: U.S. Patent 3283170A<\/a>.<\/figcaption><\/figure>\n

But, being able to tell your 1s and 0s apart really only sets a minimum for TTL’s supply rail. Why couldn’t it have been higher? It could have, and a higher Vcc, like the 10 V to 15 V used in emitter-coupled logic (ECL), might have improved the margins between logic levels and improved noise immunity. But higher voltage means more power, and power means heat, and heat is generally frowned upon in designs. So five volts must have seemed like a good compromise — enough wiggle room between logic levels, good noise immunity, but not too much power wasted.<\/p>\n

I thought perhaps the original patent for TTL<\/a> would shed some light on the rationale for five volts, but like most inventors, James Buie left things as broad and non-specific as possible in the patent. He refers only to “B+” and “B-” in the schematics and narrative, although he does calculate that the minimum for B+ would be 2.2 V. Later on, he states that “the absolute value of the supply voltage need be greater than the turn-on voltage of the coupling transistor and that of the output transistor,” and in the specific claims section, he refers to “a source of EMF” without specifying a magnitude. As far as I can see, nowhere in the patent does the five volt spec crop up.<\/p>\n

Your Turn<\/h2>\n
\"\"<\/a>
The Fender “Champ” guitar amp had a rectifier tube with a 5-volt filament. Perhaps TTL’s Vcc comes from that? Source: SchematicHeaven.net<\/a>.<\/figcaption><\/figure>\n

If I were to hazard a guess, the five volt spec might be a bit of a leftover from the tube era. A very common value for the heater circuit in vacuum tubes was 6.3 V, itself a somewhat odd figure that probably stems from the days when automobiles used 6 V electrical systems, which were really 6.3 V thanks to using three series-connected lead-acid cells with a nominal cell voltage of 2.1 V each.<\/p>\n

Perhaps the early TTL pioneers looked at the supply rail as a bit like the heater circuit, but nudged it down to 5 V when 6.3 V proved a little too hot. There were also some popular tubes with heaters rated at five volts, such as the rectifier tubes found in guitar amplifiers like the classic Fender “Champ” and others. The cathodes on these tubes were often directly connected to a dedicated 5 V winding on the power transformer; granted, that was 5 V AC, but perhaps it served as a design cue once TTL came around.<\/p>\n

This is, of course, all conjecture. I have no idea what was on the minds of TTL’s designers; I’m just throwing out a couple of ideas to stir discussion. But what about you? Where do you think the five volt TTL standard came from? Was it arrived at through a stringent engineering process designed to optimize performance? Or was it a leftover from an earlier era that just happened to be a good compromise? Was James Buie an electric guitarist with a thing for Fender? Or was it something else entirely? We’d love to hear your opinions, especially if you’ve got any inside information. Sound off in the comments section below.<\/p>","protected":false},"excerpt":{"rendered":"

The familiar five volts standard from back in the TTL days always struck me as odd. Back when I was …<\/p>\n

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