The 7400 Quad 2-Input NAND Gate, A Neglected Survivor From A Pre-Microprocessor World

By Jenny List

There are a range of integrated circuits that most of us would regard as definitive examples of their type, devices which became the go-to for a particular function and which have entered our collective consciousness as electronics enthusiasts. They have been in production since the early days of consumer integrated circuits, remaining in use because of a comprehensive understanding of their characteristics among engineers, and the job they do well.

You can probably name the ones I’m going to rattle off here, the µA741 op-amp designed by David Fullagar for Fairchild in 1968, the NE555 timer from Hans Camenzind for Signetics in 1971, and a personal favourite, Bob Widlar’s µA723 linear regulator for Fairchild in 1967. There may be a few others that readers will name in the comments, but there’s one that until today it’s likely that few of you would have considered. Texas Instruments’ 5400 and 7400 TTL quad 2-input NAND gate has been in continuous production since 1964 and is the progenitor of what is probably the most numerous breed of integrated circuits, yet it doesn’t trip off the tongue when listing famous chips, and none of us can name its designer. So today we’re turning the spotlight on this neglected piece of silicon, and trying to bring it the adulation it deserves.

Can you name this anonymous IC designer?

We could have featured a minicomputer such as a DEC PDP-11 as an example of a CPU built from 74 series logic. But the 74-driven Xerox Alto makes a greater point about 74 logic as the progenitor of modern computing devices. Joho345 [Public domain]
We could have featured a minicomputer such as a DEC PDP-11 as an example of a CPU built from 74 series logic. But the 74-driven Xerox Alto makes a greater point about 74 logic as the progenitor of modern computing devices. Joho345 [Public domain]
As semiconductor logic emerged through the 1950s and into the 1960s, there were a number of competing technologies in the field. Diode logic, diode-transistor logic, resistor-transistor logic, and others were all contenders that found their way into the early generations of solid state computers. Each technology had its adherent companies, but each came with associated limitations in the form of low speed, excessive power consumption, or demanding power supply requirements. Transistor-transistor logic, or TTL, was conceived in 1961 by James L. Buie at TRW Inc, and held the promise of reasonable power consumption at higher clock speeds, respectable speed, and a single low-voltage power rail. First to market with TTL were Sylvania in 1963, with TI following in 1964 with the 5400 metal flat-pack military TTL series, and in 1966 with the plastic-packaged 7400 variants we know so well. The series expanded to cover every possible logical function from a plethora of manufacturers, and as the emerging industry standard by the end of the decade entire mini and microcomputers were being constructed using only 74-series TTL chips. Designs using them were in the minds of the first microprocessor designers, and their influence was clear on the CPUs of the 1970s that used 74-style 5 V logic levels.

The die of a decapped National Semiconductor-branded 5400 from the 1980s Robert.Baruch [CC BY-SA 4.0]
The die of a decapped National Semiconductor-branded 5400 from the 1980s. Is that a Fairchild marking at the bottom, indicating that this chip was originally one of that company’s procucts? Robert Baruch [CC BY-SA 4.0].
So in a sense, though it wasn’t the first logic chip or even the first TTL chip, the 7400 is arguably the semiconductor progenitor of the computers we have on our desks and in our pockets today. It’s an unnoticed computing survivor from a pre-microprocessor world, and it has managed to stay with us despite the advent of the microprocessor because 74-series logic has become the “glue” that holds together so much of our digital world. It’s ubiquitous, and has outgrown its original purpose of forming the building blocks of 1960s computing, instead performing simple logical tasks where simplicity, speed, simplicity of implementation, and low cost are required. Its success also eclipses a human-level story, for if we go back to the list of circuits mentioned at the top of this page we find names alongside them. Widlar, Fullagar, Cammenzind and others like them are names that trip off the tongue in conversations about earlier integrated circuits, but how many of us can name the creator of the 7400? Certainly not us, it’s as though they have disappeared from history. It’s a simple enough device that it is likely to be ascribable to one person, but even if it was the work of a team we should be able to find the names of its members. Genuinely, the 7400’s creator or creators should be famous for their achievement, so if anyone can shed any light on early-1960s Texas we would be really interested to know.

A 7400 today

 The internal circuit of a single 7400 dual-input NAND gate. 30px MovGP0 (CC BY 2.0 DE).
The internal circuit of a single 7400 dual-input NAND gate. 30px MovGP0 (CC BY 2.0 DE).

Over the years there have been a dizzying array of 74-series compatible families that address the shortcomings of the originals with capabilities far exceeding them, and though many of the more esoteric devices are no longer made it remains available in a huge range of functions. It’s still perfectly normal to find a 74 derivative in a device manufactured in 2018, and no doubt it will continue to be so for the forseeable future. The original series may have been long-ago superseded, but they remain in production and you can still order a 7400 from all the usual sources. So we did just that, picking up a few TI SN7400s as part of a Mouser order. It’s interesting to note that at about a couple of dollars each in single quantities these are no longer a cheap part, while they would have cost tens of dollars at launch in the 1960s they have evidently passed their peak-production low price point and are now heading into the realm of small-production-run exotica. It seems that the HC series CMOS part 74HC(T)00 from the late 1980s is now the cheapest single-quantity equivalent coming in at about 30 cents.

A quad 2-input NAND gate is hardly the most exciting or exotic of components unless you have a pressing need for a bit of logic glue, so having secured a few we could hook up an LED or make an astable flip-flop if we wanted to. But the interesting story lies probably not in what can be done with a 7400, but in why we would now use a 74HC00 or any of the other families that superseded the original. Newer series almost all have either or both of higher speed and lower power consumption in their feature sets as you might expect, but they have also all addressed the original’s inherent flaw. The earliest logic gate families were digital circuits designed to represent logic 1 and logic 0 as a high voltage or a low voltage, but they also had a point during the voltage transition between logic states during which they functioned more as an analogue circuit than a digital one. The original 74 series shares this with its near-contemporary 4000 series CMOS.

In a digital circuit containing 74 series devices the moment during logic transition at which both output transistors were open caused a spike of high current to pass, which had the unintended side effects of making circuits extremely noisy, and requiring significant power supply decoupling efforts. It’s worth remarking at this point that unintended analogue properties in the 4000 series chips became popular in the world of experimental synthesisers as demonstrated by our own [Elliot Williams] in his Logic Noise series.

The 7400 then, a neglected survivor from a pivotal moment in computer design that has soldiered on unnoticed for over fifty years. You will no doubt still be able to buy 7400s for years to come, though we’re at a loss as to who would specify them in a new design, and for decades to come you’ll certainly be able to buy its derivatives. We hope this has shone a light upon it and accorded it the recognition it deserves, and in turn if we could put a name to its designer that would be a fascinating story in itself. Now, if you’ll excuse us we have a small pack of brand new 7400s to think of a project for.

7400 header image: Stefan506 [CC-BY-SA-3.0]