So why TTL logic uses 5V again? - Page 1
Forum section: electronics / general

author: AlphZeta
author_rank: Regular Contributor
timestamp: January 25, 2013, 09:14:08 pm
content: I might have learned it long time ago in school why TTL logic uses 5V, but couldn't remember any more.Does anyone know why 5V was chosen for TTL circuits? Was it just chosen and decided arbitrarily or was there any specific reasons? Back then when the logic was invented, the most common batteries (e.g. D-cells) were all 1.5V so it couldn't have been chosen because of the battery voltages.Just curious.

author: ptricks
author_rank: Frequent Contributor
timestamp: January 25, 2013, 09:26:44 pm
content: It is the lowest voltage that you can use to interface between transistors and vacuum tubes which was a great concern at the time since many of the things that TTL replaced were vacuum tube devices.

author: saturation
author_rank: Super Contributor
timestamp: January 25, 2013, 09:47:26 pm
content: https://www.eevblog.com/forum/chat/how-are-%27standard%27-voltages-determined/msg184024/#msg184024

author: AlphZeta
author_rank: Regular Contributor
timestamp: January 25, 2013, 09:56:43 pm
content: Any citations guys? I did read that thread and the Wikipedia article but was hoping could find a more authoritative source. Thanks @satuation and @ptricks!

author: free_electron
author_rank: Super Contributor
timestamp: January 26, 2013, 12:16:03 am
content: Oh that's an easy one to answer.Criteria number 1 : Base-emittor junctions of a transistor that are reverse polarised cannot have much more than 6 volts. They start 'leaking' and accumulate time based damage.Criteria number 2 : fist logic chips were bipolar and had quite a bit a static current consumption resulting in heat. higher voltage means more heat...criteria number 3 : early chip technology used for digital suffered from scaling problems. they needed quite a bit of distance to 'hold' a standoff voltage. making chips impractical and expensive ( the cost of a chip is defined in square millimeters of surface... )throw that in a heap and you end up with something that works between 3 and 5 volts. At 3 volts the transistors did not switch 'fast' enough to get nice clean pulses so they settled at 5 volts. All criteria metNow, for early MOS technology they ran into another problem. They only had NMOS transistors. There were no P-MOS ( they hadn't figured out the process of implantation yet, they were depositing doped regions through crystal growth in an oven and then etching it. ) So they stacked nmos transistors to make totem-pole systems. problem is that you now need an additional voltage to switch top and bottom. So they could have used ground , 5 volts and 10 volts ( to turn on the top transistor you raise its gate 5 volts above its source which sits at 5 volts. Problem is that this was not compatible with bipolar logic. so they flipped the stuff around. they used -5 volts and used that as the 'ground' level. to create a compatible output all they needed was a mos from the 5 volts to the output pin. turn on the top mos ang you get 5 volts out. turn it off and you get 0 volts out. the internal logic used -5v as a logic 0 and 0 volts as a logic 1. Early cpu's in NMOS technology actually have a -5 volts pin.Once they could construct both PMOS and NMOS (what we now call the CMOS process : complementary metal oxide semiconductor : meaning both n and p , although that metal - oxide ... for a long time was not true... it started like that, went away ( we used doped polysilicon as the gate no need for metal.... ) and now is back ) the negative voltage wa sno longer needed.there were other technologies around like ECL that also required a negative voltage and used 5 volts and -3 volts as their supply rails ( although the logic levels for ecl are like 1 volt and - 1.2 volts or something like that . consumption of power in ECL is a constant , you just throw current from one loop to another) so that way they maintained compatibility with existing power supply systems..it's all historical and based on practicality for early integrated circuit technology.A cray computer like the cray 1 for example did not have a 'regulator' as we understand it now. they used a rotary convertor. a motor drove a generator that made a 6 phase output voltage at 400 hertz. they rectified that and ended up with very little ripple due to the 6 phases. so they needed minimal capacitors ( the cray 1 sucked hundreds of amperes on its power rails... being a fully ECL machine )the 'regulator' just controlled the field coil of the generator to adjust the output of the generator. so they did not use a transistor to regulate the hundreds of ampere. just control the strenght of the spinning magnet an u regulate output voltage of the generator.there's all kind of trickery like that in these early machines.

author: AlphZeta
author_rank: Regular Contributor
timestamp: January 26, 2013, 12:42:21 am
content: Thanks @free_electron for the great historical insight! Now I remembered BVebo was one of the major factors led to the 5V decision. Thanks!

author: westfw
author_rank: Super Contributor
timestamp: January 26, 2013, 05:06:21 am
content: ECL was -5.2V, wasn't it? I wonder why TTL didn't use exactly the same voltage...

author: TerraHertz
author_rank: Super Contributor
timestamp: January 26, 2013, 09:14:17 am
content: I always wondered about that -5.2V for ECL. Why the extra 0.2V? Did it matter enough, so a rounded 5V wasn't suitable?

author: westfw
author_rank: Super Contributor
timestamp: January 30, 2013, 10:15:56 am
content: Probably chosen by the same people who picked 6.3 as a common filament voltage for their tubes...

author: AlfBaz
author_rank: Super Contributor
timestamp: January 30, 2013, 11:36:58 am
content: The answere is much simpler than all the posts above...They were 5V cause of the 7805

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 30, 2013, 11:51:31 am
content: Quote from: westfw on January 30, 2013, 10:15:56 am6.3v came from 3xlead-acid battery cells

author: saturation
author_rank: Super Contributor
timestamp: January 30, 2013, 02:12:05 pm
content: Freee- comments is authority on its ownbut added ideas be gleened by seeing how the gates were designed. I don't think there is a reference saying explicitly "we used 5V because ..."There is a chapter in Horowitz & Hill online you cause use as a reference, I think its on interfacing digital to analog; it describes gates from different technologies and why they need their specific Vcc and how to mix and match them.Folks may not realize TTL was patented, so it had specific design constraints. Consider Voh~ Vcc-2, so to guarantee a switching transition at >2.5V there is very little room should Vcc drop below 5V.On the top end of Vcc, BVebomakes a good case.Quote from: AlphZeta on January 25, 2013, 09:56:43 pm

author: AlphZeta
author_rank: Regular Contributor
timestamp: January 30, 2013, 02:38:33 pm
content: Thanks again @satuation for the additional information.I guess sometimes we can't reason too much about these kind of standards, as "foot" is after the king's own foot after all and we are all using that as the standard measurement today...

author: notsob
author_rank: Frequent Contributor
timestamp: January 30, 2013, 02:51:35 pm
content: I once serviced a mainframe that had a -2V supply ( I think the logic was a form of ECL )A logic low was around 0V to -0.4VA logic high was approx anything below -0.6Vwire wrapped backplane with all wires shielded.Difficult to work with, but very quick logic switching.

author: saturation
author_rank: Super Contributor
timestamp: January 30, 2013, 03:15:25 pm
content: Yes, a bigger issue about some standard references is that at their origin it can be purely arbitrary. You get to learn more about physical standards in metrology. You see a lot of this type of invention in the new metrics for measuring various parameters related to the Internet.Quote from: AlphZeta on January 30, 2013, 02:38:33 pm

author: Galenbo
author_rank: Super Contributor
timestamp: February 11, 2013, 11:23:52 am
content: Quote from: AlphZeta on January 30, 2013, 02:38:33 pmI never used foot. Inch occasionally.I always use "1/10,000,000 part of the quarter of a meridian, astronomical measure by Bessel, Delambre and Mechain"

author: N2IXK
author_rank: Frequent Contributor
timestamp: February 11, 2013, 11:32:44 pm
content: Quote from: mikeselectricstuff on January 30, 2013, 11:51:31 amWhere did the UK's early preference for 4V heaters come from? 2 lead acid cells ("accumulators"), or something else?