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betajet
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CEO
Multiple-Emitter Transistors
betajet   6/25/2014 3:43:59 PM
Nice 'blog, but I have a quibble about the LSTTL circuit diagram.  According to Wikipedia, LSTTL didn't show up until 1976 so you're a few years early.

Here's Wikipedia's diagram of a proper 1964 TTL circuit with that wonderful multiple-emitter transistor input stage that's so much fun to explain to undergraduates:



 

betajet
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CEO
Unequal output source/sink
betajet   6/25/2014 4:06:39 PM
While the pull-up transistor in the TTL totem-pole output stage is stronger than RTL's resistive pull-up, the pull-down transistor is typically 40 times stronger that the pull-up.  IIRC, a standard TTL output can sink 16 mA but source only 0.4 mA.  That's OK if you're driving other TTL gates, since TTL inputs have the same 40:1 asymmetry.

The asymmetry meant that TTL-based designs almost always used active-low drive for LEDs and active-low push-buttons.  Nowadays CMOS outputs are generally symmetric, but old-timers like me still prefer active-low if an output needs a lot of current.

Wnderer
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CEO
The Soul of A New Machine
Wnderer   6/25/2014 4:19:42 PM
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The picture of the Data General/Nova board reminds me of a great book about engineering. Tracy Kidder's ' The Soul of a New Machine'

http://en.wikipedia.org/wiki/The_Soul_of_a_New_Machine

Kinnar
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CEO
Re: Unequal output source/sink
Kinnar   6/26/2014 12:50:31 AM
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Developments never make fundamentals old, these things are still being taught the way they are in the curriculum in many countries. And it really helps to explain and test the small circuits, as a stepping measure to bring the students at the current technology usages. 

FlyByPC
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Rookie
Re: Unequal output source/sink
FlyByPC   6/26/2014 10:15:47 AM
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Wow -- FINALLY an explanation of why active-low switches and LEDs were preferred. That makes perfect sense, now. Thank you, sir!

Duane Benson
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Blogger
Re: Unequal output source/sink
Duane Benson   6/26/2014 11:38:32 AM
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re: "FINALLY an explanation of why active-low switches and LEDs were preferred."

That's quite interesting. I hadn't heard the explanation before. I missed the double emitter explanation too - although, I can certainly envision how it would work.

zeeglen
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Blogger
Re: Unequal output source/sink
zeeglen   6/26/2014 11:51:05 AM
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FINALLY an explanation of why active-low switches and LEDs were preferred.

Can still remember my first time with CMOS and how it was so nice to have the choice of LED drive.

TanjB
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Rookie
Re: Unequal output source/sink
TanjB   6/26/2014 11:59:24 AM
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I believe the origin of the asymmetry traces back to relay logic.  Relays were either on or off (subtly different from high or low) and that conditioned the way of thinking about logic.  It suited bipolar transistors very well, especially since they could only make NPN on the chip in the early days (to do both would add process steps).  Well, they could have made a chip with all PNP but that performed worse then as they still do today, so TTL was all NPN and engineers could happily continue in the mindset of a "1" being switch on, pull down.  There was not much concern about capacitance so pull-up current could be low.  External wires themselves became extra logic (a bus line served as a "wired NAND" when it had a pull-up resistor).

CMOS was exotic technology requiring the chip to go through extra process steps.  The first forms which came out in the early 70s were aimed at low power and almost seemed like magic (they ran on less than a micro-amp!).  But even these, initially, had the same asymmetric ability to sink 20mA (a standard which went back to teletypes and relays) with very little pull-up (which was fortunate since the PMOS transistors, like PNP, were lower performance than NMOS).  It was not until the late 80s when CMOS was pervasive and wired-NAND style busses finally died in pursuit of higher frequency / lower power operation that CMOS outputs became routinely symmetric and the idea of pull down faded entirely.

I'm pretty sure that version with zener diodes must have been a very late iteration of TTL.  I recall zeners being a novelty in the early 70s as discretes, and it probably added complexity for them to learn to make them on the same chip as the transistors.

DougInRB
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Manager
Re: The Soul of A New Machine
DougInRB   6/26/2014 4:23:37 PM
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Ahhh the memories....  That was required reading in one of my EE classes at Cal Poly (mid '80s)  I had never dreamed there would be so much intrigue and espionage in the design of a computer.  I recall that GI was one of the first to utilize PALs.

 

traneus
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Rookie
SN7400N in stock at Digi-Key
traneus   6/26/2014 9:26:17 PM
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Digi-Key does stock Texas Instruments SN7400N quad NAND gates (p/n 296-14641-5-ND, $1.64 for one piece) and a few other parts in the original series.

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