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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.

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!

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. 

Wnderer
User Rank
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

betajet
User Rank
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.

betajet
User Rank
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:



 

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