Breaking News
Comments
Newest First | Oldest First | Threaded View
<<   <   Page 3 / 3
Duane Benson
User Rank
Blogger
Re: Unequal output source/sink
Duane Benson   6/26/2014 11:38:32 AM
NO RATINGS
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
User Rank
Rookie
Re: Unequal output source/sink
FlyByPC   6/26/2014 10:15:47 AM
NO RATINGS
Wow -- FINALLY an explanation of why active-low switches and LEDs were preferred. That makes perfect sense, now. Thank you, sir!

Kinnar
User Rank
CEO
Re: Unequal output source/sink
Kinnar   6/26/2014 12:50:31 AM
NO RATINGS
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
NO RATINGS
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:



 

<<   <   Page 3 / 3


EE Life
Frankenstein's Fix, Teardowns, Sideshows, Design Contests, Reader Content & More
Max Maxfield

Steve Wozniak Reacts to Latest iPhone
Max Maxfield
5 comments
Funnily enough, just a few days ago as I pen these words, I was chatting with my wife (Gina the Gorgeous) when she informed me that -- as a kid -- she had never played at making a ...

EDN Staff

11 Summer Vacation Spots for Engineers
EDN Staff
20 comments
This collection of places from technology history, museums, and modern marvels is a roadmap for an engineering adventure that will take you around the world. Here are just a few spots ...

Glen Chenier

Engineers Solve Analog/Digital Problem, Invent Creative Expletives
Glen Chenier
15 comments
- An analog engineer and a digital engineer join forces, use their respective skills, and pull a few bunnies out of a hat to troubleshoot a system with which they are completely ...

Larry Desjardin

Engineers Should Study Finance: 5 Reasons Why
Larry Desjardin
46 comments
I'm a big proponent of engineers learning financial basics. Why? Because engineers are making decisions all the time, in multiple ways. Having a good financial understanding guides these ...

Flash Poll
Top Comments of the Week
Like Us on Facebook
EE Times on Twitter
EE Times Twitter Feed

Datasheets.com Parts Search

185 million searchable parts
(please enter a part number or hit search to begin)