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DPRG: Logic Family Comparison Chart and example problem between fa

Subject: DPRG: Logic Family Comparison Chart and example problem between fa
From: MIKE MCCARTY -- 93789 JMCCARTY at DEVEL.USA.ALCATEL.COM
Date: Wed Aug 25 16:32:04 CDT 1999

)john.r.strohm at bix.com wrote:
)> 
)> >)The above is not a good approach for driving bipolar TTL.
)> >
)> >While this is a true statement, and I had the same sentiments, it's
)> >probably not helpful at this point. I didn't think he had the option of
)> >redesign, but rather to MAKE THE CIRCUIT WORK.
)> 
)> Yeah, you're right.  I saw that emitter follower trying to drive TTL,
)> barfed all over the keyboard, and started writing before I read the part
)> where he said he'd already gone out to fab and then stuffed the boards
)> before he'd completed his prototype testing and found the problem.
)> 
)
)O.K. I screwed up.  But the first thirty production worked.  They used
)74HC165 chips.  There was a typo and the next 100 used 74LS165.  The
)board did not change at all between the two runs.  At this point it was
)determined it would be cheaper to change two SIPS than 3 74LS165s.  The
)decision is pure economic.
)
)It is not like I built 100 boards, with no prototype.  We are past that,
)the prototype worked, first production was 30 boards that worked, and
)the screwup happened in the second production run.

This is Mike McCarty speaking.

I know that you didn't attribute all those statments to me, but I want
to make it clear that they were not all my statements. I did not and
would not make statments critical of you in the manner of some of those
statements. I do not know what you did, and don't presume to judge what
you did. I do not know what engineering considerations caused you to
use an emitter follower. For all I know, you had problems with rise
times, and needed the impedance reduction of using an emitter follower,
or for physical reasons (like size of the sensor) you were forced to
use a sensor which had the collector permanently tied to +5V or
something. I thought driving TTL with an emitter follower was a little
unusual, but since I don't know what your requirements or design goals
were, I didn't say anything about it.

I do not know why you chose to design your circuit the way you did, so
I don't judge the results. I have no criteria for judging you or your
efforts, either for design, or process. I know absolutely nothing about
your process, how much test you did, how many design reviews, etc.

I *can* offer advice on how to make what you designed work, and that's
what I did.

As it turns out, that isn't even the way you designed it. You designed
it for the HC, and in that case it really doesn't matter. 10K impedance
is nothing to HC, unless you can't take the fall times or there is a
noise source nearby, and you need lower impedance to combat it.

Anyway, I just want to make it plain that I don't presume to judge
other people's designs, and especially not using the vocabulary you
quoted above, and which I did not use.

I'm not even an electronics expert. I'm a mathematician. Who am I to
judge the designs of a real expert?

I'm just here to learn where I can, and help where I may.

Mike
- ----
char *p="char *p=%c%s%c;main(){printf(p,34,p,34);}";main(){printf(p,34,p,34);}
I do not speak for Alcatel. <- They make me say that.
This message made from 100% recycled bits.

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