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DPRG: Logic Family Comparison Chart and example problem between famlies.

Subject: DPRG: Logic Family Comparison Chart and example problem between famlies.
From: Kipton Moravec kmoravec at airmail.net
Date: Tue Aug 24 18:22:50 CDT 1999

MIKE MCCARTY -- 93789 wrote:
[Snip a bunch of good Stuff]
> Nope, LS won't work. It requires right at 500uA pull down. 10K cannot
> provide that, because the input does not float to 5V, but quite a bit
> less, like 2V or a little less. You're only going to pull about 200uA,
> or somewhat less than 1/2 that required. For LSTTL, you could go as high
> as about 3K for the pull down.
> )Running some experiments it looked like the pull down had to be 8300
> )Ohms or less for this circuit to work.
> Not reliably, it won't. It might with that particular gate.
> )I only had 100 boards built. :(  We are now in the process of replacing
> )the pull down SIPS. (Two 10-pin SIPS are quicker to replace than three
> )16-pin DIPS).
> I think you are still not out of the woods. If you intend to use LSTTL
> parts, then you must *not* rely on an 8.2K pull down. That's too high.

That is correct, we are replacing the 10K pull down with 2.2K SIPS. It
seems to work. The voltage is measured at about .3V when not being
read(SH = Hi), and about .4V when being read (SH = Lo).  This is about
half way below the .8 V necessary for a low.

> Go look over at Fairchild. They have some nice reference material on the
> various logic families. I dunno whether they cover LSTTL, but I know
> they cover HC and HCT families. Fairchild called their ACT line FCT.

I will check them out.

I am currently choosing a transceiver like the '245 style.  I need to
talk from one board to 10-12 boards on a 23" chassis.  Currently I am
leaning towards a 74AHCT245.  Is there something better?  I am planning
on running the backplane at 2 MHz or 1 MHz. The transceivers will be
mounted as close to the connector on each board as possible.

I am probably going to have to terminate the ends on the motherboard
because the lines are so long and have just started looking into that.
The thing that is not clear from the books is whether both ends have to
be terminated.  I would think so, but all the examples show only one.


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