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DPRG: Logic Family Comparison Chart and example pr

Subject: DPRG: Logic Family Comparison Chart and example pr
From: Ferialb at www.dci.co.ir Ferialb at www.dci.co.ir
Date: Wed Aug 25 00:19:15 CDT 1999

DP>Has anyone seen anywhere a table that shows the difference between the
DP>different logic families?
DP>Like what are all the differences between a
DP>74  74AC  74ACT 74AHC  74HCT  74C  74F  74HC  74HCT  74ALS  74LS
DP>74LCX  74LVQ  74S

DP>I just got bit by the differences between a 74LS165 and a 74HC165.  The
DP>74HC165 worked and the 74LS165 did not.

 I usually use 74HC or 74HCT devices, they are well suited for most of
works I do. They have good input & output impedance (they can
source/sink up to 6 mA according to ST data sheets). Others have
mentioned differences between 74HC and 74HCT. I'm completely careless
while handling them, but I have never blown any of them.
 I always consider 74xx, 74LSxx and 74Sxx out of date devices.
some people still use 74LS, I think because they are very chip and they
used to use them (but of course you would not use them any more! :)
 I once used 74S devices when I need about 20mA output current and I did
not like to add some buffer or transistors for that. It worked, but I
did not liked that since ICs got hot and used too much current, even
with no load connected to them. I will never use them again.
 Some use 74Fxx yet but they are old and power hungry too. I expect them
to have poor input impedance too.

 Any way, I highly recommend 74HC and 74HCTs. Forget TTLs! :)

DP>I am probably going to have to terminate the ends on the motherboard because
DP>the lines are so long and have just started looking into that. The thing
DP>that is not clear from the books is whether both ends have to be terminated.
DP> I would think so, but all the examples show only one.
 I guess from "mother board" you mean an industrial back plane (which is
preferred to be passive), am I right?
 I'm not an expert in termination, but there are several ways to do
that. For a perfect termination, you should terminate both ends with
resistors. I think I have seen several good application notes on National
or ST when I was looking for RS485 terminations. You can use shunt
resistors at one or two bus ends(with or without a series capacitor), series
resistors with drivers output or clamping diodes in bus ends.
 I like this last solution because it has good DC characteristic. You can
find 1N4148 SIP modules (like resistor arrays you have used). Using series
resistors seems interesting too, it will have same effect as using
slower devices as Mike suggested. But slow ICs usually means old ICs
which means some poor characteristics too.
 Finally, I agree that 23" is not so much and there may be no need to
termination at all (considering your somehow low frequency).
 I think there are some specially designed (differential) drivers too,
but I can't remember any.

 Best Wishes,
  Hamid Reza Badili
   mailto:Ferialb at www.dci.co.ir


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