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From: koko@uthub.toronto.edu (M. Kokodyniak)
Newsgroups: sci.electronics
Subject: Re: active high TTL decoders?
Message-ID: <359@uthub.toronto.edu>
Date: Wed, 21-Jan-87 03:43:52 EST
Article-I.D.: uthub.359
Posted: Wed Jan 21 03:43:52 1987
Date-Received: Wed, 21-Jan-87 19:42:14 EST
References: <571@vu-vlsi.UUCP> <14394@amdcad.UUCP>
Organization: CSRI, University of Toronto
Lines: 62

> In article <571@vu-vlsi.UUCP> colin@vu-vlsi.UUCP (Colin Kelley) writes:
> >I'm looking for a 1-of-8 decoder (like the 74138) but with active high
> >outputs.  I've looked through several TTL data books, but I haven't been
> >able to find one.  This isn't exactly crucial, as the same effect can be
> >had with a 74138 + 1 1/3 7404s, but I hate to go adding another two ICs
> >to my design after paring the chip count down so carefully with some neat
> >ICs (like the 74259).
> 
> Get with it, guy! Where have you been the last six or so years? Use a
> PAL. TTL is obsolete and dead. For high speed, PALs come in D
> varieties which run at about 10 nS, and that gives you a trip through
> an AND-OR plane. For low cost, the plastic PALs are amazingly cheap.
> New PALs provide buried state registers for implementing synchronous
> state machines. Programmable polarity outputs mean an end to
> DeMorganing equations. There are many new innovations such as UV
> erasable, EE erasable, RAM programmed (great for debugging), *large*
> arrays, programmable sequencers etc.  All designers should have PALs
> as part of their collection of tools.
> 
> Note: PAL is a trademark of Monolithic Memories but we make them too.
> -- 
>  Social security is welfare for the elderly.
> 
>  Phil Ngai +1 408 749 5720
>  UUCP: {ucbvax,decwrl,hplabs,allegra}!amdcad!phil
>  ARPA: amdcad!phil@decwrl.dec.com


I agree with you that PAL's are great, but to replace three chips with
a PAL is overkill (like trying to kill a bird with an anti-aircraft gun).
TTL is far from dead!  Even with PAL's, some TTL glue logic will
probably be necessary.  Besides, not everyone has access to programming
tools needed for programming PAL's.

Furthermore, haven't you heard of Fast (F-series) and Advanced
(ALS-series) TTL?  These afford greater speeds than standard
TTL, if such is needed.

I don't understand why engineers and technical people always have to be
so gung-ho about technology and insist that nothing short of the
leading edge of technology is good enough for them.  I assume some
humility and am quite content riding the falling edge of technology.
Besides, I am not that lazy that I would always insist on the rising
edge just to save time and effort.  Furthermore, I appreciate that
the falling edge can sometimes save me money.

And now, I get back to the problem at hand without condescending flames.
Look up the data sheets for 74ALS538.  It is a descendant of 74LS138,
but has "three-state outputs, output polarity control [as you want],
multiple enables for expansion, data multiplexing capability",
according to the Texas Instruments TTL Pocket Book (1984).
It comes in a 20-pin DIP (as many PAL's do, but consumes less power).
It offers a one-chip solution to your problem.  However, this chip
might be a bit pricey and may be difficult to obtain through an
average consumer dealer.  (This latter statement is just speculation
on my part.)

I hope that I have been of some help.


				Sincerely yours,
				Mike Kokodyniak