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From: doug@terak.UUCP
Newsgroups: net.micro.pc
Subject: Re: PC/AT fast xtal warning
Message-ID: <1011@terak.UUCP>
Date: Wed, 22-Jan-86 12:32:11 EST
Article-I.D.: terak.1011
Posted: Wed Jan 22 12:32:11 1986
Date-Received: Sun, 26-Jan-86 05:24:51 EST
References: <604@harvard.UUCP> <1101@ptsfa.UUCP> <996@terak.UUCP> <8533@amdcad.UUCP>
Organization: Calcomp Display Products Division, Scottsdale, AZ, USA
Lines: 34

>> manufacturer's FAST (tm) logic: one part had much lower input loading
>> than Fairchild's "standard" part, making the input lines susceptible to
>> noise on a board where Fairchild's part worked fine.
> 
> Lower input loading sounds like an advantage to me. If it causes
> problems, is it possible the device was not used correctly? Remember
> most input loadings are specified as maximums and even the original
> manufacturer may improve the part and reduce the loading.

True, the specs don't give a minimum loading figure, but I don't know
anybody who terminates every etch on a board in a low impedance just in
case some chip manufacturer should decide to reduce the input loading to
a few microamps.

FAST family parts are, well, fast.  They tend to be used in applications
with very high frequency signals.  FAST also generates some pretty sharp
edges.  It's nearly impossible to keep signals from cross-coupling on
the board; you have to rely on the low impedances to swamp them out.

> >And DRAMs!  I'm beginning to think that no two DRAM chips are inter-
> >changeable, even from the same manufacturer   1/2 :-)    Aw, the spec
> >sheets are nearly identical, but their behavior is something else.
> 
> I can't say I've had much trouble. What exactly are you referring to?

I should've been more explicit.  Make that 256K DRAMs.  In simple
applications with short traces, few chips and non-critical timing,
they probably aren't a problem.  But when you try to put a few hundred
together to make a large memory subsystem (this necessarily results
in lengthy traces) and then try to run them close to rated speed, good
luck.  Some chips work, others don't.  Even though the specs are met,
some just seem more sensitive to things like noise and edge rates.
-- 
Doug Pardee -- CalComp -- {hardy,savax,seismo,decvax,ihnp4}!terak!doug