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From: stever@videovax.UUCP
Newsgroups: comp.sys.amiga
Subject: Re: Amiga Disk & HAM problems
Message-ID: <4383@videovax.Tek.COM>
Date: Mon, 11-May-87 11:46:44 EDT
Article-I.D.: videovax.4383
Posted: Mon May 11 11:46:44 1987
Date-Received: Thu, 14-May-87 05:59:23 EDT
References: <867@isrnix.UUCP> <1731@tekigm2.TEK.COM>
Reply-To: stever@videovax.Tek.COM (Steven E. Rice, P.E.)
Organization: Tektronix Television Systems, Beaverton, Oregon
Lines: 40
Keywords: capacitors decoupling
Summary: Those capacitors should have been there in the first place!!!

In article <1731@tekigm2.TEK.COM>, Jac Colby (jacc@tekigm2.TEK.COM) writes:

[ description of bit-rot and other problems caused by an added board ]

> I removed the Squeeze-Ram and all worked well.  Aminetics
> provided a fix, and so far, all is well.  Significantly, the fix
> was to add .047uf caps to the memory address decode chips to
> clean up the signals.  . . .

Those capacitors should have been there in the first place!  A good rule
of thumb is to use one 0.1 uF capacitor at *each* 74ALSxxx, 74ASxxx, and
74Fxxx chip, one 0.1 uF capacitor at each PAL, and one 0.1 uF capacitor
for each two 74LSxxx and 74HCxxx parts.  Each memory part should have
its own capacitor (0.1 uF for 64K DRAMs, 0.33 uF for 256K and 1Meg DRAMs).

If the capacitors aren't there, when the part tries to yank an output
down (transition times are on the order of 2-10 nanoseconds), the part
merely succeeds in bringing +5 volts and ground closer together for a
short time (voltage spikes in excess of 2 volts on the +5 volt supply
are not uncommon).  This can feed CRUD (Continuous and Random Unwanted
Disturbances) into the rest of your system.

One generally-reliable measure of quality is the placement of decoupling
capacitors.  If *every* part is properly decoupled (appropriately-sized
decoupling capacitor located adjacent to the part; short, wide runs or
power and ground plane connections between each capacitor and its part),
the chances are much better the product will work correctly.

					Steve Rice

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Copyright 1987 by Steven E. Rice, P.E.  All Rights Reserved.  This material
may be redistributed only where such redistribution is without charge and
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