Path: utzoo!utgpu!water!watmath!clyde!bellcore!faline!ulysses!allegra!princeton!udel!gatech!bbn!uwmcsd1!grc!gary
From: gary@grc.UUCP
Newsgroups: sci.electronics
Subject: Re: Wire-wrap Question
Message-ID: <799@grc.UUCP>
Date: 15 Jan 88 15:56:12 GMT
References: <7110004@hpcupt1.HP.COM>
Reply-To: gary@grc.UUCP (Gary Sutcliffe)
Organization: GENROCO, Inc., Slinger, WI
Lines: 44
Posted: Fri Jan 15 10:56:12 1988

In article <7110004@hpcupt1.HP.COM> glowell@hpcupt1.HP.COM (Gary Lowell) writes:
>
>Does anybody know if wire-wrap can be used to build a reliable proto-type
>at 20 Mhz clock rates and what techniques can be employed minimize
>problems ?  I am trying to determine the feasibility building a homebrew
>computer using 20 Mhz 80386 chipset.
>
>Thanks for the help.
>
>--
>Gary Lowell

I have built a few prototypes using 20 MHz clocks.  I have found it can cause
lots of little glitches that are hard to track down.  Most of these seem to
disappear when a printed circuit version is built.  We have stopped building
wirewrap prototypes of high speed circuits because of these flakey problems,
and start with a PC prototype.

However, if you still want to try, here is what I suggest:

1) Start with a wirewrap board with a good ground plane.  One side should
be solid ground (except for small areas for pins), and the other should be
power or alternating power and ground.

2) Use *lots* of bypass capacitors.

3) Keep ICs as close together as possible to keep wire lengths to a minimum.

4) Use a twisted pair for long wires and for wires that carry high speed 
signals (such as clocks), or where a large number of wires are next to each
other for a long distance.  Connect the second wire to ground at each end.
 
5) Memory arrays are a special problem because signal reflections can mess
up the close timing required to run dynamic RAM.  Consider using a memory
card from another computer and interface that to your wirewrap CPU.  
Considering the pain it is to wire up a large array by hand, you would 
probably be better off buying a memory card anyway.

Good luck! - Gary.
-- 
Gary Sutcliffe  W9XT                             (414) 644-8700
GENROCO, Inc.  Slinger, Wis.
{ames, rutgers, harvard} uwvax!uwmcsd1!grc!gary
Disclaimer (data compressed):   "f+)Je\ ^_RMv9h|O |j8gELc=tAYw$pfs!1"