Path: utzoo!mnetor!uunet!tektronix!sequent!mntgfx!mbutts
From: mbutts@mntgfx.mentor.com (Mike Butts)
Newsgroups: sci.electronics
Subject: Re: Wire-wrap Question
Message-ID: <1988Jan18.121135.245@mntgfx.mentor.com>
Date: 18 Jan 88 20:11:30 GMT
References: <7110004@hpcupt1.HP.COM>
Organization: Mentor Graphics Corporation, Beaverton Oregon
Lines: 44
Summary: If you're careful it'll probably be fine...

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.
> 

In my experience, wire-wrap can work well for small high-frequency runs,
IF YOU'RE CAREFUL.  Your main enemies are inductance and stray coupling.  
There are several considerations which will help:
1) I believe the part of your system which actually runs at 20MHz will be
small.  Place the parts so as to minimize the 20MHz run lengths.
Keep in mind, though, that edge rates will be fast for many signals,
regardless of their actual frequency, so be mindful of the following.
2) Use twisted pair for the 20MHz runs.  That is, use a twisted pair
of 30ga. wires, with the ground wire wrapped to ground posts as close
to the source and destination pins as possible.  Needless to say, keep
your runs as short as possible (within reason).
3) Run fast signals directly point-to-point, not in neat up and down
channels, to avoid crosstalk.  The board may not look pretty, but the
signals will look better.
4) Use a board with a ground plane, and ideally with power and ground
planes.  (I know Augat makes such boards, they or others may even have 
app. notes on this subject.)
5) Use high-frequency 0.1 mfd bypass caps very liberally - one on each
chip and more on the LSI.  Place them close to the Vcc and Gnd pins.
You might even consider soldering them to the posts.
Caps are cheap, and bad bypassing will give you the most hateful kind
of flakey bugs.

I've never worked with 20 MHz microprocessors, but have seen successful
experience with multi-board FAST-TTL systems running 8MHz globally and 16 MHz
in small parts.

Good luck and have fun!  Here's to the home-brewer!



-- 
Mike Butts, Research Engineer                         503-626-1302
Mentor Graphics Corp., 8500 SW Creekside Place, Beaverton OR 97005
...!{sequent,tessi,apollo}!mntgfx!mbutts OR  mbutts@pdx.MENTOR.COM
These are my opinions, & not necessarily those of Mentor Graphics.