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Message #73213 of 126777
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Date: 2005-03-07
From: William Chops Westfield
Subject: Re: [EE] Wire wrapping insight needed!

On Mar 6, 2005, at 7:28 PM, Jake Brownson wrote:

>> Mr Design, go for it. Us 'ole guys have been proven wrong before...
>> I am not afraid of being proven wrong...I can take it.
>
> So what do you "ole guys" use for prototyping?

PCBs.  At $100 or less for a prototype board (actually, several
boards) and less than a week turnaround, WW doesn't make sense.
And that's supposing you don't make your own PCB.  (do you know
how little engineering time, or even technician time, you can
buy for $100?)  (At some professional engineering level, the prototype
boards have fine pitch packages and 12 layers, and the need to
use a PCB for the prototype as well as production is "obvious."
The point is that it makes a lot of sense for smaller circuits, too.
I spent some significant dollars on an LPKP circuit board router
(mechanical etching thing) so that I could go from computer to
finished board at a high level of 'convenience.'  I'm still not
sure it was a good idea.  I think in the several years I've owned
the machine, I could have had every board I've made as a hobbyist
on it (which is not as many as I'd like, but still) made
professionally and still had money left over...

Arguably, back when WW was common, designing PCBs took more skill
than WW (hey, places hired high schools students like me to do WW
on prototypes, but the PCB layout guy was a skilled ancient :-)
But now there are cheap (or free) high quality packets like CadSoft
Eagle, and I don't think that it's a lot harder to come up to speed
with Eagle than it is to come up to speed from nothing with WW,
especially if you're talking about a 20MHz design and needing
twisted pair.  I WW'ed my senior design project, but that was
three or four chips on the periphery of an 5Mhz (?) system.
I WW'ed a keyboard encoder (ala TV Typewriter cookbook) that
was perhaps a dozen SSI chips.  And the aforementioned summer
job in HS.  None involved twisted pairs.  I'd be real nervous
using WW with signals that ran faster than about 2MHz.

The rats-nest vs neatly-routed wire argument is somewhat controversial.
The job said "be neat" (but things were slower then.)  The "serious"
designs allegedly tell you what length of wire to use for each
connection, cause the wire's delay is part of the system design.
(Cray-1 backplane, for example.)  debugging is simplified with a
neater layout, I think.

You can combine techniques.  Lay out the high frequency parts, power,
and decoupling and parts of the design that are "certain" on a PCB,
then use WW sockets for an wrap the rest.  This gives you the sort
of 'evaluation board' that many vendors now offer.  And in fact I've
seen an increasing number of projects in the 'senior design project'
class built on top of such evaluation boards (which is yet another
possibility.)

BillW

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