There is a huge difference between a CAD-system and PCB-design software.
A CAD system such as AutoCad and similar are using the aproach that you
always use the scale 1:1 wich means that one drawing-unit equals one
real-world-unit so it does not matter if you are using metric or imperial.
The reason is that you normaly want the drawing on paper, so you scale it at
print-out, or let the CAM-SW handle the rounding, CAD SW do not work with
1/2" representation but with 12,7 representation and when translating the
drawing, especially a mecanical one, you normaly don't work with 0.01 mm
tolerances, which means that rounding is quit easy and non-destructive.

PCB-design software on the other hand does work in absolute measurements
i.e. the database is basad on either mm or inch and not drawing units. And
when working on a scale of 0.01 mm or even less tolerances, scaling is an
issue, and the same thing applies, it dont work with 1/2" but rater 0.5 in =
12,7 mm and as I mentioned earlier its, a quite difference between 0,508 mm
and 0,5 mm, its nearly a 2% rounding error.

With best regards

Tomas Larsson
Sweden
http://www.naks.mine.nu for downloads etc.
ftp://ktl.mine.nu for uploads. Or use the free www.yousendit.com service.

Verus Amicus Est Tamquam Alter Idem 

> -----Original Message-----
> From: piclist-bounces@mit.edu 
> [mailto:piclist-bounces@mit.edu] On Behalf Of Olin Lathrop
> Sent: Monday, February 20, 2006 2:44 AM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] Metric units
> 
> Gerhard Fiedler wrote:
> >>> All CAD where I've looked into this use an integer representation 
> >>> that is a sub-multiple of both mm and mil.
> >
> > Can that work? What base unit would that be? I don't think such a 
> > number exists.
> 
> It certainly exists and some CAD systems use exactly this 
> method.  As someone else already pointed out, 100um is a 
> sub-multiple of both a mm (10) and inch (254).  By factoring 
> out the common 2, you get 200um as a base unit for both (1/5 
> mm and 1/127 inch).  You can keep scaling that down until you 
> get the resolution you want.
> 
> 
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