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From: geda AT psjt DOT org (Stephan =?utf-8?Q?B=C3=B6ttcher?=)
To:  <geda-user AT delorie DOT com>
Subject: Re: [geda-user] A fileformat library
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Date: Wed, 06 Jan 2016 15:37:35 +0100
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	(Peter Clifton's message of "Wed, 6 Jan 2016 12:22:33 +0000")
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"Peter Clifton (petercjclifton AT googlemail DOT com) [via
geda-user AT delorie DOT com]" <geda-user AT delorie DOT com> writes:

> Indeed, although it realistically isn't actally that huge a task from the
> data point of view. The most obvious missing thing I can see, is a physical
> layer stack model. Pcb layers don't explicitly map 1-1 to physical layers -
> and at the very least, we need to inforce a defined ordering of layer
> groups, before a to-from layer group notion makes sense. (I sort of did
> this in the 3D branch, but it needs to be more explicit).

I do not agree that we need a 3D physical model in PCB.  That would be a
different tool.  PCB as is describes abstract elements of a printed
cicuit.

A Via needs to tell which layers/groups it connects to, and what kind of
pads there are on those layers.

It may be nice if the file format ensures that the layout can be
implemented on a pcb.  And that requires to define a layer order and
restrict vias to continuous sets of layers.  But who knows.  Maybe that
is a restriction that some day prevents some use for somebody?  Maybe it
is up to the DRC check to verify that there are no unphysical vias.
Layer order becomes a part of the design rules.  If the user can choose
vias from a library of known good vias, that will not be a problem.

The distinction between Pin and Pad can be removed. A Pad is a Via
inside an Element with only one layer and no hole.  No Via connects to
an outline layer or a documentation layer, no more need for a
rule/script in the Makefile to remove Pins and Vias from a layout to
export documentation layers.

Currently, all my PCB files to not have the layers in physical order.  I
like to have at least top and bottom layers always in the same colors,
so those are on layers 0 and 3, no matter how many layers the board
shall have, which may not be known from the start.

-- 
Stephan