Re: gEDA-user: PCB format wishlist

[John Doty <jpd@xxxxxxxxx>]

On Sep 6, 2010, at 11:05 AM, Andrew Poelstra wrote:

> On Mon, Sep 06, 2010 at 10:43:50AM -0600, John Doty wrote:
>> 
>> On Sep 6, 2010, at 10:33 AM, Andrew Poelstra wrote:
>> 
>>> On Mon, Sep 06, 2010 at 09:53:36AM -0600, John Doty wrote:
>>>> 
>>>> Need some geometric shapes. Need to be able to attach material properties
>>>> to them (including "vacuum" for holes).
>>>> 
>>> 
>>> How about?:
>>> trace      (inc. arcs, pads)
>> 
>> Trace? Pad? Forget them: they aren't primitive geometric concepts. Remember,
>> all shapes should support any use by any material, or vacuum, in any layer
>> or collection of layers. A "trace" is a particular (usually itself composite)
>> shape of conductive material.
>> 
> 
> I don't think we want to drop all the way down to geometric shapes - at some point,
> why not just call numbers and strings primitives and forget this whole discussion?

Choosing the right level for the primitives is important. I wouldn't drop below a "planar stack of geometric shapes" here. But I wouldn't go higher for primitives either. One might very well wish to draw arbitrary shapes in silk, or one might require holes of arbitrary shape. I once worked with a board that had different numbers of layers in different places, and with different conductive materials for different traces (not designed with pcb!). A well-factored design should be able to express this kind of thing.

A format that would be equally at home specifying the layout of a printed circuit board, a VLSI chip, or a Mondrian painting would be a good target. I don't think we need to go as far as spherical circuits or Picasso ;-).

> 
> A trace has a length, width, and curvature, which are different properties
> than that of a polygon (which is a set of points, so I suppose isn't really
> a primitive), or a circle, which has a radius and sweep angle.
> 
> Perhaps "line" is the word I should be using.
> 
>>> polygon    (inc. rectangle, etc)
>> 
>> What's the distinction here?
>> 
> 
> No distinction. I'm just indicating the scope of these primitives.
> 
>>> circle     (inc. quarter-circle, half-circle)
>> 
>> Versus arc?
>> 
> 
> A circle could be considered as a "filled arc". I have been considering arcs
> as "curved lines", which I think makes more sense.
> 
>>> via
>> 
>> Definitely not primitive. A hole in one or more layers with conductive material in it.
>> 
> 
> Again, while geometrically a via is not primitive, I think that in PCB
> terms, a via is primitive. It can exist on several layers, which the
> other shapes do not, so it doesn't make sense to build it out of other
> shapes.

It's often necessary to align shapes on different layers: that's not a special property of vias. So, the machinery of composition of objects from more primitive objects needs to be able to control that. Then it would make sense to use that machinery to compose vias. The reason we can't have blind and buried vias is a consequence of the lack of such factored, orthogonal design in pcb at present.

> 
> 
> Andrew
> 
> 
> 
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John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@xxxxxxxxx




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