Re: gEDA-user: General Layers questions

[John Doty <jpd@xxxxxxxxx>]

On Mar 16, 2011, at 11:09 AM, Stephan Boettcher wrote:

> John Doty <jpd@xxxxxxxxx> writes:
>> 
>> The "layer" concept should be physical, not a metaphysical
>> abstraction. Objects in a layer may contain holes, but a "hole layer"
>> is nonsensical, a toxic conceptual shortcut. An "outline" layer is
>> similarly bad: the insulating layers may all have the same shape
>> sometimes, but not always.
> 
> So, a via needs a separate hole in each copper and insulating layer?  And
> each layer needs its own discription of it's shape?

Just as a program needs to know that every object in an array of integers is an integer. The composition machinery is responsible for keeping track: you don't need to declare each array element separately.

> 
>> Trying to model things that aren't layers as if they were layers is
>> one common mistake in this kind of tool. Equally common is leaving out
>> layers: the insulating layers in a PCB are just as important as the
>> copper, and have their own properties (shape, thickness, material,
>> etc.). They're a critical part of the layer stack.
>> 
>> The description language needs to be able to express "feature p in
>> layer x is aligned with feature q in layer y" in order to build up
>> composites. This is the geometrically sensible way to describe the
>> result of drilling through several layers. But the geometric
>> description language should not be tied to any particular fabrication
>> procedure.
> 
> This is all too physikal for my taste.

I assert that if you do it any other way, you wind up with the following catastrophe: the code for every layer type needs to incorporate a specific definition of its interaction with the code for every other layer type. A total collapse of factoring, poisoning flexibility and maintainability. But if layers correspond to actual geometric layers, this can be avoided, I believe.

> 
> Why are you so attached to the concept of drilling?

I'm not. Indeed, I would urge our developers to purge the idea that a hole implies a drill from their minds when considering geometry. Export to instructions for a specific fabrication technology is a different problem from geometry capture, and in well factored software these will be kept separate.

>  For the design of a
> layout, all that matters is that there are conductive connections
> between layers. 

At the netlist level, that's all that matters. But at the layout level, it's geometry that matters.

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




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