At 07:50 AM 9/25/01 -0700, Dwight Harm wrote:
>I want to have a non-rectangular void on a ground plane, but it appears I
>can't put a polygon there, only a rectangular fill.  When I place a polygon,
>my plane layers don't show up in the drop-down box for choice of layer.

Right. Protel reserves inner-plane layers for split planes. They *could* 
distinguish them, I forget if there is a distinguishing field in the 
database, but it is not high on my list of priorities, I'll explain why.

Place the polygon blowout that you want on a mech layer (or any non-copper 
layer, but using a mech layer is best). Be sure to uncheck "remove dead 
copper," since all the "copper" will be dead and your polygon might become 
one of those infamous invisible polygons, a nuisance and a half. Use a wide 
draw width and 0 grid. Cross-hatching is overkill. The draw width should 
only be as narrow as necessary to accomplish the outline you want, this 
will keep the number of primitives down, an advantage from a number of 
points of view.

(If one is using a mix of metric and inch grids and draw sizes, and the 
gerber output is in a different mode than the PCB database, the latter will 
be approximated to the chosen resolution and very small gaps might appear 
in the gerber output; displayed gaps, if you see any, are display 
artifacts, which you can tell by zooming in, they do not get larger than a 
pixel wide no matter how far in you zoom.)

Then select everything on the mech layer you used -- it is simplest if it 
is an otherwise empty layer at the time. (Edit/Select/All on Layer -- which 
will use the current layer). Then explode the polygon with 
Tools/Convert/Explode Polygon to Free Primitives. Then globally edit those 
primitives to put them on the inner plane you want to use.

There is a way to keep your polygon and move it too. Unlock the primitives 
-- this is an option when you build or repour the polygon. Then, assuming 
that they are selected as noted above, double-click on one of them to pop 
up the edit box. Globally edit the selected primitives to the inner plane.

The polygon remains on the mech layer you used, but all its primitives are 
moved to the inner plane. If you rebuild the polygon, the primitives will 
be back on the mech layer. This would allow you to edit the polygon.

But leaving the polygon that way could be hazardous, since this combination 
of parameters is quite likely to be untested (a polygon on one layer with 
its primitives on another). I'd recommend, before the final plot of a job, 
exploding the polygon.

>So I've tried changing to having an non-plane, just an inner layer with a
>large polygon connected to ground (DGND).  But it seems like the autorouter
>doesn't "know" about it -- it routed DGND with traces.

Yes. The autorouter currently behaves in that way. It may have been written 
before polygon fills were added to Protel, I'm not sure, but that is a 
known deficiency of the router, it adds redundant traces. It adds them, as 
I recall, even if you have routed the pins on the *same* layer.

>I found a bit of a workaround -- leave the DGND plane there for the
>autorouter, then remove it later.  I think another option would be to build
>my void out of multiple rectangular fills.
>Am I missing something?

You could also simply draw the void with track, only if it is a complex 
shape would it become appropriate to use the polygon technique I described. 
The workaround you used is reasonable. Be sure to run DRC *after* 
eliminating the DGND plane.

>(I've also found I have to turn off the display of the layer with the large
>polygon if I want scrolling to be reasonably responsive.)

Yes. Another reason to explode the polygon if you go that way, but your 
polygon may have been large because you probably used a fairly small draw 
size. It would not be unreasonable to use a 50 mil draw or wider. Polygons 
with small draw widths are naturally large; using a pour grid and 
crosshatching could almost quadruple that size. I never crosshatch.

Abdulrahman Lomax
Easthampton, Massachusetts USA

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