At 07:47 PM 9/11/01 -0700, G. Allbee wrote:
>In this revision of the board I added a slot in the middle of the board.  I
>remembered an email from this group some time ago that said something like
>put a through hole on each end of the slot and then draw lines on the
>perimeter of the slot.  So this is what I did, however I used lines on
>multilayer figuring they would also work as keepouts and prevent routing in
>the slot.
>I discovered that the multilayer lines were preventing the autorouter from
>routing and when changed to a mechanical layer the router would work like
>normal..  Hmmm - a bug?

Thanks to Mr. Albee for reporting the source of tthe problem to us, with 
its solution.

Technically, this is not a bug; rather it is a situation where a program 
has been fed data which was not anticipated by the programmers. It is, 
indeed, a shortcoming, because a truly complete program would detect and 
report such data conditions. But we do not expect Protel to be "truly 
complete" because it would take unlimited programming resources, and we 
cannot afford to pay for that!

We *can* hope that the documentation will continue to become more complete 
and more accessible, and we may also hope that future releases will be able 
to detect this condition; but there will always be such traps; with 
continued work they will become rarer. As to the documentation, we should 
not depend completely on Protel for this: we have better resources, we only 
lack coherent organization, so maintenance and depth of the FAQ depends on 
an occasional volunteer. (More accurately, we *are* organized, but the 
organization is developing slowly.)

The multilayer was not intended for anything other than pads which exist on 
all copper layers; it could be argued that Protel should not permit any 
other primitives on that layer. The purpose of this layer is to create 
copper on all copper layers. Mr. Allbee perhaps expected that this would 
serve as a keepout, which, indeed, it should (i.e., it should function 
identically to a set of copper primitives matching it in location and size 
on every copper layer).

Mechanical layers have no effect on routing; they are pure drawing layers 
at present. The lines in the old suggestion that Mr. Albee remembered were 
lines which would appear on a documentation layer; they should have been on 
the fabrication drawing, which is, most simply, the drill drawing layer, or 
it is a mech layer assigned to the purpose of a fab drawing.

I used free track in the past for layer-specific keepouts, but this is no 
longer necessary, since Protel now has a keepout checkbox for lines, arcs, 
and fills. These are respected by the autorouter, I think; someone will 
correct me if I am wrong. Such keepouts do not photoplot. For a multilayer 
keepout, use the keepout layer!

The autorouter is old technology, we need to remember. It was not written 
to handle arcs, nor does it handle the full rule set, far from it, and it 
also gets a little crazy with off-grid pads. One of the recent service 
packs added a warning to design rules that are not respected by the router.

The autorouter is a relatively good router within its limitations, 
especially considering the price; we understand from Protel staff comments 
at recent PCB Design Conferences that the coming router upgrade will be far 
more rule-compliant as well as better and more powerful in other ways.

Abdulrahman Lomax
Easthampton, Massachusetts USA

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