On 01:52 PM 6/02/2002 -0500, Abd ul-Rahman Lomax said:
>At 02:00 PM 2/6/2002 +1100, Ian Wilson wrote:
>>Negative clearances on components do not work - would be nice if they 
>>did.  Alternatively a rule that allowed component clearance violations to 
>>be ignored for specific components or regions would be helpful.
>Here is what we should have. There should be two placement outline layers. 
>Track on these layers would represent the limits of reserved board space; 
>a closed outline figure would be required. Crossing of this track (not 
>mere contact) would generate a placement violation, as would enclosure of 
>one component's space by that of another except if the following feature 
>is added:
>A further refinement would be that additional closed figures would be 
>allowed *within* the outline, representing free space under the part. 
>(crossing of such tracks would be an error.)
>The placement outline layer would be an associated layer, i.e., it flips 
>with the part.
>The placement outline would also be used by any autoplacement routines.
>The design rules should also allow exceptions, as Mr. Wilson suggested.
>Abdulrahman Lomax
>Easthampton, Massachusetts USA

But then why stop there - the design rules should allow us to check that 
the component fitting under the one with some "off-pcb" space is not too 
high.  I may be able to fit an 0805 under a ROM socket possibly even an S0 
in some parts but not a fat tant.  Or what about fitting small components 
under the curve of a large axial cap?  The height available varies with 
distance from the centre line of the component.

I seem to remember that one of the PCB packages has a design rule system 
that can deal with addition and subtraction etc.  Using this (and with some 
extra design rule functions like DistanceToComponentBorder, and/or Overlap 
etc that give us measures off the component(s) being DRC'ed) we could get 
more elaborate.  Cost would be a significant slow down in DRC and great 
scope for complex design rules not doing what we actually wanted.

Ian Wilson

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