At 01:10 PM 2/21/2002 +1100, Ian Wilson wrote:
>There are a few workarounds.  The one that I think is most documentable 
>but sometimes subject to Gerbering issues is the Lomax Virtual Short.

Note that Lomax himself now considers as at least equally satisfactory the 
use of mech layer shorts merged in the gerbers through CAM Manager 
definitions (which can be named, helping with documentation for future 
generations), the shorts being part of a special jumper component, just as 
with the virtual short.

Note that Schematic control of the short is a very important part of any 
solution. Schemes which do not automatically create and separate nets 
except at one point, the visible and controllable short between nets, do 
not satisfy this criterion; specifically this would be an argument against 
the modification of split planes as some have used.

>Basic method: make a really small gap between two small pads (0.1 mil), 
>give each pad a name and then create a special clearance design rule to 
>allow such a small gap between these pads.  Issues to watch for are gerber 
>rounding and aperture matching.  So set a tight apt matching tolerance and 
>set gerber to include more than the standard 3 decimal figures.

It is best if the pads in question are part of a jumper which appears on 
the schematic; the whole process becomes automatic at that point. Want a 
single-point ground? Put a single-point ground jumper on the schematic. 
With the virtual short you will need to set a design rule allowing the pads 
of that component to be so close to each other; with the mech layer 
solution, you still need to set up a special gerber definition and, 
preferably, to name the mech layer or layers used appropriately.

The gap should be smaller than 0.1 mil in my opinion. I've used 0.002 or 
0.004 mil. Protel can get a little flaky in the sub-mil region, so one may 
need to experiment (examples have been given in the past of sizes and 
definitions known to work).

PCAD has tienet polygons. I consider that solution, as far as I understand 
it, as inferior to either of the workarounds we have at present.

I've described in the past various alternatives, I think, as to how Protel 
could make this a directly accessible feature, instead of merely a 
workaround. Instead of going down that road again, I'll just state what I 
consider desireable.

I want to place a symbol on a schematic; it may have any number of pins, 
and these pins will be kept separate for netlist generation. However, the 
footprint which is associated with this symbol may have pads which are 
shorted together without creating any DRC error.

This, I think, would actually be quite simple to implement, it is really 
only a little jiggering with the DRC routines. Perhaps the routines would 
recognize something about the name of the symbol, in the type field 
perhaps, since that is fixed to be generated from the symbol name, which 
allows shorts between the nets of the pads to take place within the pad 
areas, whether by the pads themselves shorting or by track connected to the 
pads (provided that they only short within the pad area, not anywhere 
else). No special rule should be needed, because it is extra work to create 
such a rule and errors may take place during that. More than one name 
should be possible for this symbol, so perhaps the name would have a 
controlled prefix, such as PCBSHORT.

Protel support is distinct from Protel engineering. While we would wish 
that support personnel would read and be familiar with this list, I don't 
think that they are at this time. I might be wrong about that, at least 
with regard to some. I've many times said that it is completely natural and 
to be expected that this list can provide better support than Protel; I 
would suggest, in fact, that Protel abandon much of its direct support and 
direct the funds freed up by this to software maintenance and development. 
Basically, issues that were not resolved quickly on this list would then be 
referred to support personnel, who would be very closely connected to 
engineering.

This list generally answers questions more quickly than Protel support 
could possibly manage unless they were to throw a *lot* of money into the 
effort. And that would be silly.

[EMAIL PROTECTED]
Abdulrahman Lomax
Easthampton, Massachusetts USA

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