At 08:48 AM 12/20/01 +1000, Damon Kelly wrote:
> > If you want one footprint that can except two different
> > devices you can
> > do that with a special footprint (named ssop14-ssop16??).
>This option causes DRC errors - or at least it does in my implementation. I
>sometimes require an SO-14/SO-16W footprint, to allow alternative op-amps,
>and I always seem to get DRC errors of the type "extra pads on net" etc...I
>can live with them, but it's irritating.
>Is there anyway to prevent the DRC from tripping over the extra pads?

Last time I looked, Protel does not generate a DRC error for pads where the 
pads have the same name. If one of the pads has no name, it will create a 
DRC error. However, in either case, track connecting the pads, if it is 
part of the footprint, will create short errors until the track is assigned 
the appropriate net, either as part of the synchronization process or using 
Design/Netlist Manager/Menu/Update Free Primitives.... -- which also 
updates component non-pad primitives. Technically, this is a minor bug, but 
we use it.

To summarize, there are three ways to handle double-padding:

(1) Place two symbols on the schematic with a note that only one of them is 
to be inserted. If the two symbols are placed on top of each other, the 
note may not be necessary. Name one of them, for example, U14A, the other 
U14B. Each one of these may then have its own footprint. There will be two 
separate components on the PCB, they can be placed in any arrangement that 

When one symbol is placed atop another, one may wish to hide some of the 
various text displays....

(2) Use a single symbol on the schematic but place a double-pad pattern on 
the PCB. This footprint has two pads for each original schematic pin. Each 
of these pads will have the same name. If the Synchronizer is used to 
transfer net information, the pads will be properly assigned nets; if track 
or other primitives are added, Update will be required. If Load Netlist is 
used, there is still a bug which can cause assignment oscillation.

(3) Create a schematic symbol with double pins. I won't go into detail on this.

Protel does not support multiple components with the same reference 
designator. It will cheerfully allow you to create them, but it does not 
properly handle net assignments. I can see arguments for leaving it this 
way, but it could also be argued that allowing multiple components with the 
same refdes (and which would be assigned identical nets) would be useful 
for double-padding. On the other hand, a single footprint for 
double-padding is schematic-controlled and thus will survive 
synchronization with unused component removal, etc., and I think it's 
better to leave it as-is.

Protel does not seem to support free primitives in Unions. If it did, this 
would provide another solution to this problem (though Update would still 
be required); one could simply add pads and create a union with the 
original footprint. Free primitives in Unions would be useful for other 
reasons. My guess is that they did not do it because it would have added 
another field to all the primitives, whereas component Unions only involved 
the component primitive, not all its parts. Maybe next release.

Abdulrahman Lomax
Easthampton, Massachusetts USA

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