At 05:40 PM 2/13/01 +1100, Geoff Harland wrote:

>The idea of putting tracks on a Mechanical layer, and then including the
>contents of that (Mechanical) layer with just *some* of the Gerber files
>produced, to wit, the (Gerber) files produced from the internal power plane
>layers (only), raises the issue of a hobby-horse of mine.
>The fact that the Gerber files produced from the *other* layers will not
>include the contents of the Mechanical layer concerned implies that two sets
>of Gerber files need to be produced; these sets are distinct because the set
>of Mechanical layers which are included with each Gerber file depends upon
>which layer that the Gerber file is produced from.

Yes. That's the idea.

>  And when two sets of
>Gerber files are set up in a (CAM) Configuration file, then there are
>assorted implications when it comes to actually producing these Gerber
>For starters, it will typically be the case that each set of Gerber files
>will use a different set of embedded apertures (assuming that the RS274X
>option is selected). And the first file to be produced that lists the set of
>apertures used (with the corresponding set of Gerber files) will be
>overwritten by the second file of the same nature, that lists the (typically
>different) set of apertures used with the corresponding (other/second) set
>of Gerber files.

This is only a problem if the photoplotters use the aperture lists provided 
instead of the embedded RS-274X apertures. But, yes, this could be a 
problem indeed.

> >From my recollections, a report file that is produced at the same time as
>the first set of Gerber files is similarly also overwritten when the second
>set of Gerber files is subsequently produced (by an updated version of the
>report file).

I haven't checked this, but I would be surprised if it wasn't.

>In the circumstances, my inclination would be to deselect the RS274X option
>and invoke the feature in which an aperture list is created whose contents
>are determined by the contents of the PCB file. Then I would use that
>aperture list when subsequently generating the Gerber files. The resulting
>Gerber files would be of RS274D nature (no embedded aperture definitions),
>and an aperture file would be produced for each Gerber file produced.
>Because all of those aperture files have identical contents, it is not
>necessary to retain more than one of those files. And although I have not
>had cause to use it "in anger" yet, I have written a Perl script which will
>parse the contents of the aperture file, and then add appropriate embedded
>aperture definitions to all of the Gerber files, so converting them from
>RS274D format to RS274X format.

That is one solution, slightly more cumbersome than what I would suggest. I 
would write a little script or batch file that deletes the offending files 
entirely. They are unnecessary in normal use; in the event that someone 
needs an aperture list, it could be reconstructed from the files. Possibly 
I would put those delete commands together with a command that zips up the 
whole CAM directory and puts it somewhere convenient for storage and 

>Although this sounds convoluted, the outcome of doing this is that *all* of
>the Gerber files use the *same* set of embedded apertures. (Theoretically, a
>PCB manufacturer should be able to handle different Gerber files using
>different sets of embedded apertures. But my preference would be to not push
>my luck. And I consider that it is probably easier to preview a set of
>Gerber files when all of them use the same set of embedded apertures.)

Any fabricator that does not know that RS-274X files in a file set can have 
distinct apertures deserves to lose business. And it will be *his* bad 
luck, not the designer's, though delay hurts everyone. RS-274X files fully 
specify the plots. That, in fact, was the whole idea behind embedded 
apertures in the first place. Used to be that we were limited to only so 
many apertures per job. We are still limited to 999 apertures, but now it 
is per film, with RS274X, not per job. (Note that fills generate flash 
apertures, so if you use a lot of different fills, one might approach the 
limit on a complex job.) Older plot machines might be limited to 99 
apertures, which was an older standard.

>Before the release of the CAM Manager server, users could control whether
>Gerber files were of RS274D format or RS274X format, and regardless of how
>the aperture list was determined (whether by the user or by Protel, based on
>the PCB's contents). But as I have mentioned in earlier postings (the last
>time was some time last year), there has been some loss of functionality
>with the CAM Manager wrt Gerber file generation. The status quo is that if
>you specify a set of apertures, then the Gerber files produced from these
>will always be of RS274D format (no embedded aperture definitions); that
>remains true even if the set of apertures concerned was created by Protel,
>based on the PCB's contents. OTOH, it is possible to create RS274X format
>Gerber files, but only if you forsake all control over which apertures are
>used in these files.

Yes; however, it has been years since I was concerned over what apertures 
were used. The only problem here, really, is that having multiple CAM 
Gerber set-ups will leave only the last setup with an aperture table in the 
file set. Since that table is now little more than disk padding -- if one 
uses RS-274X, I think the gains from being able to specify differing mech 
layer inclusions is worth the minor problem. However, we need to know about 
that file problem, because it could confuse the heck out of some poor 
technician at the fabricator, and so I thank Mr. Harland for his thoughtful 
comments, even though I have come to expect this from him...

..... lest we forget what a blessing this list and its contributors are for 
Protel users.

Abdulrahman Lomax
P.O. Box 690
El Verano, CA 95433

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