Re: gEDA-user: Composite/negative layers in gerbers

[Chris Emerson <geda-user@xxxxxxxxxxxxxxxx>]

On Tue, May 02, 2006 at 08:49:08PM -0400, DJ Delorie wrote:
> > My board house[1] has complained about my gerbers having "negative
> > plots" and "composite layers".
> 
> Can you send us examples?  Which PCB are you using (hid or pre-hid)?

I'm using pcb 20050609, and one of the boards at
http://www.tartarus.org/~chris/tmp/pcb/

What Olimex say is that they can't DRC or (more importantly for me)
panelise them.

> > 1) (Main question) How can I tell if my gerbers are negative and/or
> > composite so I can check?
> 
> In the gerber file itself, look for the strings IPPOS and IPNEG,
> and/or LPC and LPD.  For pre-hid PCB, also search for LNCUTS.

They've all got IPPOS, but they do have LPC, LPD, and/or LNCUTS.

> A generic trace layer will have IPPOS (positive polarity) and LPD
> (draw the "dark" parts).  IPNEG is a negative polarity layer,
> sometimes used for ground/power planes if certain conditions are met.
> LPC means we're drawing "cuts" or "clears" to erase previously drawn
> stuff, such as when we make clearances for traces through polygons.

Thanks for the description.  I think I found the RS-274X spec somewhere,
so I'll have a read and see if I can make sense of it.

> > 2) What can I do to affect whether pcb uses negative and/or composite
> > outputs?  Is that what the "Invert positive/negative" tick box on the
> > printing page does, at least for the negative stuff?
> 
> There's not a lot you can do, because there's not much else we can do
> to make clearances.  The Gerber spec itself says to use LPC to make
> clearances.  The gerber HID doesn't use IPNEG yet, so that might be an
> option for you at the moment.

Well, IPNEG doesn't seem to be the problem so that probably doesn't
help.  I'll maybe see what Eagle produces, as they certainly accept
that.

Thanks,

Chris