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

[Karel Kulhavy <clock@xxxxxxxxxxxxx>]

On Wed, May 03, 2006 at 08:09:59AM +0100, Chris Emerson wrote:
> 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)

Olimex is doing DRC? Why, when the DRC is easily done by PCB? Isn't
it OK for them if you do PCB's built-in DRC and tell them it has
passed?

> panelise them.

The company which had no problems with my files also did panelization
themselves. Especially when friend did Ronja Twister boards with batches
of 100, they must have been definitely panelized into large panels,
because otherwise it would be unnecessarily expensive.

> 
> > > 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.

Yes I had it wrong. I should have said "LPC" instead of "IPNEG". I don't
remember the Gerber anymore.

> 
> 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.

Since the incident with the company that said it accepts RS274-X and
didn't, I put a URL of the freely available copy of RS274-X
specification directly into the README file that accompanies the data
and is sent to the manufacture. You can take it from there:
http://ronja.twibright.com/schematics/README
> 
> > > 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.

Eagle doesn't use LPC AFAIK. It uses hatching.

CL<
> 
> Thanks,
> 
> Chris