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From: Bdale Garbee <bdale AT gag DOT com>
To: DJ Delorie <dj AT delorie DOT com>, geda-user AT delorie DOT com
Subject: Re: [geda-user] Free rotate and exported centroid (XY) file question
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Date: Fri, 15 May 2015 10:01:50 -0600
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DJ Delorie <dj AT delorie DOT com> writes:

> If you go by the IPC spec, there's no way to know the *starting* angle
> of the footprint when you load it.

Well, on some level the only thing that really matters is the relative
rotation of different instances of the same part in a single design.

The reason I say this is that someone, somewhere in the manufacturing
process chain, has to generate a mapping from how the part appears at
the input of the placement robot to how the PCB image is oriented.  I
have never yet run into an SMT assembly house that didn't assume they
had to do that themselves... which makes sense since they're the ones
who know what machines they're going to be using, how the parts load in
those machines, etc.  This is in part why board houses insist that
footprints have robust pin 1 / cathode / whatever markings in the silk
or fab drawings, since a real human is going to want to look at that
data in almost every case...

> Or, you could hack in something to keep a rotation attribute around,
> and update it as needed.  Attributes don't need file format changes at
> least.

I think this is the right approach... maintain an explicit rotation
angle attribute for each placed part.  Anything else seems likely to
break in some circumstances.

FWIW, I've only once or twice placed a part off 0/90/180/270, and in
each case an explicit pin 1 marking allowed the assembly house to figure
it out without much help from me.

Bdale

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