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Date: Tue, 08 Jul 2014 23:39:48 -0400
From: Dan McMahill <dan AT mcmahill DOT net>
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Subject: Re: [geda-user] Silkscreened component values, mailing list, and
 gEDA development
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Reply-To: geda-user AT delorie DOT com

On 6/30/2014 8:02 AM, Peter Clifton wrote:
> On Sun, 2014-06-29 at 15:30 -0500, mskala AT ansuz DOT sooke DOT bc DOT ca wrote:
>
>> Finally, are there any plans for the diode schematic symbols and
>> footprints to ever agree with each other about which pin is the cathode?
>
> I think diode-3.sym agrees with the ACY series diodes in PCB (or at
> least, has reversed pin 1 & 2 from diode-1.sym.
>
>
>> I have found bug reports about this dating at least as far back as May
>> 2005, but nothing seems to have been done about it in nine years.
>
> Its not like we can arbitrarily change any of the existing symbols..
> doing so would break otherwise working designs.
>
>> If I
>> will be required to triple check every symbol and footprint and define my
>> own a large fraction of the time because there's a significant chance of
>> the default ones being as catastrophically broken as diodes are, then that
>> may be a dealbreaking issue and is at least something I'd like to know
>> about before I invest any more time in gEDA.
>
> Can I strongly suggest that whatever tool you end up using, you DO spend
> time checking footprint / schematic pin-outs match, hidden power pins
> (if any) connect to the nets you expect, and that physical footprints
> actually match the components you bought.
>
> Doing all the above is a routine part of designing PCBs that will save
> you no end of cost and time where inconsistencies arise. I know
> designers who basically throw out huge component libraries from big
> commercial EDA vendors, because they are not reliable enough to trust
> out of the box.
>
> If you haven't found it yet, http://www.gedasymbols.org/ is a useful
> place to find and share symbols for gEDA.

The issue of a reliable symbol/footprint library is one that is there on 
all systems.  I've been burned by using the bundled library on a very 
high dollar system before.  The problem with a diode is this.  Diodes 
come in many many different packages.  Unless every one of those 
packages agrees on which is pin 1 (anode or cathode) (and they neve 
will) then using a generic symbol for the diode will eventually cause a 
problem.  This is why I tend towards the approach of a symbol per 
complete part number (including the package codes) that is attached 
correctly to the footprint with the pin out correct.  I used a fairly 
simple build system to help generate all of those symbols from a 
template symbol and a text database.  It is some effort but the result 
is symbols I can trust.  Of course I haven't had time in several years 
to do any board designs so my library reflects that.  If you're 
interested though it should be up on gedasymbols.org.

http://www.gedasymbols.org/user/dan_mcmahill/

Among the many problems is that you won't even find universal agreement 
across vendors on how package pins are numbered.  For example:

#         ___x_
#        /     \
# TO18:  |3   1|     <-- bottom view (supposed to be a circle)
#        \  2  /
#          ---
# NOTE:  Philips and ST number their TO-18's differently.
#        I will use the JEDEC numbering which is what Philips uses.

SOT-23's are notorious for this as well.

Others have other approaches but for me having an ascii database that 
defines the mapping between symbol pins and footprint pins for each 
complete component has worked reasonably well.

-Dan (now returning to the rock I've been hiding under)