Re: gEDA-user: gEDA 1.2.0, opamp-1.sym pin numbers?

[Andy Peters <devel@xxxxxxxxx>]

On Sep 23, 2007, at 7:40 AM, Dan McMahill wrote:

> Peter Clifton wrote:
>> On Sat, 2007-09-22 at 20:35 +0200, Stefan Salewski wrote:
>>> I think that opamp-1.sym will not be compatible with SOT23-5  
>>> footprints
>>> -- in TI datasheet of OPA656 pin 1 is output for SOT23 package.  
>>> Maybe
>>> opamp-1.sym is compatible with SC70-5 configuration, which I have  
>>> newer
>>> seen.
>>>
>>> Or opamp-1.sym and opamp-2.sym are only abstract representations  
>>> of an
>>> OpAmp, without any relationship to a concrete footprint?
>>>
>>> In any case, there seems to be no symbol in gEDA related to an  
>>> ordinary
>>> OpAmp in DIP8 (or SO8) package.
>>>
>>> This may look very strange for new gEDA/pcb users.
>>
>> There are various "heavy" symbols, such as lm741-1.sym.
>>
>> It is a well known problem though, the mapping between a light symbol
>> and a real package is not something which is easily solved.
>>
>
> which is why I'm in favor of a heavy symbol generator for op-amps like
> the one I demonstrated for bipolar transistors.  You use a small  
> set of
> symbols and then have a text file which is essentially a spread sheet
> that for each actual part number (the full vendor part number  
> including
> package code) you give the mapping from symbol pin to the footprint  
> pin
> # as well as list the actual footprint.  Its pretty simple, you don't
> have to continually wonder if you have the right footprint or right
> pinout, and you only have to maintain a small number of graphical
> symbols.  Adding new part number is then a snap.
>
> I'm more and more convinced it is the way to go.

This is one MAJOR reason to use heavy symbols.

I'd rather select a part number from a library, knowing that it  
always has the correct footprint and vendor part number (or numbers)  
every time, rather than go through an intermediate step to match ALL  
parts on the schematic with the proper footprints.  That's just too  
easy to screw up.

Sure your library ends up having lots of symbols.  That can be  
handled with a reasonable library directory structure.  Creating,  
say, a new resistor is as easy as copying an existing resistor symbol  
and changing the part number and value fields.

I've been in favor of the heavy symbols including a manufacturer part  
number, but this is limiting because multiple manufacturers make  
compatible devices, so how do you choose which manufacturer's number  
to use?  Either you allow multiple vendor part numbers or you create  
your own part-number system.  After using the latter system for a  
couple of years, I see its advantages.  All that's needed is a way to  
take a BOM generated by the schematic and run it against a database  
to get vendor part numbers (and prices and whatever else).

To further complicate things: for cases like 0805 resistors, where  
you have a lot of different values, perhaps the simplification is a  
"light/heavy" symbol.  This can work where the only variation is the  
resistance value; everything else (footprint, tolerance, etc) stays  
the same.  The part number in the symbol is a "base" part number.   
When you place an instance of the RES0805 symbol on the schematic,  
you modify the value as needed.  Then you generate the BOM, and your  
database lookup uses the base part number and the value to get a  
manufacturer part number.

It's funny how complex this can get ...

-a


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