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Message-ID:	<55AAEF63.2060003@sbcglobal.net>
Date:	Sat, 18 Jul 2015 17:29:23 -0700
From:	"Girvin R. Herr (gherr375 AT sbcglobal DOT net) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
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To:	geda-user AT delorie DOT com
Subject:	Re: [geda-user] Difference between attached and detached attributes
References:	<55A813B5 DOT 4040609 AT sbcglobal DOT net> <alpine DOT DEB DOT 2 DOT 11 DOT 1507190000470 DOT 2893 AT nimbus> <20150718234406 DOT GA14479 AT visitor2 DOT iram DOT es>
In-Reply-To:	<20150718234406.GA14479@visitor2.iram.es>
Reply-To:	geda-user AT delorie DOT com

On 07/18/2015 04:44 PM, Gabriel Paubert (paubert AT iram DOT es) [via 
geda-user AT delorie DOT com] wrote:
> On Sun, Jul 19, 2015 at 12:09:52AM +0200, Roland Lutz wrote:
>> On Thu, 16 Jul 2015, Girvin R. Herr (gherr375 AT sbcglobal DOT net) [via geda-user AT delorie DOT com] wrote:
>>> I have been doing some symbol work in gschem and I got confused
>>> again about attached attributes vs detached attributes.  The
>>> manuals do not go into detail about usage.  Hint, hint.  Can
>>> someone tell me what is the difference between them and where I
>>> should apply one vs the other?
>> An attribute is a specially formatted text object which can be
>> attached to a component, net, bus, or pin, or not be attached at
>> all.
>>
>> An attribute which is not attached to an object is called a "floating" or
>> "toplevel" attribute.  Floating attributes are used for two purposes: in a
>> symbol, they contain default attribute values.  In a schematic, they
>> contain information global to that schematic page; for example, the VHDL
>> backend searches for a toplevel "module-name" attribute in the schematic,
>> and John Doty's spice-noqsi backend searches for "spice-prolog" and
>> "spice-epilog".
>>
>> An attribute which is attached to a component overrides the value from the
>> symbol's floating attribute.  (There are some exceptions to this like the
>> "graphical=1" attribute which can't be properly unset by attaching an
>> attribute to the component.)  You can delete an inherited attribute by
>> attaching an attribute with the value "unknown" to the component.
>>
>> You can't attach an attribute to a specific pin of a component's
>> symbol, so you can't override pin attributes when you instantiate a
>> symbol in a schematic.  There is a special slotting mechanism,
>> though, which allows you to select one of a set of pre-defined pin
>> numbering schemes.
>>
>> It is important to understand that the attributes inherited from the
>> symbol (both floating and attached to a pin) live in the symbol file,
>> whereas the attributes attached to the component live in the instantiating
>> schematic file.  You don't have a way to change the symbol from the
>> instantiating schematic; the way to change inherited attributes is to
>> attach an attribute with the same name and another value to the component.
>> (You could obviously change the symbol by embedding it and hacking the
>> schematic file, but AFAIK, there is no way to do this in gschem.)
>>
>> When creating a component, some floating attributes from the symbol are
>> usually copied to the schematic and automatically attached to the symbol,
>> hiding the inherited attributes.  This is called "attribute promotion".
>> The default set of promoted attributes is: footprint, device, value,
>> model-name, and symversion.
>>
>> The menu items "Attributes/Attach" and "Attributes/Detach" are used
>> to attach floating attributes to components, nets, buses, and pins,
>> and to convert attached attributes back to floating attributes.  You
>> don't need these for normal gschem usage.  (If you still want to try
>> them, take extra care not to accidentally lose invisible attributes.
>> You can type "e n" to make invisible text temporarily visible.)
> "e n" is a big hammer and often results in an extremely cluttered
> display: if you select a component, typing "t t" will toggle the visibility
> of the attributes of the component which is much less invasive. Typing
> "t t" again obviously reverts the operation.
>
>      Gabriel
>
Gabriel,
That's a good tip.  I made a note of it for future use.
Thanks.
Girvin Herr

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