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Message-ID:	<55AAD92F.6090406@sbcglobal.net>
Date:	Sat, 18 Jul 2015 15:54:39 -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>
In-Reply-To:	<alpine.DEB.2.11.1507190000470.2893@nimbus>
Reply-To:	geda-user AT delorie DOT com

On 07/18/2015 03:09 PM, 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.)
>
> Roland
>
>
Roland,
Thank you.  This is a good explanation of attributes and their uses.  
This explanation should go into the attributes document for others to see.
Thanks again.
Girvin Herr

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