Re: gEDA-user: Resistor valuesâ

[al davis <ad252@xxxxxxxxxxxxxxxx>]

On Saturday 25 December 2010, Vanessa Ezekowitz wrote:
> * If the part in question can usually be described by a
> single value, for the purposes of the signal flow in the
> schematic that is, then give it a default of "value=0".

No.  Zero is almost always wrong.  The only sensible default 
value in this case is a copy of the reference designator.  If 
the reference designator is "R1", the default value should be 
"R1".

Justification ...  For simulation, all modern simulators, and 
some not-so-modern simulators have a means of assigning values 
to parameters.

In a spice format, you could say:
.param R1=10k
or something like that.
Some let you use parameters in other ways, making it even more 
useful.

> * If it is a discrete part that is specified entirely by its
> part number rather than a measurement, like a diode or a
> transistor, then pick a reasonable default; "value=1N914" or
> "value=2N2222".

As I recall, those are specific devices that were popular 40 
years ago.  Not sure how relevant they are today.  Unless you 
link to something, they are just arbitrary strings, no better 
than any other.

The best default would be something that is more universally 
meaningful, and not specificm like "D" or "diode" for a diode, 
or "NPN" for an NPN transistor.

For a simulator that reads spice format, you could then say:
.model D D
.model NPN NPN
to make the names meaningful.  It might be useful to make an 
"include" file to define these things.

> * If the part is something like a logic IC, use the standard
> name of the part in the fastest commonly available series
> for that particular gate; "value=74F74" or "value=74HCT245".

Same as diodes.  Names like 74F74 have no meaning, unless you 
look it up, and are completely wrong most of the time.

> * If none of these fits, then leave the "value=" attribute
> off entirely, since the user would have no choice but to get
> creative anyway.
> 
> The point here is that every one of the component types in
> question has a value, therefore the "value=" field will end
> up being utilized by nearly everyone who instantiates the
> symbol for that component.  Otherwise, schematics with a lot
> of such symbols become nearly impossible to read, let alone
> debug.



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