Re: gEDA-user: Resistor valuesâ

[Vanessa Ezekowitz <vanessaezekowitz@xxxxxxxxx>]

On Sun, 26 Dec 2010 23:55:04 -0500
al davis <ad252@xxxxxxxxxxxxxxxx> wrote:

> 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.  

Exactly my point - it is *supposed* to be wrong.

I chose zero here because anyone who sees it in their schematic file should instantly think, "Oops, I forgot to set the value of that part".  From my own experience, it is easier to spot something that is visibly flat out wrong than to look for something that is just not there.

Setting it to zero by default could even be used to signal Gschem to add an extra highlight to those symbols bearing it.  Perhaps the default, highlight-sensitive string should be exactly "0.0" or some variation of that, since no sane user would type anything but a single "0" when they mean "zero".

> The only sensible default value in this case is a copy of the reference
> designator.

No, that's wrong too.  If I wanted to see the refdes's, I'd keep PCB in that view mode instead of putting it in value mode, and in the schematic, doing this would just lead to doubled refdes's everywhere initially, which could get confusing in densely packed schematics.

> 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.

Where does this tie in with what default the symbol might have in its "value=" attribute?  If you're directly assigning that 10K value to R1 within your spice code using its refdes as the basis for the assignment, then it seems to me you're already overriding whatever value was specified in the Gschem schematic and/or the symbol library.

> > * 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.

I mentioned the 2N2222 and 1N914 because they are easy to use and easy to find.  Being general purpose components, they make excellent teaching aids, and I use them myself from time to time for the occasional LED driver or small signal amp (nothing big by anyone's standards here).  That said, the examples I gave could have applied to any of a hundred other part classes, though.  Would my argument been any different if I'd named some modern power MOSFET or the latest Atmel microcontroller instead?

Discrete transistors aren't as popular as they were a few decades ago in computer technology, but they are still in common use in the analog world (power supplies, amplifiers, radios, telephones, hobby electronics).

If my suggestions aren't to peoples' liking, then try something a little more logical: if you don't want to go with something well known for a given symbol's default because that something is "too old" or "outdated", then at least go with whatever is the number one selling item in that class (as measured by end-user distributors like Radio Shack rather than wholesalers and manufacturers).  You're virtually guaranteed to get it right most of the time that way.

> 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.

If that were the case then there's no point at all, because the symbol file itself, by definition, already tells you what it is.  You (and the simulator) already you're using a diode or an NPN transistor or whatever by virtue of the fact that you chose to instantiate the symbols for those parts.

The original poster was asking about adding values to resistor symbols so that they show "R1" and "10k" on the schematic diagram only.

I was expanding on that idea to handle named parts as well -  what *kind* of diode or transistor the PCB will end up having stuffed into it at build time.  Since "value=" has no meaning at all with such things, I suggested it be re-purposed in those cases.

> 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.

I'd say that one would need to know precisely what kind of transistor one is using in a circuit before a simulation begins, or it may end up with invalid results.  I don't know whether the "value=" attribute is the right place to put that information, but I can't think of a better place.

> > * 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.

Fair enough, but you're missing the point I was trying to make with the named parts in particular:  PCB can display component values or refdes's (or names, for that matter), and my point was to give Gschem something to display alongside the refdes's in the schematic, and to give PCB something to display in the "value" mode instead of "unknown" all over the place.  It doesn't matter if "74F74" conveys enough detail to make it possible to simulate it or not - find some other attribute to represent that information.

-- 
"There are some things in life worth obsessing over.  Most
things aren't, and when you learn that, life improves."
http://starbase.globalpc.net/~ezekowitz
Vanessa Ezekowitz <vanessaezekowitz@xxxxxxxxx>


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