kicad-developers team mailing list archive

[Wayne Stambaugh <stambaughw@xxxxxxxxx>]

On 3/30/2016 7:24 AM, Paul "LeoNerd" Evans wrote:
> On Wed, 30 Mar 2016 12:22:48 +0100
> "Paul \"LeoNerd\" Evans" <leonerd@xxxxxxxxxxxxxx> wrote:
> 
>> <for ease of replies; continued in part 2...>
> 
> and now on to the more generic "where KiCad could be improved" part of
> this problem...
> 
> -----
> 
> As a further consideration, this leads into the area of KiCad that I
> dislike, and that's the way that schematic symbols and part footprints
> are so intertwined like this. Nowhere is it more apparent how annoying
> it is, than in the area of transistors.
> 
> Suppose I'm just sketching out a circuit, and I decide I need some
> bipolar NPN transistor. I haven't decided yet exactly what part I'm
> going to use, so I certainly don't know its pinout yet. It'll be some
> sort of small-signal NPN perhaps. I don't want to commit to a
> particular pinout, I just want to place "an NPN transistor" in the
> schematic. Later once I've played around with maybe half a dozen of
> them on my breadboard I might finally decide on some specific part,
> maybe a BC549, so now's the time I commit to the shape. I want to
> notate that at the time I run cvpcb *and not earlier*. Once I've
> associated footprints, only now does any consideration of which
> schematic pin (B/C/E) is which physical pin (1/2/3). Depending on exact
> parts, these logical-to-physical associations could be done any of six
> different ways.
> 
> The problem is that KiCad doesn't let us arrange schematics and
> footprint parts in a nice generic way. In KiCad we're forced to either:
> 
>   A) make six different NPN transistor schematic symbols
> 
>   B) make six different e.g. TO-92 transistor footprint parts
> 
> This isn't very nice.
> 
> Taking a look at e.g. what Eagle does here, they have a 3-way split:
> 
>   *) A single schematic symbol of simply "NPN transistor", with pins
>      named B, C, E.
> 
>   *) A single PCB footprint of simply "TO-92", with pins numbered 1, 2,
>      3.
> 
>   *) A "component" which associates a schematic ("NPN transistor") with
>      a footprint ("TO-92") and provides the pin mapping E=1 B=2 C=3
> 
> This is great, because adding new kinds of NPN transistor means never
> having to draw that schematic symbol again - you just associate it with
> new footprints (or maybe the same footprint with different pin
> mappings). Adding new TO-92-shaped components means never having to
> redraw that footprint - you just associate it with new symbols (maybe
> some 2-pin current sources that come in TO-92 cases with one pin
> unused). Overall, it minimises the amount of work needed to add new
> parts.
> 
> I would love KiCad to have something similar like this. I find it very
> disheartening to have to copy an existing schematic symbol or footprint
> part when all I want to do is change the pin mappings between them.
> 
> How I think it could be done, is to somehow start to break the enforced
> requirement that schematic and part pin names must match. One idea is
> that a footprint part could in fact just name another part and provide
> some pin renamings. For example, under this hypothetical scheme, I
> could define a footprint part for a BC549, which is simply
> 
>   (component Transistors:BC549
>     (module Housings:TO-92)
>     (pin-mappings (map 1 E) (map 2 B) (map 3 C))
>   )
> 
> and then nobody has to *copy* the TO-92 footprint, because it's only
> referenced by name. Someone then draws an NPN transistor schematic
> symbol whose pins are given the obvious B, C and E names; someone draws
> the TO-92 footprint whose pins are numbered 1, 2 and 3. Then when I
> select "Transistors:BC549" in cvpcb it now knows how to associate those
> virtual pin names into physical pin numbers.
> 
> The upshot of this model is that it doesn't break anything currently -
> existing 1:1 mappings that go directly from schematics to parts
> continue to work unaffected. But now it means people can place "a
> transistor" or "a diode" or whatever generic parts, and only later at
> cvpcb time do they have to know what pinout details apply to the
> selected component.
> 
> Who's with me here?

You are a little late to the party.  This has already be discussed and
most likely will be part of the new schematic and symbol library
formats.  I just haven't had time to update the file format documents.
However, this still does not prevent you from shooting yourself in the
foot.  It only allows you to do so more conveniently.

> 
> 
> 
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