On Thu, 19 May 2011 15:57:12 +0200
Kai-Martin Knaak <kn...@iqo.uni-hannover.de> wrote:
> Stephan Boettcher wrote:
>
> > The way to promote gedasymbols and to fix the default library is to
> > remove the default library, except for a small set of very generic
> > symbols.
>
> ack.
> This set of symbols should provide the ability to start working as is
> and generally be examples for complete working symbols. That is, they
> should contain footprint attributes.
Not to get into the whole light/heavy symbol debate, but since we're
talking about making this uniform, simpler, and easier for new users, I
think there would be less opportunity for error if symbols did not
include a footprint attribute, UNLESS there is only one footprint
possible. For instance, what footprint does a resistor symbol use?
Or, what footprint does an SPDT switch use? A battery? A transistor
(and of course many logical-physical pin mappings for transistors!).
Speaking of transistors, this brings to mind the mapping of schematic
pins to PCB footprint pins. Because I don't want to create a different
PNP, NPN, N-MOSFET, P-MOSFET, etc. symbol for each package pinout, I
use logical pin names ("numbers") in my symbols:
G, D, S (gate, drain, source) for MOSFET
B, C, E (base, collector, emitter) for BJT
P, N (P-doped and N-doped terminal) for all types of diode incl. LED
--> anode and cathode are less appropriate because they refer to
actual current flow (which may be reverse biased, esp. for
zener)
The I have corresponding footprints such as
SOT23__MOSFET_1G_2S_3D - MOSFET in SOT-23 package with gate on pin
1, source on pin 2, and drain on pin 3.
The result is that you need fewer variants of symbols and there is less
of the "magic" pin 1 = +, pin 2 = - assumption between symbols and
footprints on these generic parts, where pin numbering is not
standardized. It is much harder to make a footprint-symbol pin
mapping error since there is a single logical-physical mapping that
takes place in just one step (selecting the footprint).
If you think that the current gschem library's polarized capacitor and
diodes are sufficient, consider that gschem's “led-3.sym” has the
opposite polarity (pin 1 is negative terminal) of led-1.sym and
led-2.sym!! The casual user is very likely to overlook this. Even if
the pin numbers were shown, "pin 1" has no meaning for an LED, in
contrast to an IC package.
Regards,
Colin
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