Re: gEDA-user: GND and Vcc pin on IC 74245

[Wilbert Knol <w.knol@xxxxxxxxxx>]

On Friday 09 September 2005 23:45, David D. Hagood wrote:
> Wilbert Knol wrote:
> > Idea and I have always been totally intrigued why anyone would
> > want to do this - I have yet to see a single, good argument in
> > favour.
>
> OK, try this - there is not always a one-to-one mapping from
> entities on the schematic to ICs. Consider a 7400 quad NAND gate
> chip: this part is not rendered as a box with 4 NANDs in it, but as
> four separate NAND gates "floating around" on the schematic. OK,
> so, where do you put the power pins?
> 1) On every device, and have the problem of somebody connecting
> U1A's Vcc to the +5 digital net, and U1B's Vcc to the +5 analog
> net? 

Agreed. Unworkable.

> 2) On one device in the package? Now one device is "special" 
> and has extra pins, so you cannot auto-number them.

Again, I agree. 

Incidentally, this is why the argument that 'gschem allows you to have 
a choice' doesn't wash. There is no workable alternative to hidden 
power nets.

> 3) On a hidden net so that the normal behavior is that the parts
> "Just Work".
> I agree - when you are dealing with a device for which there is a
> one-to-one mapping between schematic and board hidden nets are not
> really needed, but for some devices they really do make things
> cleaner.

This is stating two bad alternatives to (somehow) justify an even 
worse solution - see my previous posting why I strongly dislike 
hidden nets. 

Suggestions:
 
1. Have the power pins show up only on a virtual, extra gate, or even 
just a box to symbolise the package.

2. Or make them show up only on the slot (gate) of one's choice, and 
attach an attribute to the gate. This attribute  - a list, similar to 
the present 'slotdef' attribute - exempts the power pins from being 
auto-numbered, and identifies them as being unique to this slot.

Anyway, this is a minor gripe. Gschem is still a great tool.


Wilbert.