Re: gEDA-user: How to deal with single/dual parts?

[Dave N6NZ <n6nz@xxxxxxxx>]

Peter Clifton wrote:

> I contend that the intended meaning for a 4-NAND gate chip, is that
> there are 4 slots. Not however many permutations of gate/pin swapping,
> or whatever else you want to do.
> 
> I hold that a "slot" is related to a multiplicity of identical units
> within the chip, and the slotting mechanism was _not_ intended as an
> arbitrary way of fudging different combinations of pin-numbers into the
> netlist in various situations.

That's overly restrictive in the real world.  First off, not all chips 
have N identical sections.  Secondly, there are ways to map functions 
into related, but not identical slots.

To use an ancient example to show that your idea was broken as long ago 
as the 1970's, consider a design with 5 2-input NAND gates and one 
3-input NAND gate.  I want that slotted into 4 sections of a 4x2 NAND, 
with the 3-input and the left over 2-input NAND slotted into a package 
of 3-input NAND gates with one of the inputs replicated.

Of course, that begs the question of how to show pin numbers on the 
schematic.

Symbols should show function.  There is a mapping from function onto 
implementation.  Implementation details are back-annotated onto 
schematics to show where to hang the 'scope probe.  If any of those 
pieces is done poorly enough, the system isn't usable.

Another piece that is missing in gEDA is that some attributes might be 
associated with the schematic, but other associated with a particular 
implementation of the schematic.  The CAD system at Amdahl when I worked 
there may moons ago (on the 5995 I-Unit) stored attribute sets 
separately from the schematic.  The schematic topology and attribute 
sets were separate files and individually rev'd.  You could pull up the 
schematic for a particular gate array, revision <x>, and join it 
(relational join on unique instance identifier) with attribute set 
<name> revision <x>. And a 2 input NOR gate in the schematic might be 
implemented with different macros and also placed differently depending 
on which attribute set you used.

-dave


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