gEDA-user: pcb refdes name restrictions?

[Stuart Brorson]

On Mon, 30 Oct 2006, John Luciani wrote:

> On 10/30/06, Peter Baxendale <peter.baxendale at durham.ac.uk> wrote:
>> Another dumb question. I teach a class of undergraduates about ECAD and
>> this year abandoned commercial tools in favour of geda. Students being
>> students, they tend to try things I wouldn't think of doing. Today, a
>> couple of them decided to be creative and on their schematic used names
>> like "CONNpower" and "CONNsignal" for refdes values. Whilst I thought it
>> unconventional and probably inadvisable, I couldn't offhand see why they
>> shouldn't do that.
>
> A reason not to have long refdes values is clutter. Names that are seven and
> eight characters get difficult to place (legibly) on dense schematics and
> PCBs. A seven character refdes will probably take up more board area than 
> most
> of you SMD components.

I'll add a second reason.  In a very common design flow, you first
create a schematic with refdeses R1, R2, R3, C1, C2, C3, etc.  Then
you lay out the board.  Then when the layout is done, the layout
engineer *renumbers* the refdeses from e.g. upper left to lower
right.  The new refdeses are then backannotated into the final
schematic.  The idea is that refdeses with similar number all lie
close to each other so that when it comes time to service the board
(or during DVT) you can more easily find the components.

Note that you can't do this with things like CONNpower and
CONNsignal.  How do you renumber alpha refdeses?  Admittedly,
CONNpower and the like are easier to deal with than J1, J2, etc, but
if you've got a board with thousands of components on it, then you
can't give each a unique alpha refdes, and the above renumbering
scheme is extremely convenient.

Since we can't do backanno in gschem/PCB, this point is moot, however.

Stuart