At 08:21 AM 9/24/01 -0500, Mark E Witherite wrote:
>         The issue with the virtual short is that it must be cut to test 
> the two nets.

Right. And if you know you are going to cut it, and especially if you are 
going to cut it before assembly, it is desireable to fabricate it "cut," 
i.e., open, i.e., it is a simple jumper or zero-ohm resistor instead of a 
virtual short.

For those who might be popping into this discussion late, a "virtual short" 
is a footprint which is made such that Protel sees it as two or more 
separate and non-connected pads, does not generate a DRC error, and yet it 
fabricates as a short. There are several methods for doing this, they have 
been detailed elsewhere.

On the schematic, a jumper is placed between the nets to be shorted. So, 
from a schematic point of view, there is no difference between a virtual 
short and a jumper.

A virtual short may easily be made in such a way as to be easily severable, 
should this be necessary. So the decision as to whether or not to use a 
virtual short or a jumper/zero-ohm resistor depends on what one wants for 
the as-fabricated default.

It is *not* recommended to generate a single ground net for 
otherwise-isolated single-point grounds. The reason for this is as Mr. 
Witherite explained: there will be no easy way to discover additional 
connecting points, nor is there any control over where the connection is 
made, which can also be crucial. DRC won't care. Using a component to 
accomplish the connection forces a single-point connection and makes it 
simple to control the location of that connection. An additional advantage 
is that one may sever the connection in trouble-shooting the board; in 
development, one can also test the effect of putting resistance or 
inductance into the connection; if the footprint has been properly chosen, 
one can change the nature of the connection from a direct short to a 
resistor or an inductor without requiring any revision to the artwork.

Abdulrahman Lomax
Easthampton, Massachusetts USA

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