At 09:31 PM 6/4/2002 -0400, Mitch Berkson wrote:
>I'd like to draw graphical dashed lines in a schematic to indicate guard
>traces which should be placed during PCB layout.  Is there a way to do 
>this in Protel 99SE SP6?

Others have noted how to place a dashed line, and also that such a line has 
no incorporated intelligence, it is only a note to the PCB designer.

However, it is possible to go further than that.

I assume that the guard traces are to be grounded, since they would be 
positively harmful if they were not. So one would place a jumper at the 
ground connection (whether this is at the driving end or the receiving end 
depends on the design, but if it is to be grounded at more than one end, it 
should be grounded *frequently* along its length to a solid ground, 
probably a plane).

this jumper is what we have called a "virtual short." There are two ways to 
implement it.

(1) a jumper footprint can be created that has a gap of, say, 4 microinches 
between rectangular pads, and a design rule allows that particular 
footprint to have a gap of 2 microinches. If boards were always fabricated 
according to nominal values and to the limits of the technology (which is 
well above the microinch region), this gap would never appear on the 
finished boards. However, we have seen cases where round-off error in the 
gerber generation, coupled with poor control of the aperture assignment and 
other conditions, has resulted in a gap on the order of 1 mil on the 
gerber, which has then been opened by an assumption-making fabricator to a 
fabricatable dimension.... The technique is still useful, but one would 
want to ensure that the aperture assigned is rounded up, and this is enough 
of an opportunity for error that I now prefer the second method.

(2) a jumper footprint is created that has a shorting bar placed on a 
mechanical layer. One of the copper layers is chosen as appropriate to 
which to add the short, and the gerber definition for this layer is 
eliminated from the CAM manager gerber control file (most of us only have 
one just file for a design). A different file is created by copying the 
first, and all other layers are eliminated from that file except for the 
copper layer in question; then the mechanical layer dedicated to the short 
is merged in photoplot with the copper layer. this additional CAM control 
file is given a descriptive name so that future generations do not become 
confused. (Note, however, that if the shorting bar is forgotten, it should 
be a simple matter to short the jumper footprint manually on the PCB, 
assuming that the short lives on an outer layer. I wish all 
design/fabrication errors were as easy to fix....)

The mech layer used for the short is also given a descriptive name, such as 

(Note also that if all the CAM files are checked for plotting and left that 
way, the copper layer with the short will routinely be plotted with all the 
other files; this is pretty much set-and-forget.)

A combination of the two techniques *could* be used, which would leave it 
very unlikely that the gap would ever be present.

Both these techniques fool Protel into thinking that the net to be 
controlled is isolated from ground; one controls the grounding point by 
controlling the placement of the short, which is highly desirable behavior.

Protel does not have, unfortunately, a signal pair routing constraint, 
which would make this process even more controllable. But there is 
Tools/Outline_Selected_Objects, which would add the guard traces, which 
could then be selected with "select connected copper" and globally edited 
to the appropriate subground net (i.e., the apparently isolated net created 
by placing the virtual short on the schematic, which would likewise be 
given a descriptive name, such as "DETECTOR SHIELD" or the like.) After 
editing the outline to the appropriate name, the outline would be broken 
and the virtual short moved into position. DRC will then correctly check 
the routing.

Similar techniques allow the creation and control of other kinds of 
subnets, such as star grounds, single-point connected analog/digital 
grounds, etc; also  the process has been used to create RF components where 
copper is physically connected (at DC) but needs to function and be treated 
as if it were not.

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