On Tue, 5 Feb 2002, Abd ul-Rahman Lomax wrote:

> Looking at Mr. Wiseman's files, I discover that there is indeed a zero
> aperture in the RS-274X list. Wondering what the source of this file might
> be, I looked through the PCB file and found two fiducials with arcs with
> zero width, used apparently to keep tracks out. (These arcs should have the
> keepout attribute, but they don't.)

it's not clear that keepout is useful here, and, since using keepout for
features other than board edge has been seen to irritate the autorouter, I
tend to not use it. I'm prepared to be convinced otherwise, but at the
moment, simple copper does fine.

> Editing the arc to 1.5 mils draw causes the pads in question to be drawn
> correctly. Deleting the arc also causes the pads to be drawn.

this is the giveaway, isn't it...

> It appears that there may be a minimum match width, not too surprising. I
> don't have time to check further today, but the match routines could be old
> enough that they allow a 1 mil variation no matter what.

1 thou, on a board where I'm using 4 thou track and gap, just won't do. 2
thou boards are coming my way soon, by the look of things. Time for Protel
to get this fixed, and maybe generate a patch / SP?

> The virtual shorts being used here depend on very precise photoplotting,

I generally boost the long axis of the shunts by a thou before plotting,
to be safe. However, I do not expect (or see) any problems plotting at
this size. The whole board's 4-thou track & gap - two thou overlap across
a 6 thou feature is enormous.

> Elsewhere I described an alternate method of doing a virtual short, that
> is, a jumper footprint that appears as open to DRC but is actually shorted
> in plot. This alternative involves placing a track, as part of the VS
> footprint, on an otherwise unused mech layer.

I used to do this, and still do sometimes for jumper links. However, it's
extra steps at photoplot time, and extra steps = extra opportunities for
mistakes. Since, by the time a board this complex goes out, I've normally
been working 20-hour days for a week, the simpler I can make things, the
longer the PCB shop will let me sleep before phoning for clarification :)

> There are several bugs here, interacting. When a zero aperture is assigned
> to draw the pads, they are not drawn, they are missing. The pads themselves
> are not completely drawn even when the aperture is correct, though this may
> be harmless in this case. The zero-aperture arc, however, is drawn. I
> placed a zero-width track and it is also drawn correctly, the problem is
> just in the pad-drawing routine.
>
> Aperture match is not exact even if match is set to 0.

I think that summarises concisely and accurately - I thank you all for
your time & effort - this list continues to be amazingly useful!

Steve

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