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Re: Bringing the IBM PC XT into the 21st Century

Posted: 20 Sep 2020, 21:41
by DMA
wcass wrote:
07 Sep 2020, 06:27
I draw them out as 2D DXF file and import them into DipTrace.
Wait, whole sense card? Does it count as one pin, or it's just "some copper fill" for diptrace?
wcass wrote:
07 Sep 2020, 06:27
This is all presuming that you are using a 0.8 mm thick 2 layer PCB with no vias. You improve signal detection by reducing the distance between the copper layers, so going to 0.6 or 0.4 mm PCB is helpful.
One can also electrically connect the top pad to the line, using a via. That will result in 3-4x better SNR. This also means the pad under the ex-floating pad is not needed. It will probably be beneficial to put the ground plane under both pads, but I didn't experiment with that.

4-layer PCB surely allows for awesome things like "frameless" layouts and can have an integrated ground plane (which enables whole new worlds in "rest of the keyboard" design - you can make everything from transparent acrylic, for example), but we don't yet have data on 4-layer PCB longevity in curved-plate designs.

Sem-related - my experiments with pad layouts had shown that only bottom 10mm of the pad are critical for model F - the rest can be freely used for any other purposes. You MUST keep top "dots" of the footprint - they lift the flipper's "legs" to ensure proper contact area when the flipper is down - but you can run traces thru those (ANY traces - rows, columns, ground, LEDs - whatever), because those dots are too far to interfere with sensing.

Re: Bringing the IBM PC XT into the 21st Century

Posted: 20 Sep 2020, 23:05
by wcass
DMA wrote:
20 Sep 2020, 21:41
wcass wrote:
07 Sep 2020, 06:27
I draw them out as 2D DXF file and import them into DipTrace.
Wait, whole sense card? Does it count as one pin, or it's just "some copper fill" for diptrace?
It is multiple "imported object". The entire import does not add to pin count.
DMA wrote:
20 Sep 2020, 21:41
wcass wrote:
07 Sep 2020, 06:27
This is all presuming that you are using a 0.8 mm thick 2 layer PCB with no vias. You improve signal detection by reducing the distance between the copper layers, so going to 0.6 or 0.4 mm PCB is helpful.
One can also electrically connect the top pad to the line, using a via. That will result in 3-4x better SNR. This also means the pad under the ex-floating pad is not needed. It will probably be beneficial to put the ground plane under both pads, but I didn't experiment with that.

4-layer PCB surely allows for awesome things like "frameless" layouts and can have an integrated ground plane (which enables whole new worlds in "rest of the keyboard" design - you can make everything from transparent acrylic, for example), but we don't yet have data on 4-layer PCB longevity in curved-plate designs.
Curving a PCB puts the "inside" layer in compression and the "outside" layer in tension. This is made worse as the thickness increases. I think that vias might be susceptible to damage from these forces, so i try to keep the PCB thin and though hole count low.
DMA wrote:
20 Sep 2020, 21:41
Semi-related - my experiments with pad layouts had shown that only bottom 10mm of the pad are critical for model F - the rest can be freely used for any other purposes. You MUST keep top "dots" of the footprint - they lift the flipper's "legs" to ensure proper contact area when the flipper is down - but you can run traces thru those (ANY traces - rows, columns, ground, LEDs - whatever), because those dots are too far to interfere with sensing.
I plan to take advantage of this when i do a pad card for Unicomps upcoming Mini M. I will likely run a 6x16 matrix with the row traces going through the "dots" - giving wide clearance to the tenon holes.