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PICList Thread
'[OT]: I-facing PalmIII Touch Screen to a PIC16F877'
2001\06\30@043156 by Tim Thompson

Hello all,

I was wondering if anyone out there knows anything about the touch screens
used in 3COM Palm IIIx's. I recently recieved a Palm IIIx thats LCD was
smashed, so I had the idea of trying to interface the touch screen panel to
a PIC. The panel consists of a thin clear plastic sheet with conductive
strips on the right and left sides. The Palm connected to these 3 strips in
6 places: top, middle, and bottom of both side strips. The 6 points came
together at a 4-pin connector. It was difficult tracing the connections
from the 4-pin connector due to the fact that the glass was smashed
something fierce, but I beleave the pinout is as follows:

Pin1: Left side middle
Pin2: Both sides top
Pin3: Right side middle
Pin4: Both sides bottom

I beleave the touch screen is of a resistive type, because i think
capasitive types require human touch, which rules out the
little pen dealie that the Palm uses. Anyway, if anyone can give me any
information on how to use this thing please let me know!

Tim Thompson

P.S. Info on the EL backlight would be apreciated too, such as power
requirements, etc.

Remember, 'kill' doesn't kill processes, users kill processes.

-- hint: To leave the PICList

'[OT]: I-facing PalmIII Touch Screen to a PIC16F877'
2001\07\04@100453 by M. Adam Davis
Typically resistive touchscreens have 4 contacts, two on each layer.
Imagine it as follows:
Take a sheet of plastic, and spray resistive paint on one side.  Attach
an electrode one each of two opposite sides of the plastic.

This forms one sheet of the touchscreen.  If you connect one electrode
to + and one to ground, then you will see a voltage gradient across the
surface of the plastic.

If you take another similar sheet, and place it facing the first sheet,
but turned it 90 degrees, then you have a basic touchscreen.

When nothing is touching the screen (pressing the two sheets of plastic
so the contact each other at a given point) then the fours contacts form
two resisters of constant resistance, the top sheet and its two contacts
forming one resister, and the bottom and its two contacts forming the
other, with no connection between the top and bottom.

We'll label the contacts A and B for the top layer, and 1 and 2 for the

When the screen is being touched, the top and bottom are connecting at
one point, such that a resistance is formed between the bottom and top
contacts.  You can probably imagine the rest of the method there.

I would suspect the palm is of the same basic design, but I can also see
a method of doing this from your description, as long as the electrodes
are resistive as well.  Otherwise, according to your description, all
the wires on the connectore are connected, since the long strips on the
sides in the touchscreen are generally of low or negligible resistance.
Use a tester to determine which wires in the connector have resistance
to other wires when there is nothing on the screen, and then test
touching the screen.  If you hook a tester between one side and a top or
bottom, then you'll see the resistance change as you move about the screen.


Tim Thompson wrote:

{Quote hidden}

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