Since many, if not all Simpad SL4 models suffer from touchscreen issues
(resistance isn't linear anymore, preventing it from being calibrated properly
), I will attempt to fix the issue by replacing the touch panel entirely.
I connected the SL4 touch screen to an Arduino to confirm that the problems are caused by the touch panel
(the glue binding the upper and lower panes together apparently causes issues after 10 years or so
)
After quite a while of searching, I found
a 4-wire resistive touch panel on Aliexpress which has a large enough active area and fits in the Simpad SL4 case. The panel is overall smaller and thinner, but there is a lot of wiggle room.
Let's get started!
Here's the SimPad with the replacement touch screen
A few T7 screws removed and I'm in. Keep in mind that the battery is attached to the back shell with velcro.
Disconnected the battery and touch panel.
Figured out the pin output of the Siemens touch panel, conveniently it's the same on the replacement
The old panel is glued on with pretty sticky glue. I managed to get in with a razor blade and a lot of silicone oil, trying to not break anything
And it's off.
I connected the new panel using regular 2.54mm pin header connectors, they fit fine in the touch panel connector.
The problem: it doesn't work well. I can't properly calibrate the screen, and I suspect it's because the new touch screen has a much lower resistance than the stock panel.
(Stock panel has 2.0-2.2k resistance between Y1 and X2, while the new one has about 200-600 ohm
)
I will try soon whether simply adding resistors solves the issue.