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Fixing a Simpad SL4 touchscreen

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Karpour Page Icon Posted 2017-04-05 12:19 PM
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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.
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Karpour Page Icon Posted 2017-04-05 1:02 PM
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I found the chip that handles the touch panel input, it's the Philips UCB1300BE located near the RESET button.
I think the problem with the replacement not working is due to the software expecting certain resistance characteristics.

Current ideas for fixing the touch panel:
- add resistors to make it appear more like a stock panel
- put an Arduino in between which reads the new touch panel and passes the values through, pretending to be a touch panel itself
- same as above, but intercept the serial bus between cpu and touch panel controller to pass values.
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Alt Bass Page Icon Posted 2017-04-05 1:23 PM
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Did you use the usual trick to override calibration limits? It's
HKLM/HARDWARE/DEVICEMAP/TOUCH/MaxCalErrorWORD


Set it to gigantic value below 0x80000000.

Edited by Alt Bass 2017-04-05 1:31 PM
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Alt Bass Page Icon Posted 2017-04-05 1:24 PM
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Holy sweet little baby Jesus, it parses smile codes inside code tag too....

HKLM/HARDWARE/DEVICEMAP/TOUCH/MaxCalError:DWORD


Edited by Alt Bass 2017-04-05 1:26 PM
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CE Geek Page Icon Posted 2017-04-06 5:33 AM
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LOL
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Karpour Page Icon Posted 2017-04-06 3:23 PM
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I will try that and get back with results! Of course it'll be a bit of a challenge to do that without a properly calibrated screen, bit I think I'll manage
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Alt Bass Page Icon Posted 2017-04-06 9:33 PM
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You can use ActiveSync to control it.
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Alt Bass Page Icon Posted 2017-04-06 9:37 PM
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Aaand you can write a BAT file which executes a command line registry manipulation program which you place next to BAT file.

In the worst case you can compile a program for that in eVc.
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C:Amie Page Icon Posted 2017-04-07 8:25 AM
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Quote
Alt Bass - 2017-04-05 1:24 PM

Holy sweet little baby Jesus, it parses smile codes inside code tag too....

HKLM/HARDWARE/DEVICEMAP/TOUCH/MaxCalError:DWORD
The board didn't have a code tag, I hacked one into it very quickly a few years ago - emphasis on VERY quickly ;)
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Laci Page Icon Posted 2019-12-31 2:12 PM
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Did you ever get the touchscreen to work properly?
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Karpour Page Icon Posted 2020-01-05 5:04 PM
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Sadly not, I had to shelf this project due to having no time. The 2 ways this could work is

1. Change registry values (which is hard when touchscreen is the only input available)
2. Somehow add a circuit in between the new touchscreen and the onboard touchscreen controller that adjusts the values so it's in the expected range and can be calibrated
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Laci Page Icon Posted 2020-01-07 3:22 PM
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Thanks for your reply. I did the registry trick and while it now accepts the calibration settings the top half of the touchscreen is still as unusable as it was before.
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Laci Page Icon Posted 2020-08-08 8:56 PM
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Quick update: I managed to fix my Simpad by replacing the touchscreen with a DMC AST084A080A touch panel. Bought it for less than 10€ from a local ebay seller here in Germany.
The panel has a different pinout and seems to be a bit less sensitive than the original, but overall works fine on Windows CE 3.
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stingraze Page Icon Posted 2020-08-12 9:26 AM
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Cool. I didn't know different touch panel parts work on those devices.
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