This explains how to internally interface a GD-0912-R (or a similar board). This resemble a similar mod done for the XD-0608-R (http://forum.bongofish.co.uk/index.php?topic=1928.0
The mod is primarly targeted to connect to a Teensy, but this is good info to connect to any TTL-serial TX/RX device along with 5V power supply. (like a FTDI breakout board or Arduino) -- connecting the 5v supply removes the need for an external power adapter brick. This can easily be powered off the USB.Warning: This mod will permanently disable the external serial & power connections. Playing with electronics is not without risks. You can break or burn anything including your fingers. .
In short, the mod consists of: Connecting the serial signals from the back of the ADM202 chip to get the TX/RX signals (and hard-wire 2 incoming signal); connecting the 5V at the 5V regulator output.
There are 5 serial signals.
| Internal connector | | ADM202 pins
Direction | PC DB9 | (board marking) | CheckPoint | RS --- TTL
<--tablet | 2 (RXD) | 7 (TXD) | CP1 | 14 <-- 11
-->tablet | 3 (TXD) | 8 (RXD) | CP2 | 13 --> 12
-->tablet | 4 (DTR) | 6 (/DSR) | CP3 | 8 --> 9
| 5 (GND) | 4 (GND) | |
<--tablet | 6 (DSR) | 5 (/DTR) | CP4 | 7 <-- 10
-->tablet | 7 (RTS) | 2 (CTS) | CP5 | R2R --> TR1R
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First remove the ADM202 chip: Dremel or cut the legs then desolder every leg (beware to not lift the "pads" underneath and break the trace. If you break a pad that you need you can find a spot to solder somewhere on the board to connect to that same signal). You could alternatively just cutout the legs that we care about and make sure you are not soldering to the leg (just the pad).
Pin 9 of the ADM202 should be grounded (this is DB9-DTR).
Pin 10 of the ADM202 should be left unconnected. (this is DB9-DSR)
Pin 11 of the ADM202 should be connected to RX (D2 on Teensy) ( data coming out of tablet )
Pin 12 of the ADM202 should be connected to TX (D3 on Teensy) ( data going into tablet )
Remove the TR1R transistor. The pad that is "alone" is the one we care about. We want to ground the signal. (This is DB9-RTS)
Cut the regulator output leg (near TP1) -- btw TP1 is a Test Point and connects to that same leg. This is our 5V VCC line.
Find a spot to connect the ground -- typically anywhere on the ground plane is good enough or you might try pin 4 of the connector (labeled GND in the markings). (Keeping it "closer" to this area of the board is probably better)
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I did the two ground by soldering a tiny piece of wire (insulation removed) to a nearby ground (pin 9 and TR1R transistor).
Always watch out where and in which position you solder a wire to avoid connecting unrelated traces and vias (i.e. holes).
Next step is to get the firmware on the Teensy. (see next post).