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I did some reverse engineering on the pins of 2560 used on TouchShield Slide and this is what i've found.
Feel free to update the table because i don't guarantee that the table is 100% correct (i check the pins 3 times... but who knows)

Update: To use the MediaTek MT3329 GPS only with Arduino i implemented NewSoftSerial in the

library. Click here to download the MediaTek MT3329 GPS for Arduino.

Unzip this archive in

YourFolder\Antipasto44\hardware\arduino\cores\touchshield\src\components\

You will find a modified core of TouchShield Slide (TSS) to enable the use of Serial2 and Serial3,

In my case i use Serial2 to connect a MTK3329 GPS to TSS, that let's you upload more than 33 images

in to the memory (thanks to openmoco guys) and also 4 extra libraries DigiText,

GButton, EEPROM and GPS_MTK3329 - ( the first two libraries can be seen in action here)

MediaTek MT3329 GPS library for TouchShield Slide is using Serial2 of the shield at 38400 bps.

(see GPS_MTK3329.cpp)