APM 2.5 Free UART how to add serial data (Stratologger Perfectflite)?

Hi

I wanna read telemetry from Strattologger Altimeter (http://www.perfectflite.com/StratoLogger%20manual.pdf) from one free uart on APM 2.5.

1) Wich is the free uart i can use.  Actually im using 3dr radio, GPS. ???

2) Can i plug directly de TX from St

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