Setup your Raspberry Pi to communicate with the SurfaceRT.
To enable the UART on your Raspberry Pi to be used as host you need to configure it:
sudo raspi-config
Go to 5 - interfacing options
Go to P6 - serial
Would you like a login shell to be accessible over serial?
Answer: 'No'
Would you like the serial port hardware to be enabled?
Answer: 'Yes'
Reboot your Raspberry Pi.
See Raspberry Pi for information on how to physically connect your RPI and your SRT.
To establish a UART-Connection to the Surface RT use
RPI2: minicom -b 115200 -D /dev/ttyAMA0
RPI_Zero minicom -b 115200 -D /dev/ttyS0
To escape from minicom use STRG+A and then X.
Further information: https://www.raspberrypi.org/documentation/configuration/uart.md