1. Introduction:
The Two-way Radio System developed by FrSky provides an additional return link that would be used for monitoring the Rx’s voltage, temperature, location, speed and so on.

2. Receiver module:
a) Model: D8R, D8RHV
Compatible with FrSky Two Way module DFT/DJT.
b) Model: V8R, V8RHV, V8R7, V8R7HV, V8R4
V8 receivers work with DFT/DJT, just in receive-only mode.

3. Features
a) Advanced Continuous Channel Shifting Technology (ACCST SYSTEM) achieves high reliable link in interference rich environments. 
b) Easy to bind and instant link-up.
c) Excellent reboot times.
d) All channels failsafe.
e) Quick response.
f) Very smooth servo movement.
g) Alarm on events of receiver (low battery voltages, poor reception, etc.)
h) Error-free link, by using 48bits CRC algorithm.
i) Low power consumption.
j) True two-antenna diversity.
k) Firmware upgradable.

4. Function of Two Way communication
a) Two way communication between receiver and transmitter
b) Receiver is able to accept 2 analog inputs, which can be used to monitor battery voltage, temperature sensor, etc., and transfer them to transmitter. Users can set the Alarm Level of these analog voltages, when exceed or below these levers, sound alarm is issued.
c) Receiver is able to accept serial data streams from simple 3-wire RS232 port, such information as GPS, altitude, and user developed data can be transferred to Tx without error, and sent at the RS232 port on the Tx.

5. Receiver specifications:
Model: D8R
Weight: 16.2g
Dimension: 54*27*17mm
Operating Voltage Range: 4.0V-7.2V
Operating Current: 100mA
Specified Range: 1.5km (Ground Range)
Resolution: 11bit (3072)
Latency: 22ms
Number of Channels: 8CH
Analog voltage: 0~3.3V

More detail: http://www.frsky-rc.com/download.asp?id=22