Well, I'm no expert on this telemetry stuff, but I did read some of the manuals over at FrSky and I have shamelessly copied the following:
4
Function of Two Way Communication
4.1
Two way communication between receiver and transmitter
4.1.1
The receiver is able to accept up to 2 analog inputs, which can be used to monitor battery voltage, temperature, etc., and transfer the data to the transmitter. Users can set the Alarm Level for each of these separate analog voltages in the transmitter. The alarms can be programmed to a voltage above or below a settable threshold or disabled.
4.1.2
The receiver is also able to accept serial data streams from its simple 3-wire RS232 port. Such information as GPS, altitude, and user-developed data can be transferred to the transmitter without error. This serial data is then available at the RS232 port on the Transmitter.
4.1.3
Technical data about the protocols are also available at the download portion of the FrSky website. 1
So... as I read it, the analog inputs should be usable for battery Voltage or Temperature, but other devices appear to put out the information in a digital format, so they would have to go to the digital port. I'm only guessing here, but I suspect that said port could either handle a single device, or, by using the hub, would allow multiple digital devices to be used on that one port.
The D6FR with it's built-in battery voltage sensor and one external analog telemetry port (A2) should be able to monitor the receiver battery and a second battery pack or a temperature. At least that's how I read it.
Regarding the other questions, I have no clue. I guess I need to order a bunch of this stuff and start playing with it.
Bruce or whomever, please feel free to step in and correct me if necessary!
Regards
Christian