What’s this about?
This post shows how to wire up and configure RFDesign RFD868x or RFD900x telemetry units to a Pixhawk 2.
Prerequisites
You will need:
- Two RFD868x or RFD900x units
- FTDI/USB cable (comes with the units)
- Pixhawk2 telemetry cable with 6 pin servo style plug.
- Spare 2 pin or 3 pin male servo style plug.
- Internet connected PC etc with a groundstation installed.
What to do
I am always suspicious when something for UAVs claims to work out of the box. So it goes without saying that these don’t, at least in this context! We will go through:
- Reloading the firmware for the telemetry units.
- Configuring the units (to conform to UK regulations)
- Re-wiring the telemetry cable.
Reload Firmware
I prefer to use the configuration software direct from RFDesign on a PC.
- Download and install the latest version of the modem tools.
- Download the latest version of the SiK (non-mesh) firmware.
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Run the modem tool (it’s called RFD900 Tools)
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Connect the first of the telemetry units to the PC using the FTDI cable. The LED on the unit should flash green.
- Check for the correct COM port and click Connect, then Load Settings
After a few seconds, the data will populate in the left panel, as below. - Click Upload Firmware and follow instructions using the .bin file saved earlier.
- Reconnect and load settings.
- I found the default (max) transmit TX Power set to maximum, corresponding to 30dBm. I reduced this to 10dBm – don’t forget to save the settings.
- Disconnect, unplug and repeat for the second unit.
Rewire the telemetry cable
There’s a useful video of this for the standard Pixhawk at: https://www.youtube.com/watch?v=mScWEMlZXL0. However, I still found it necessary to use the datasheets for the Pixhawk 2 and the RFD900 to be sure as the end of the video is a little fuzzy.
For information:
To rewire:
- Remove all the wires from the RFD connector.
- Remove the jumper pin from the RFD unit.
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Reconnect wires as in the video. The 5V supply and GND wires can go in the 3pin servo socket. The rest can re-use the 6-pin socket that you have just taken the pins out of.
- Here’s a summary and my hand-written notes!
| Pixhawk 2 Pin | Function | RFD Pin | Function |
| 1 (the red wire) | 5V out | 4 | 5V in |
| 2 | TX | 7 | RX |
| 3 | RX | 9 | TX |
| 4 | CTS | 3 | CTS |
| 5 | RTS | 11 | RTS |
| 6 | GND | 2 | GND |
The RFD connectors are below, displayed left to right in the same order as the diagram above.
Make the connections
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At the RFD end, plug in as shown:
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Power…
- Then signals…
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For the Pixhawk 2, make sure the connection is made to the TELEM1 port as this has a dedicated power supply for telemetry units.
Power Up
Once powered up, the LEDs on the units will flash green initially, then solid green once they connect to each other. I found that the connection was made almost immediately.
I am hoping these units give a noticeably more resilient telemetry connection with my ground station, so that at all times there are at least two reliable and independent means of control of The Groundhog. They can of course be used for more than telemetry, aspects which will be explored in the future. I’ll report on the telemetry performance of the units in the coming weeks.
Mike Isted 