Pixhawk 4 SPKT/DSM Connection Spektrum Receiver/Transmitter

Pixhawk
1

Hello and thank you to all ahead of time for any help,

I am currently assembling a HolyBro S500 v2 kit with the hopes of using a Spektrum DX6e transmitter and a AR620 or SPM4650 receiver. I had feverishly ordered everything thinking I had everything I needed until I came upon using the DSM/SPKT connector for the receiver. Closer inspection of the kit upon arrival yielded a cluster of wires that weren’t intuitively usable with a Spektrum receiver. According to the PX4 documentation from this link, Radio Systems · PX4 v1.9.0 User Guide, all Spektrum DSM RC receivers are compatible and must be connected to the SPKT/DSM input.

With the understanding that the cables have to connect via the SPKT/DSM port, I thought there would be a turnkey cable available from HolyBro/ Spektrum with the kit but that does not seem to be the case. For hooking up the receivers, I searched the forums and looked at the Pixhawk 2 and Pixhawk 4 pinouts. The number of pins went from three on the Pixhawk 2 to five on the Pixhawk 4. I am currently continuing forward with trying to make a custom wire with a 5 pin JST-GH connector to 4 pin JST-GH connector to the SPKT/DSM port, but am unsure about which of the new pins are necessary. I know that there were initial SRXL2 incompatibilities with Pixhawk that have been resolved since its release.

Pixhawk 4 DSM RC Port
Pin Signal Volt
1(null) VDD_5V_SBUS_RC +5V
2(yellow) SBUS* +3.3V
3(null) RSSI** +3.3V
4(red) VDD_3V3_SPEKTRUM +3.3V
5(black) GND GND

  • Connect SBUS or DSM/SPKT receiver’s signal wire here
    ** Sends the RC singal strength

SPM4650 Pins
1 S
2 N/C (No Connect)
3 +
4 -

Included Pixhawk 4 DSMX receiver cable
5 pin JST GH to 3 pin Molex Picoblade

The supplied DSMX receiver has cables on the 2,4,and 5 pins.
I was going to strip the molex picoblade end of the wire and solder it onto the SPM4650 pins with the following connections:
DSM/SBUS RC Port => SPM4650
2 (SBUS* +3.3V) => 1 (S)
4 (VDD_3V3_SPEKTRUM +3.3V) => 3 (+)
5 (GND) => 4 (-)

Would this be an appropriate connection for the Pixhawk 4 and a Spektrum receiver or have I missed the boat entirely?

Any circumferential information, sources, or advice would be appreciated as the PX4 documentation did not specify any recommended Spektrum transmitter/receiver pairs so this would be a chance to expand upon that.

Chris

2

Hey !
I have the very same question using R-XSR receivers from FrSky.
Did you find the answer ?

I bought the PX4 Vision which provides several cables to connect to the DSM/SBUS connector on the Pixhawk.
But the 4 cables used that are coming from the R-XSR are the following one:

  • GND
  • +5 V
  • S-Port
  • SBUS_IN

or it seems that I should use the SBUS_OUT to send command to the Pixhawk4.

Did you just rewire the connector ?

Thanks for help !

3

I believe the connection I provided is correct. However, the Spektrum RX require two cables totaling $30 in order to update. One of them I don’t have so I went with FrSky setup instead.

This wiring diagram from the documentation shows that you should be connecting to the DSM/SBUS RC or PPM RC with your receiver.
https://docs.px4.io/master/en/assembly/quick_start_pixhawk4.html

Your receiver is both an S.Bus and CPPM. The default signal is CPPM according to the manual I found here: https://www.frsky-rc.com/wp-content/uploads/Downloads/Manual/R-XSR/R-XSR%20ACCST%20-Manual.pdf

You can change the signal by following the video on this page:

An interesting note from the manual: “SBUS/CPPM output (BLUE LED stay lit indicates R-XSR in SBUS mode, otherwise in CPPM mode)”

You do not need to worry about S-Port (Smart Port) unless you want the telemetry broadcast from the RX too. Tutorials on how to connect telemetry this can be found here: FrSky Telemetry · PX4 v1.9.0 User Guide

I believe SBUS_IN is not used unless you are setting up a Master/Slave connection with another RX.

I think GND,+5V, and SBUS_OUT on the RX side is all you will need but whether you use the DSM/SBUS RC or the PPM RC with the Pixhawk is up to you.

Here is an interesting article if you would like more than enough info on telemetry signals:

I hope you found this informative and helpful. Feel free to follow up with either your success or failure as it will certainly help anyone in the future.

Chris

4

Sorry for delay, but thanks Chris your answer did help me a lot !