In the Analog input section of the Remote control RSSI page (), there is an example of connecting a FRsky X8R to a Pixhawk. In the picture, a 3 pin RC servo cable is used between the receiver’s RSSI out and the Pixhawk SBUS in.
I don't have an X8R, but this is a dangerous practice for at least some FRsky receivers. If you connect all three pins for monitoring RSSI, the center conductor can carry power back to the Pixhawk. On my Pixhawk (admittedly a clone), this results in powering the servo rail thru the FRsky LR9 radio receiver.
In other words, even though the Pixhawk is not powering the servo rail, it does power the RCIN connector so that the RC receiver will be powered. And the SBUS port that monitors analog RSSI evidently does have its 5v pin connected to the servo rail power bus. Thus, the servo rail can inadvertently be powered via the RSSI monitoring cable.
On my maiden flight, I had over .9 volt of excursions on my Pixhawk VCC signal. I added capacitor filters for the second flight with some improvement, but did not solve the problem. I finally realized that the servo rail was being powered from the RSSI signal in addition to my UBEC and fixed the problem today.
Since your example picture shows all three conductors for this cable, I believe that you should warn users to make sure they are not inadvertently powering their servo rail thru this connection.
In the Analog input section of the Remote control RSSI page (), there is an example of connecting a FRsky X8R to a Pixhawk. In the picture, a 3 pin RC servo cable is used between the receiver’s RSSI out and the Pixhawk SBUS in.
I don't have an X8R, but this is a dangerous practice for at least some FRsky receivers. If you connect all three pins for monitoring RSSI, the center conductor can carry power back to the Pixhawk. On my Pixhawk (admittedly a clone), this results in powering the servo rail thru the FRsky LR9 radio receiver.
In other words, even though the Pixhawk is not powering the servo rail, it does power the RCIN connector so that the RC receiver will be powered. And the SBUS port that monitors analog RSSI evidently does have its 5v pin connected to the servo rail power bus. Thus, the servo rail can inadvertently be powered via the RSSI monitoring cable.
On my maiden flight, I had over .9 volt of excursions on my Pixhawk VCC signal. I added capacitor filters for the second flight with some improvement, but did not solve the problem. I finally realized that the servo rail was being powered from the RSSI signal in addition to my UBEC and fixed the problem today.
Since your example picture shows all three conductors for this cable, I believe that you should warn users to make sure they are not inadvertently powering their servo rail thru this connection.