SBUS and PWM connections to Matek H743 Mini

My first Ardupilot setup for a heli uses a PixRacer board which has conventional 3-pin connectors for servos and ESC. For my next heli I’m trying a Matek H743 Mini which only has single-wire connections for the receiver SBUS input and for the servo PWM outputs.

The Matek board is rated for 2S to 8S power input, so I propose to power it direct from my 6S flight battery. The FrSky receiver and the servos will get their 6v power from the ESC’s BEC and a backup stand-alone BEC (isolated from each other by diodes), which are also powered by the 6S flight pack. Since the receiver, servos, and Matek board all have a common Ground, it seems to me that everything will work if I connect only the signal wire from these devices to the Matek board. Is that good practice, or should I also take a ground wire from each servo and the receiver direct to the Matek board?

If you look at the Matek wiring diagram, there are ground and 4v5 pins near Rx6 that would be perfect for powering a receiver. That way the receiver is powered when you connect the FC to a computer via USB.

If you’re going through the trouble of making a servo harness it wouldn’t be a bad idea to tie all the servo grounds together and tie that to the FC ground. The issue is really the common ground between the FC and the BEC output.

Thanks Chris, I’m aware of the ground and 4v5 pins close to Rx6, and was initially planning to use them. But my receiver (FrSky X4R) is rated for 4.0 to 10.0v, so 4.5v seems to me to be a little too close to its lower limit for comfort. Rx6 and its adjacent ground are good for the SBUS connection from the receiver.

This heli is already flying with A.N.Other flight controller, and power is supplied to the FC, receiver and digital servos from the ESC’s BEC and a stand-alone BEC, paralleled via Schottky diodes for redundancy, which gives 6v and plenty of amps capacity. So my plan now is to provide 6v to the receiver and servos from this power board, which has six 6v power outputs. Using it to power the Matek board as well is not possible, for 6v is below its specified voltage range, so it will get power directly from the 6S LiPo pack that powers everything.

So my real question boils down to, is a single signal-wire connection from the Matek board to ESC and servos acceptable practice given that everything already has a common ground?

Joshua Bardwell says to connect the signal grounds in this video: ESC Signal Ground: Yes or No? | WHY THE ANSWER IS YES - YouTube

If you look at Matek’s suggested wiring, the battery connects to a 4in1 ESC, and the ESC connects to the FC via signal, power, and ground. Essentially the ESC grounds are tied together with the signal ground to the FC.

Servos would presumably also benefit from connecting the grounds to get a better reference point, but you could probably ground the whole servo bus (assuming you’re using one) rather than taking each servo directly to the flight controller.

There is a possible downside, if you develop a bad ground connection in the main BEC ground cable that is powering the servos, it’s possible that the current going through the servos could end up going through the ground connection to the flight controller and then to the battery (which might be too much current for a small-gauge signal wire). In your case I think a redundant ground through the second BEC should prevent this from being an issue.

Thanks again Chris. Joshua’s video is, as always, very instructive. Taking the Matek suggestion for connection to a 4-in-1 ESC as an example, my system will, I think, replicate that because all my servos will have a common ground connection to the FC via my BEC distribution board. Your suggestion of another ground connection from the BEC board sounds a sensible one though, and I think I’ll make that a decent sized soldered one, rather than using a plug at one or both ends.