The new X4R has been released with two versions available, the X4R (white) and the X4R-SB (black). Both have the same form factor as the D4R II.
The X4R version has four PWM channels and the new smart port functionality however it does not support S-BUS. The S-BUS version has three PWM ports and one S-BUS port and also supports the new smart port functionality.
Neither of the units support PPM however Frsky does sell a very small SBUS to PPM decoder. Neither of the units have a RSSI port however there is a RSSI pad on the board to which an RSSI lead can be soldered.
In this post we look at how to add S-BUS functionality to the standard X4R (4 PWM) version. In summary, if you make the mod, you will get 4 PWM channels plus S-BUS.
There are only four standard servo style ports on the X4R, these are all PWM ports. On the S-BUS version, one of these is sacrificed for the S-BUS connection.
On the actual RX board there are two wire traces that feed to the same servo port. Each on these wire traces feed from their own pin on the IC. One pin sends a PWM signal whereas the other sends a S-BUS signal.
Obviously the port can’t be fed the two signals simultaneously as they will conflict. Frsky gets around this by having a break in each trace. It then uses a 0 ohm resistor as a bridge (switch) for one or the other trace. Accordingly, if one switches the resistor from one trace to the other you have effectively changed over the X4R from one type to the other.
In this PDF link are the instructions provided by Frsky to complete this conversion. Essentially, you remove the resistor from one side of the board and place it on the other trace located on the other side of the board.
In this blog post, we do not wish to change the X4R over, we want the best of both worlds, 4 PWM ports and ALSO S-BUS. Essentially we will add S-BUS functionality to a X4R (4PWM) version by adding a servo lead to the S-BUS trace.
In the picture below, you can see the two pads (A and B) that would be bridged with a zero ohm resistor to make the S-BUS version. In our case, we don’t want to bridge these as this will conflict with the PWM signal already being sent to the servo port.
To access the S-BUS functionality we need to tap into the S-BUS trace together with the positive and negative rails. We have used one end a standard servo extension lead. We chose the male plug as we wish to plug this directly into a flight controller board.
To install, cut the servo lead in half and solder the Signal wire to pad labelled with ‘A’. Also connect the +ve and -ve wires to the corresponding output pins.
Please note that this pad is extremely small and it will require a precision soldering tip and a steady hand.
Special thanks to whimsical in completing this mod.