Why would you want to ‘uninvert’ the inverted FrSky SBUS or smartport signal from your receiver? There are four different kinds of processor chips used in our multicopter flight controllers: F1, F3, F4 and F7. F1 and F4 processors are “older” processors and do not have inbuilt inverters to rectify raw inverted FrSky SBUS signals. If you want to use FrSky SBUS/Smartport with an F1 or F4 flight controller, you must uninvert the SBUS/Smartport signal coming from the receiver.
However, not all F1 and F4 flight controllers require uninversion. Many F1 and F4 flight controllers now have built in hardware inverters that uninvert the incoming SBUS/Smartport signal. Be certain to check if your particular flight controller actually requires uninversion of the SBUS/Smartport signal before making any unnecessary modifications.
This guide will teach you how to modify the following receivers to obtain uninverted SBUS and Smartport signal.
Be warned that the following modifications require very accurate soldering skills, a fine iron tip is recommended.
The X4R-SB is an older model FrSky receiver and is the bulkiest on this list. The only advantages of the X4R-SB in comparison to modern FrSky receivers is that the antennas use IPEX connectors and are easily replaceable, also, X4R-SB receivers are often sold cheaply as they are outdated stock. Get the FrSky X4R-SB receiver here.
There are a couple of different points from which uninverted SBUS signal can be tapped. The first is the middle leg of the highlighted transistor. The other is the circled pad, which is far more convenient.
On the X4R, uninverted Smartport signal is located on the singular pin of the isolated transistor.
The XSR is the successor of the X4R-SB, housing the functionality of the X4R-SB in a compact and convenient package. The XSR can be bought more cheaply than the X4R-SB although, the antennas are soldered on and are irritating to replace, depending on your soldering skill level. You can purchase the FrSky XSR receiver from here.
Similarly to the X4R-SB, uninverted SBUS is located on the middle pin of the referenced chip on the XSR, solder to it to get an uninverted SBUS signal.
Again, much like the X4R-SB the uninverted Smartport telemetry signal is positioned on the single leg of the circled transistor chip.
The FrSky XM+ is one of the relatively newer FrSky SBUS receivers. The XM+ is much smaller than the X4R-SB and the XSR, and boasts SBUS and antenna diversity. However, the XM+ lacks telemetry. Buy the FrSky XM+ receiver here.
To retrieve uninverted SBUS from the XM+ receiver, simply solder a wire on to the middle leg of the three pin side of the highlighted chip.
The FrSky XM is a smaller version of the XM+, dropping the dual antenna system to obtain its smaller stature. Like the XM+, the XM lacks Smartport telemetry. Get the FrSky XM receiver here.
Make a solder joint at the middle leg of the selected transistor to access the uninverted SBUS signal.
Although a singular solder joint may not seem a difficult task, making one in such close proximity to many tiny SMD components is particularly trying. Before attempting any of the modifications, assess your confidence in your soldering ability. If you would like to improve your soldering skills to be able to undertake useful hacks like SBUS signal uninversion, learn how to solder like a champion with Blair Murphy.