I am building a quadcopter using a FrSky Taranis X9D radio. It came with a FrSky X8R receiver. I wanted to keep my wiring clean by using the S.BUS feature on the FrSky receivers, I purchased a smaller FrSky X4R-SB receiver. The X8R has 8 PWM channel pins and the X4R-SB has 3 PWM channel pins, but if I use S.BUS (which is serial, not PWM), I can access 16 channels using only 1 pin, on both X8R and X4R-SB. The X4R-SB is much smaller, making it more ideal. (do not confuse the X4R-SB with the D4R-II, this is important, D4R-II uses CPPM, not S.BUS)
(update 10/25/2014: a follow up hack for Smart Port)
I want to use a Naze32 flight controller, which is open source and does have code to interpret S.BUS protocol. S.BUS is UART communication but it is inverted and the Naze32’s UART cannot accept inverted input. Some flight controllers, such as the Pixhawk, has a dedicated inverter just to solve this problem, but the Naze32 does not.
The first option is to buy a “S.BUS to CPPM converter” but CPPM is not a serial bus like S.BUS and thus does not have the advantages of being a serial bus. CPPM uses timing, timing needs to be measured (measuring things = possible error) and the signal edges can be affected by capacitance, noise, etc. Also having such a converter means there will be a tiny bit more latency in the system. These two disadvantages are probably too insignificant to notice performance wise. But I still didn’t want to spend another $13 + tax + shipping just to solve a problem that shouldn’t have existed in the first place.
The second solution is to buy an “inverter cable” which is a cable that has a NOT gate inline and then shrink wrapped. Or I can just buy a NOT gate and make the cable myself. I still didn’t want to spend the money. I opted to hunt down the inverter on the X4R-SB circuit instead, and connect a wire to the input of the inverter (labelled as “A” in the datasheet). This provides me access to the un-inverted signal that I can directly connect to the Naze32.
See the pictures below to understand how this hack was done.
And just in case I confused you even more, all you need to do is connect a wire to the “A” pin.
I have tested it with Naze32 Rev5 and firmware f4d556c68876ccd5902bddf1cade32f1bb382c9f. Works like a charm.
It is probably possible to perform the same hack on a X8R but the X8R is constructed using two PCBs and the inverter is covered up by one of them. Separating the two PCBs is very difficult and risky.
The Smart Port (I think it’s also called S.PORT) is another inverted serial bus available on the X4R-SB and X8R but it is bidirectional. Since whatever you want to connect to it will need a bidirectional circuit anyways, it is not worth it to perform another surgery on the Smart Port. Also, the Naze32 can use SoftSerial to transmit in an inverted fashion, so a dedicated inverter isn’t even required. (SoftSerial would not work well for taking inputs, but outputs is OK)