March 12, 2014

S.BUS decoders

I have started getting used to using S.BUS in gyro controlled flyers (like FBL helis and multirotors). With S.BUS there is only need for one cable from the receiver to the controller which makes the setup simpler. Also, when using one of the two Futaba telemetry receivers available and picking the cheaper one you must use the S.BUS if you need more than three channels.

When collected the parts needed for the Tri-copter build I naturally picked the R7003SB telemetry receiver since even the cheapest helicopter gyros have S.BUS support nowadays and I had already stumbled upon RC911's build for the KK 2 board which supported S.BUS. However, it turned out S.BUS was not released for the KK 2.1 board yet. To solve this, you either but the pricey R7008SB receiver or you buy an S.BUS decoder. I picked the latter and installed a Corona S.BUS decoder in the Tri-copter.

However, the Corona takes up a lot of space, all the cables that is and I quickly removed it in hopes of a S.BUS release from RC911. Which never came. And now I have gotten what I thought was an alternative, the Frsky decoder. I have now tried out the FrSky S.BUS decoder and cannot make it work because I don't have a Futaba channel changer or the Frsky channel changer. I thought that I would be able to use either the Corona channel changer or my Futaba reciever, which has an S.BUS programmer built in. But it turns out it doesn't work.

The Frsky decoder seems to work like four S.BUS servos in parallel on the bus. While programming, you push the button to select one "servo" at the time to listen to the bus, and while installed they are all active. However, the "servo" does not support the full range of features that a S.BUS servo should according the specs and only supports the command for changing the channel. Using the Corona programmer in programming mode or in channel changing mode does nothing. Hooking the decoder to the Futaba transmitter only causes it to crash.

From the image below you can see why I had hopes for this decoder to be a better fit on my Tri-copter. It is really small and perfect for a tight installation. It is a simple device wrapped in a piece of paperboard with four servo PWM outputs. Some silly engineer must have thought it was a great idea to let one of the outputs point upwards instead in putting the four outputs in two rows, which would have been even better. Also, the outputs are very exposed and needs some insulation before installation.

I won't be using this product. It didn't gain my trust and I won't buy a programmer just to get this one device working. Instead, I'll be doing some cutting and soldering on the servo leads on the Corona device to make it fit nicely on the Tri.

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