SBUS SERVOS

[Ace]

I understand the principal of SBUS and its benefits particularly for a complex wing set up.

What is not clear is if you can use the 4 ch FrSky decoder + channel selector, Bangood, 16ch & FrSky Redundancy 10/20 with say Spectrum?

[Frank Skilbeck]

Posted by Ace on 06/08/2018 07:25:58:

I understand the principal of SBUS and its benefits particularly for a complex wing set up.

What is not clear is if you can use the 4 ch FrSky decoder + channel selector, Bangood, 16ch & FrSky Redundancy 10/20 with say Spectrum?

No, there isn't a standard serial bus protocol and different manufacturers use different protocols. S-Bus is not compatible with Spektrum.

For Spektrum you'd need to look at one of the manufacturers that covers different protocols, e.g. Powerbox.

While I can see the appeal of an SBus system my worry would be that the loss of a single connection would cause all servos that are fed from that hub, and prefer to use multi-pin connectors.

[Ace]

Hi Frank, Sorry should have been more specific.

I have a Spektrum DX9 TX, however I use mixture of receivers (Spek, Redcon, Lemon & Orange) My Orange 12's have a SBus port. The Redcons have PPM

Other than the 8 channel converter do I have other options?

**LINK**

[Biggles' Elder Brother - Moderator]

Hi Ace,

the FrSky converter will work with any SBus system. So if your Rx outsputs SBus it will work fine.

BEB

[Chris Bott - Moderator]

Ace yes, if it's S.Bus then all of the options I list above will work.

I believe that all except the FrSky one are OK to use with analogue servos too. (Of course I can't be 100% sure and these days specs seem to change by each manufacturing batch - but the ones I bought all gave a 20mS frame rate)

[Chris Bott - Moderator]

Frank I've heard others worry about using a single connection and it's a legitimate concern, but consider this:

Anyone flying with a Rx battery (i.e. not BEC) is likely have a single connection between batt and switch harness and another single connection between switch harness and Rx. These are servo power carrying connections too.

Where I've used S.Bus to a warbird wing (6servos). I used a bigger sized connector between fus and wing, and fed servo power directly to that connector rather than through the Rx.

So there are different ways to arrange things.

S.Bus also allowed me to use channels 9-16 in the wing. These are not available as separate servo ports on the Rx.

[Bob Cotsford]

I've used Xtreme Power System X10+ decoders on a few models now and it's claimed to run all the mainstream BUS protocols including Futaba/FrSky, JR, Spektrum, HOTT, Jeti etc.. With petrol models using high torque servos I use two leads, the usual S-Bus signal lead plus a second power lead (one battery, two leads - one receiver and one wing) so that the servo power load isn't carried just through the S-Bus lead. You could use a similar method with any of the decoders using a Y lead on one channel to plug in a second power connection.

It is a second connection, but that's better than half a dozen separate plugs!

I usually keep the standard orientation on the power connection so that from the fuselage I have a male lead carrying S-Bus and a female carrying power. Makes connecting up the wing quick and simple. Of course your average club size model can happily live with the 6A rating on a standard 4 channel FrSky decoder, even with electric retracts. Just remember that if you use a Y or extension lead it needs to be heavy duty too.

[Frank Skilbeck]

Posted by Chris Bott - Moderator on 06/08/2018 09:54:57:
Frank I've heard others worry about using a single connection and it's a legitimate concern, but consider this:

Anyone flying with a Rx battery (i.e. not BEC) is likely have a single connection between batt and switch harness and another single connection between switch harness and Rx. These are servo power carrying connections too.

Where I've used S.Bus to a warbird wing (6servos). I used a bigger sized connector between fus and wing, and fed servo power directly to that connector rather than through the Rx.

So there are different ways to arrange things.

S.Bus also allowed me to use channels 9-16 in the wing. These are not available as separate servo ports on the Rx.

Chris yes, but it's not just the power concern, it's a single point failure that leads to a loss of all downstream devices, where if you use a multipin connector the loss of a single wire will only affect that servo. I suppose some of this comes from my involvement in safety assessments of instrumentation systems, where we avoid a single connection isolating critical functions. But again in what we do the risk is pretty small and the convenience outweighs this.

I do share you concerns over the Rx battery via switch to the Rx having had problems (fortunately found on the bench), so now on larger models I use FET switches and either dual feeds to the Rx or heavy duty connections.

[Chris Bott - Moderator]

Frank I think we're in great danger of completely agreeing on this.

I guess that if a model is deemed big enough to be concerned about a single S.BUS connection then we should maybe be thinking about redundancy throughout. 2 receivers? 2 Power supplies? Maybe that redundancy bus unit with 2 separate S.Bus inputs? Maybe just 2 decoders - one in each wing?

Another way to reduce wiring to a wing might be to use another Rx in there bound to Ch9-16.

Or stick to multi way connectors. I have that in a full house glider and it works very well.

[Chris Bott - Moderator]

Bob I'd forgotten all about that X10+. It's a nice looking unit. Do you buy them from that website? How have you got on with import duty etc?

[Tim Ballinger]

Now I’ve got just too many options .....

Tim

[Biggles' Elder Brother - Moderator]

Posted by Tim Ballinger on 06/08/2018 13:08:27:

Now I’ve got just too many options .....

Tim

Well, its better than the alternative!

BEB

[Bob Cotsford]

Chris, I've not had any problems in the past ordering one or two units at a time. The ones I have are V1 which is essentially just a different board layout. Since giving up larger models I find myself with a couple of units going spare if you'd like to have a play. PM me your address and I can pop one in the post for you to try out.

When I first came across the XP boards the Bangood offerings weren't available, neither were redundancy receivers so this was ahead of the game at that time.

[Ace]

Cheers all for the clarifications.

Time for tea and Ginger Nuts to contemplate pros & cons options.

[David P Williams]

Posted by Chris Bott - Moderator on 05/08/2018 20:11:58:

Chris - I have a couple of this type - do they output at 20ms for standard servos from a Frsky RX 9ms input? The documentation I have tracked down isn't very clear or informative. (I tried the Frsky decoders and fried a servo or two!)

Thanks, David

[Chris Bott - Moderator]

David I've always thought so but your question has prompted me to actually measure it.

So first of all here's 2 oscilloscope traces. Probably showing that my old uncalibrated 'scope is a tiny bit out in timing measurements but is should be plenty good enough for our purposes.

My X8R is bound in D16 CH1-CH16 (18ms) mode.

The bottom trace here is a standard servo output from the Rx. 'scope is reading about 17mS so maybe that's how far out the 'scope is.

The top trace is the output from an FrSky 4Ch S.bus decoder that's connected to the same Rx.

That clearly shows that pulses are at twice the rate, i.e. 9mS between pulses.

Next, I'll measure each of the decoder types I have.

FrSky 4 Ch decoder - top trace.

[Chris Bott - Moderator]

Other S.Bus decoder types - All from the same X8R set up as above.

X8R in 16Ch (18ms) mode. = 18mS 3.3v pulses

FrSky 4 ch decoder = 9mS 3.3v pulses

Bangood 8 Ch decoder = 20mS 3.3v pulses

Bangood 16 Ch decoder = 22mS (This may have been an adjustment I made many moos ago - I can't remember) 3.3v pulses

Mike Blandford's Arduino 16 Ch decoder = 18mS. 5v pulses

So all appear to be good for analogue servos except that FrSky one.

[David P Williams]

Chris - thank you so much for that, I can continue to use them in my new model with some confidence.

[Tim Ballinger]

Chris,

Excellent work. We can all make an informed choice now and sleep easier in our beds.

Until I guess someone nudges FrSky into giving folks the option as well.

Tim

[Chris Bott - Moderator]

Thanks chaps. It's something I've wanted to know for a while too.

I'd like to do it with the Rx bound in 9mS mode too. But I did the tests using a Rx that's in a model and it has analogue servos neatly plugged into it.
If I spot another easy to get to Rx I'll do those measurements too.
Although, don't you have to go down to only transmitting 8 channels, to get 9mS mode?

[Mike Blandford]

In ersky9x, I have a mode available where all 16 channels are sent to the XJT module every 9mS. The XJT actually monitors all 16 channels and doesn't always send the channels in order. Each RF frame, sent every 9mS, contains 8 channels, nominally 1-8 or 9-16. However, each channel position has a bit to indicate whether it is from 1-8 or 9-16, so the first channel in the frame is either 1 or 9, the second is either 2 or 10 and so on up to the last channel that is either 8 or 16.

What the XJT does is to note the "last value sent" on a channel, then when it is about to send a frame it looks to see if channel 1 or channel 9 has the larger difference from the "last value sent", and sends that channel. This means that sometimes channel 1 is sent in consecutive RF frames. I have seen this happen when using a logic analyser on the SPI bus to the CC2500 RF chip on the XJT.

By sending all 16 channels to the XJT every 9mS, this gives the XJT all the information to get channel changes sent as early as possible, so minimising latency. I believe openTx may have or get this at some point.

To make best use of this, you really need to use SBUS as that also outputs frames every 9mS. For minimum latency, you therefore need to use the 9mS PWM output and so a digital servo.

I have some changes (unpublished) to my arduino SBUS decoder that try to make use of the 9mS update rate. Basically, it normally sends pulses only every 18mS, but if it sees a change on a channel, and a pulse is not normally sent in that 9mS period, it sends a pulse, then returns to an 18mS period. I still need to do some more testing on this.

Mike

[Chris Bott - Moderator]

OK now I'm confused. I've brought in another model that has digis in the fus.

So, I've bound that to my Horus X12 and the OpenTx Bind section clearly says:

D16

Channel Range 1-8 (9ms)

But the results are exactly the same as above. Starting with Rx servo port pulses at 18mS.

Nothing has changed.

[Chris Bott - Moderator]

OK thanks Mike. I've just read your post twice and I think I understand.

So are my tests, using OpenTx, valid?

I just chose a random channel to check for frame rate and I didn't move any controls.

Do you happen to know if anything changes, in OpenTx, when you switch between 18ms and 9ms settings and re-bind?

PS - I really must get ersky9x onto my Taranis soon, I am very curious.

[Mike Blandford]

The Rx (X8R) servo pulses are always at 18mS. Some of the D8R receivers had an option where you press the bind button on the Rx for 6 seconds to switch the outputs between 18mS and 9mS.

On openTx selecting 8 channels (9mS) simply sends channels 1-8 to the XJT every 9mS. The XJT still sends all 16 channels, but since channels 9-16 are always in the centre position, changes on channels 1-8 will always be sent before channels 9-16, so you get lower latency.

Mike

[Tim Ballinger]

I have to say I hav really enjoyed the informed responses in this thread , possibly more so than any other thread I have read. Again many thanks.

Tim