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SBUS SERVOS


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Jim Drew on his RCGroups thread said that his X10+ decoder uses the S-Bus frame rate, and that as this is always 9ms for FrSky his X10+ outputs servo pulses at 9ms intervals. My first experience with SBus was using it to drive retract doors in a Macchi 200 usin FrSky decoders, it took me a few smoked analogue micro-servos to get the message that it's safer to just use digitals with SBus!

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  • 2 weeks later...
I don't know why I've only just thought of this but I just tried my multimeter, which has a frequency measuring option, on a servo tester.
It works a treat, measuring 50Hz.
This corresponds nicely to a 20ms pulse rate from the servo tester.
My meter also has a duty cycle setting. This showed a reading from 5% to 10% depending on the testers knob position. Corresponding well to the 1ms to 2ms of the servo pulse length.

I've found multimeters on Amazon at around ?15 with this facility, so if you want to confirm your decoder outputs, there's a cost effective way to do it.
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  • 2 months later...

A bit of a postscript...

Acquired the bangwood 8 ch decoder following all the discussion in this thread. Added it to the Sbus of my newly acquired FrSky radio and really pleased with how it has tidied up my wing wiring. Not counting the motor power ,just the single lead to connect to the fuselage. I think the before and after photos say it all.

Before

ed661ac3-5caf-4acd-b522-883443b5c669.jpeg

After

f4180ebe-67a8-41af-b055-b2dd917f07c8.jpeg

Edited By Tim Ballinger on 18/10/2018 11:56:42

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  • 1 month later...

It's been a long time but I've finally got round to testing Bob's X10+ decoder.

img_20181128_214403__01__01.jpg

It's a bit fiddly getting it set to S.BUS mode but then, it works well.

I can confirm that when connected to a FrSky receiver it's output is indeed at 9mS pulse rate. So it's not suitable for analogue servos.

Edited By Chris Bott - Moderator on 28/11/2018 23:12:17

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Posted by John Duncker on 29/11/2018 15:42:09:

and you have a nose wheel steer servo which needs to be deactivated when the gear is up.

I am wondering how this applies to S.Bus, if at all possible this is usually done in the transmitter programming, or possibly in the receiver setup.

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  • 4 months later...

To put it even more simply. Each SBus servo has its own IP address. Just like your internet at home. Each servo can be programmed to perform the task in hand. Its all digital technology. Thats how one cable can be used to manage a number of servos, A number of signals can be sent down one cable, they are looking for the IP address to deliver that signal to. Its brilliant stuff. I have been using it for years. Never ever had an issue. Just a few headaches at the begining when learning the system.

Once you get your head round it you will never use analogue again.

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  • 1 year later...

Sorry to bring up an old thread but I need help or a strait jacket.

 

I acquired an FrSky 4 channel SBus to PWM decoder.

I have tested it on all my SBus equipped receivers and in every case I get 9ms on the decoder outputs, including when servos are connected to the outputs.

 

I have connected up a range of servos (pico, naro, HS55 and a standard) and all except 1 work perfectly (faulty servo) so I have left them to cycle and cook and all seem OK, no heat build up or jitters.

I tried 2 different retract units separately, one does nothing, the other cycles continuously.

 

After hearing of servos burning out, I am now not sure what to do or trust, & why the difference with the retract units?

Can anyone shed some light please?

 

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I think it depends entirely how the input circuitry of the servo or retract unit detects the pulse width of the signal. A digital circuit is likely to detect the start of a pulse and count time intervals until the end of it. It's unlikely to matter how fast the pulses turn up one after another. 

 

An analog circuit though, could be very different. I'm imagining a simplistic resistor feeding a capacitor. The capacitor charges for the duration of the pulse. The longer the pulse, the higher the voltage goes. Then that voltage is used to position the servo. In between pulses the capacitor is discharged and it might expect 16 mS to do so before the cycle starts again. If a new pulse comes along after only 9mS, everything is upset. 

It's very probably nothing like this in reality, but you can see how a signal that isn't what the unit was designed for, might mean the circuit doesn't perform as expected. 

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Be wary with analogue servos.  I've had some that seemed ok for half an hour but went into a frenzy, burning out in the process, the second or third time they were powered up.  Now I stick to using digital servos on Sbus.  I've not had any problems with electric retracts, Lado, Eflite, JP and HK have all worked ok off an Sbus decoder.

 

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