• This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn more.
Pumpkin drop event

So many protocols...

ElectriSean

Eternal Student
Mentor
#1
What is a protocol? Essentially, it is the language that two devices use to communicate. This can be a confusing topic, as there are many different protocols used for different things in our hobby.

Transmitter protocols
Modern 2.4Ghz transmitters use a digital protocol to talk to the receivers in the aircraft. These are proprietary, meaning that each manufacturer has it's own, and you need to 'keep it in the family' so to speak. Spektrum transmitters only work with Spektrum compatible receivers. Futaba transmitters only work with Futaba receivers etc. These receivers also need to be bound to a transmitter before they will work. Receivers can only be bound to one transmitter at a time, so they will ignore signals from other transmitters. This allows multiple people to fly at the same time, without worrying that someone else will be able to take control of the aircraft.

Common Transmitter protocols:

ACCST - FrSky
DSMX/DSM2 - Spektrum
FASST/S-FHSS - Futaba
AFHDS - FlySky/Turnigy
SLT - Tactic

Receiver Protocols
Receivers get their input via the transmitter protocol, and send their outputs in a few different ways. By far the most common in fixed wing aircraft is PWM (Pulse Width Modulation)

PWM
This is a universal protocol, and is what most servos expect. Because it is universal, almost any servo can be used with any receiver, brand matching is not required. The exception to this is the Futaba SBUS system, which we will discuss in a bit. Many flight controllers can accept PWM input, but it is not common anymore as there is fairly high latency (the time it takes for your transmitter stick movements to become control changes) and you need to run one signal wire per channel.

The gory details about PWM - https://en.wikipedia.org/wiki/Servo_control

CPPM
CPPM stands for Combined Pulse Position Modulation. It is also a universal protocol, with the advantage that it can carry the signal for up to 8 channels over a single wire. It has very slightly worse latency than PWM, but it is more widely used in multirotors because of the simplified and cleaner wiring.

The gory details - https://en.wikipedia.org/wiki/Pulse-position_modulation

Serial Protocols
Serial protocols are digital, and have around half the latency of PWM/CPPM. They can also carry the signal of up to 16 channels over a single signal wire, making them ideal for multirotors. Serial protocols are proprietary, but as far as I know all flight controllers are able to use any of the protocols provided they are set up properly. Because they are serial, they need to be connected to a UART (Universal Asynchronous Receive/Transmit) port on the flight controller.

Common Serial Protocols:

SBUS - FrSky and Futaba. Futaba also makes SBUS compatible servos.
SPEKSAT - Spektrum compatible sattelite receivers.
IBUS - FlySky and Turnigy.

ESC Protocols
ESC protocols determine how the flight controller talks to the Electronic Speed Controllers that run the motors. Fixed wing aircraft almost exclusively use PWM, and as far as I know all ESC's except the KISS family understand PWM input. For modern multirotors, PWM is rarely if ever used due to the latency discussed above. Not all ESC's support all protocols, and many older ESC's will need a specific firmware. Modern BLHeli_S and BLHeli_32 capable ESC's can use most or all of them.

Common ESC protocols:

PWM - The old school analog original.
Oneshot125 - Roughly 8 times faster than PWM, has active braking and other improvements.
Oneshot42 - The original KISS protocol, roughly 3 times faster than Oneshot125.
Multishot - About 10 times faster than Oneshot125, still analog.
DSHOTxxx - Finally a digital protocol. Can be faster (DSHOT1200), more accurate, and able to send more than just motor control signals. Details - https://oscarliang.com/dshot/

I hope this is helpful, I know when I fist started learning about all this I was extremely confused. Questions, comments and especially corrections are welcome :)
 

JimCR120

Got Lobstah?
Site Moderator
#2
Nice. I have long wanted to tackle something like this but often find my knowledge lacking. Thank you for this. It will surely be referenced often.
 

PsyBorg

Wake up! Time to fly!
Mentor
#3
Nice Work Sean. :applause:

Things like this and other basics should have a totally separate forum section where they can be posted without the ability to reply so they can be organized. If clarification is needed then people can contact the author directly or start their own thread asking for help. This way people can be directed to the section for research and teaching without having to sort thru a bajillion posts.
 

rockyboy

Skill Collector
Mentor
#4
Excellent gathering of info here - sticky thread candidate for sure! I know I'll be pointing people over to this for years to come :)
 
#8
My biggest problem when I had my Taranis, and they may have changed this I’m speaking from almost 2 years ago. The issue I had was I absolutely hated the module hanging out of the back with an extra antenna. I thought it looked dumb, gave it an awkward feel. Just wasn’t a fan of it.
 

rockyboy

Skill Collector
Mentor
#9
You don't need to use the external module if you use the FrSky protocols and receivers with any of the currently sold FrSky Taranis or Horus radios. You do need the external module with the Turnigy 9x radios as it doesn't have any internal radio board.

I leave a DIY Spektrum module plugged in the back of both my X9D and QX7 all the time so whenever I grab a plane to go fly, I don't have to think about what type of receiver I have inside it. Just turn it on, select the model in the transmitter, power up the plane and go fly.
 

rfd

AMA 51668
#11
right now, the jumper t8sg is the ONLY tx that will cover 90-99% of all tx/rx protocols. protocols alone are just ONE aspect of a complete radio system. one should look at one's current and possible future radio needs, list them out, compare all to the systems currently available and then make an educated choice. this is super hard and daunting for a newbie, and probably for at least more than half of the folks around the globe who are, in some manner, flying some type of r/c aircraft.

so what are radio attributes? here's a starter list ...

  • protocols addressed and required
  • channels available
  • number/type of feature switches
  • model templates
  • programming intuitiveness
  • components quality
  • build quality
  • support
  • price
 

rfd

AMA 51668
#12
i'll add - this topic is important and shouldn't be segregated in only the multirotors board - it applies to all manner of aircraft.
 

ElectriSean

Eternal Student
Mentor
#13
This thread is not intended to be yet another discussion about what the best radio is, or how to pick one. It is more a discussion and disambiguation of all of the protocols one might encounter in the world of multirotors. The section on transmitter protocols is the smallest and least confusing, as most people know what radio they have and therefore what protocol they are going to be using.
 

rfd

AMA 51668
#14
This thread is not intended to be yet another discussion about what the best radio is, or how to pick one. It is more a discussion and disambiguation of all of the protocols one might encounter in the world of multirotors. The section on transmitter protocols is the smallest and least confusing, as most people know what radio they have and therefore what protocol they are going to be using.
it's great that yer addressing this issue. and though perhaps it wasn't yer intention, whilst protocol attributes are important, that's just one part of what radios offer, and knowing just the protocol isn't enuf to make a wise purchase decision, for pro and newbie alike. your thread is a great seed to grow a more complete dialogue concerning r/c radios.
 

ElectriSean

Eternal Student
Mentor
#15
There are many good threads about radio systems here, including Jim's epic and soon to be stickied discussion - http://forum.flitetest.com/showthread.php?37966-Which-Transmitter-You-Should-Buy over in the Radio section. I posted this in the multirotor section because the bulk of the information deals with the protocols that flight controllers use to communicate. Definitely check out Jim's thread, it covers everything you seem to think this one is lacking ;)
 

rfd

AMA 51668
#16
There are many good threads about radio systems here, including Jim's epic and soon to be stickied discussion - http://forum.flitetest.com/showthread.php?37966-Which-Transmitter-You-Should-Buy over in the Radio section. I posted this in the multirotor section because the bulk of the information deals with the protocols that flight controllers use to communicate. Definitely check out Jim's thread, it covers everything you seem to think this one is lacking ;)
long since posted on that thread. thought about it a lot and offered to chart the radios, but had second thoughts as that's not the right way to go, and neither is that thread. yer thinking is from a narrowed perspective and audience, but could have a wider and better impact. but i can see none of this matters to you and that's ok, too.