PWM, PPM, SBus, DSM2, DSMX, SUMD
When it comes to radio transmitter and receiver, acronyms are often mentioned: PWM, PPM, SBUS, DSMX and so on. These are the different radio communication protocols and technologies. In this post we will explore the differences of these radio signal types. We will also have a look at some of the technology, and see how it makes flying more reliable and safer.
PWM
This is the most common receiver output signal. In the old days when there were only RC fixed wings planes, the receivers were used to control the servos or ESC directly with standard PWM signal, one channel for each servo. Until today the same technology is still being used. Multirotors require at least 4 or 5 channels (sometimes even more) and you will see the same numbers of servo leads connected between the receiver and flight controller.
PWM radio receiver is the most common and usually the cheapest option.
PWM stands for pulse width modulation, the length of the pulse specifies the servo output or throttle position. The length of the signal pulse normally varies between 1000 & 2000µs (micro seconds), with 1000µs being the minimum & 2000µs the maximum.
PPM
PPM is also known as PPMSUM or CPPM. The advantage of PPM is that only one signal wire is needed for several channels (typically 8 channels max), instead of a number of individual wires. So you should only connect the ground, power and signal cable.
A PPM signal is basically a series of PWM signals sent one after another on the same wire, but the signal is modulated differently.
PPM is what they call “analog signal in time domain”, channels are sent one after another and not at the same time. Therefore tt’s not as accurate or jitter free as serial communications, but it’s more widely available and supported by many Flight controllers.
Check out this post for a more detail difference between PWM and PPM.
PCM
PCM stands for pulse code modulation, it’s a data types like PPM. However PCM signal is digital (ones and zeros) and the PPM signal is analogue, which is the length of time the signal is on. PCM has the potential of signal error detection even error correction, but this still depends on the product you buy.
PCM is more reliable and less susceptible to interference, but additional conversion is required so the equipment is more expensive in theory. But it looks like PPM is still the mainstream in RC radio at the moment.
Serial
Serial Receiver is a digital loss-less protocol that uses only 3 wires (signal, power, ground) for multiple channels. As the name suggests this type of receiver requires serial port on the flight controller. This includes SBUS, XBUS, MSP, IBUS, and SUMD.
SBUS (S.BUS) – Frsky, Futaba
SBUS is a type of serial communication protocols, shared by Futaba and FrSky, that supports up to 18 channels using only one signal cable.
SBUS is an inverted UART communication signal. Many flight controllers can read UART input, but cannot accept inverted one (such as the Naze32) and an inverted is required. But some FC such as Pixhawks has built-in dedicated signal inverter for this purpose.
XBUS – JR
XBUS is used by JR, which supports up to 14 channels in one signal wire. One of the advantages is the tiny time delay between each channel.
MSP (multiwii serial protocol)
Protocol that was created as part of the multiwii software. Basically it allows you to use MSP commands as the RC input and it supports 8 channels in one signal cable.
Flysky IBUS
IBUS is the new flysky serial protocol. It’s a two way communication: one port for servo data output and one port for sensors.
Graupner Hott SUMD
The Graupner SUMD is a serial protocol like Speksat and SBUS. The channels are encoded into one digital signal and there you have no latency. But we are talking ms of difference, I doubt anyone can tell the difference. Advantages of SUMD:
- VS SBUS – SUMD doesn’t require signal inverter.
- VS PPM – SUMD has better resolution and zero jitter while PPM has only 250 steps and always 4ms jitter.
Graupner SumH
SUMH is a legacy Graupner protocol. Graupner have issued a firmware updates for many recivers that lets them use SUMD instead.
Spektrum DSM2 and DSMX
DSM2 signal is more resistant to noise, interference and other transmitters transmitting on the same frequency. It also finds a backup frequency at start-up in case the primary frequency fails. This lower the chance of losing signal greatly, however if both channels becomes unusable you may still lose the connection.
DSMX was based on and improved from DSM2, which also uses the same econding scheme. The difference is the DSMX signal is able to switch to a new frequency channel in case of cut out within a couple of milliseconds, so in theory you wouldn’t even notice the glitch.
DSM2 is still a popular technology, if you are away from sources of radio interference (such as wifi, microwaves, and wireless security cameras), it should work just as well as DSMX. But DSMX is just more reliable.
Spektrum Satellite
A Spektrum Satellite is an additional antenna and receiver circuit that usually gets connected to the “main” receiver to improve link reliability by providing diversity reception.
