Understanding Radio Communication Protocols
Did you ever hear the story about the guy who bought a separate quadcopter and transmitter? He was so enthusiastic about getting stuck into his new hobby, that he only wanted the best pieces of kit. That’s why, after days and nights of choosing the right quad, he went out and bought the most advanced transmitter on the market. Only, when he got home, it became obvious that something was wrong: the transmitter and quad both seemed okay on their own, but they wouldn’t work together. The guy eventually sighed and gave up fiddling with them. He had to do some research to find out what the problem was.
After a quick internet search, he began to curse himself as he realized what he had done. The transmitter and the quad couldn’t work together, because they couldn’t talk to each other. It was at this point that the guy realized he should have read up on radio communications (RC) protocols before parting with his cash.
RC protocols are basically languages used by different devices to communicate with each other. Back in the day, you may remember that RC devices such as toys and planes could sometimes be controlled from another person’s transmitter. Since then, different brands and models have begun to use their own, unique protocols. This, however, can turn into something of a double-edged sword, as the story above illustrates. As the guy in that story found out, if you’re going to build your own quadcopter, you need to make sure that your transmitter uses a protocol compatible with the drone’s flight controller and radio receiver. Doing so, however, involves negotiating a minefield of unfriendly acronyms related to protocols.
PCM (Pulse Code Modulation)
The first of these is PCM, which stands for pulse code modulation. This is a variation of pulse position modulation (PPM), a form of signal modulation which has its origins in telegraph time-division multiplexing. While PPM is analog, PCM is digital. It can detect signals and correct errors. In general, it is preferred to PPM as it is less prone to interference.
PWM (Pulse Width Modulation)
Another form of pulse modulation is PWM, which stands for pulse width modulation. This was one of the first protocols to be used with RC airplanes. Back in those days, receivers could manage servos with PWM signals. These receivers are still in use today thanks to its low cost and its widespread familiarity with amateur enthusiasts. The signal pulse usually varies between 1-2K microseconds.
Spektrum RC transmitters and receivers commonly use one of two DSM protocols. DSMX is the world’s first wideband, frequency-agile signals protocol. Like its twin, DSM2, it is hardly vulnerable to noise and interference from other transmitters. The main difference between these two is that DSM2 starts up with a backup frequency in case of primary signal failure. Apart from that, the exact way in which they ‘decide’ to switch between channels is also slightly different. The Spektrum receivers and transmitters used with DSM are typically geared up for 2.4Ghz signals.
If you buy anything from the brands Futaba or FlySky, you’ll need to familiarize yourself with the SBUS protocol. This protocol supports up to 18 channels using only one signal cable. For those who are familiar with universal asynchronous receiver and transmitter (UART) signals, SBUS is quite easy to understand, as it is an inverted UART signal. This gives Futaba and FlySky the opportunity of securing brand loyalty for their excellent transmitters and receivers, as other flight controllers find it difficult to decipher inverted UART inputs. As the technology is under patent, competitors can’t currently use it.
Sticking with Spektrum for a moment, the main receiver of Spektrum Satellite is directly connected to an extra antenna. This offers diversified reception to enhance link dependability.
Another protocol which can handle several channels through a single cable wire is CPPM. This protocol uses the same PWM signals discussed earlier, relayed one after the other in a series. As CPPM is an analog single-time domain, these signals can only be sent one at a time, rather than simultaneously. As a result of this, it has some of the advantages of SBUS, but with fewer channels and less precision. Its advantage over SBUS is that it is more widely used, and therefore more likely to be compatible.
The latest and most up-to-date of series protocols is IBUS FrSky. This system has one port designed specially to deal with servo data input and the others designed to maintain efficiency in sensor connections. This means that the IBUS can function, put simply, as a two-way RC system.
If you’re easily put off by talk of protocols and signals, and confused by acronyms and the sneering way they are bandied about by those who think of themselves as tech-savvy, you may be tempted to give up your idea of building a drone and simply purchase a ready-to-fly model off the shelf. Doing so would make your life easier in the short term, but removes a large part of the joy of drones. Even if you wanted to get more involved on a technical level at a later date, you might find it difficult to do so, as standard controllers typically don’t say which protocol they are using.
Alternatively, a good way to choose which protocol you should use, is to start with the transmitter you like, find out which protocol it uses, and then find compatible counterparts. Just don’t be like the guy in the story – make sure your transmitter and receiver can talk to each other. Should this happen, however, you may be able to make use of a converter box. Failing that, you’ll be looking at rewiring and replacing parts of your kit.