RC Radio Protocols Explained: PWM, PPM, SBUS, and CRSF
Between your radio transmitter and your drone’s flight controller sits a language — a protocol that turns stick movements into signals the flight controller understands. PWM, PPM, SBUS, and CRSF are the four you will encounter most often. Knowing the difference helps you wire receivers correctly and get the lowest possible latency out of your setup.
PWM: One Channel, One Wire
PWM, or pulse-width modulation, is the oldest and simplest approach. Each channel gets its own physical wire, and the signal is a repeating pulse whose width encodes the stick position — typically 1000 to 2000 microseconds. PWM is still common on fixed-wing models and single-channel functions like servos, but it is bulky for a multi-channel quadcopter because every channel needs a separate connection.
PPM: All Channels on One Wire
PPM, or pulse-position modulation, compresses all channels onto a single wire by sending each channel’s pulse in sequence, back to back. It solved the wiring problem for early flight controllers but came with a cost: latency increases as more channels are packed into each frame, and the analog timing can jitter. PPM still works, but it is largely a legacy format today.
SBUS: Serial and Faster
SBUS is a digital serial protocol originally from Futaba. It carries up to 16 channels over a single wire at a fixed baud rate, with less latency and more resolution than PPM. SBUS is inverted at the signal level, which meant older flight controllers needed an inverter to read it — a common source of confusion on first builds. Most modern flight controllers handle the inversion in firmware, so wiring is usually a simple three-wire connection to a UART.
CRSF: The Modern Standard
CRSF, the Crossfire protocol from TBS, is now the backbone of ExpressLRS — the long-range link that has taken over the FPV hobby. CRSF is a bidirectional serial protocol: the receiver sends channel data to the flight controller while simultaneously receiving telemetry back, all over the same wire. That telemetry link is a game changer, feeding RSSI, link quality, and battery voltage straight to your radio and on-screen display.
CRSF delivers lower latency than SBUS and supports advanced features like receiver-based LQ warnings that let you know when your link is weakening. For most new builds, CRSF over an ExpressLRS receiver is the clear default.
Choosing and Wiring the Right Protocol
The rule of thumb is simple: use CRSF whenever your receiver supports it. Wire the receiver’s TX and RX to a spare UART on the flight controller, select CRSF in the receiver configuration tab, and enable the appropriate serial RX option in firmware. If you are flying an older receiver or a fixed-wing setup, SBUS is a solid fallback, and PWM remains the right choice for individual servos and simple models.
Latency and Why It Matters
Latency is the delay between moving a stick and the drone reacting. It is the sum of several steps — the radio link, the protocol encoding, the flight controller loop, and the ESC update rate. PPM adds noticeable delay because every channel has to be transmitted serially, while SBUS and CRSF are dramatically faster. For freestyle flying, lower latency means a more connected, responsive feel; for racing, it can be the difference between clipping a gate and clearing it. CRSF’s low latency combined with a fast flight controller loop is one reason ExpressLRS builds feel so crisp in the air.
Telemetry is the other major benefit of a modern protocol. CRSF returns signal strength and link quality to the radio in real time, so you know when to turn back before you lose the link entirely. SBUS is a one-way street by comparison. If long range or flying around obstacles is in your plans, a bidirectional protocol with live telemetry is worth the small additional setup effort.
Understanding these four protocols takes the mystery out of receiver wiring. PWM is one wire per channel, PPM packs everything onto one wire, SBUS digitizes it with a serial link, and CRSF adds full telemetry and modern latency. For an FPV build, CRSF over ExpressLRS is the standard that gets you the best range, the most data, and the lowest delay.