# Understanding FPV Flight Controller Pinouts and Wiring Diagrams
A flight controller pad diagram looks like alphabet soup the first time you see it: RX1, TX3, SDA, SCL, LED_STRIP, BZ-, CUR, RSSI… Miswire one pad and your quad won’t arm, your receiver won’t talk, or worse — you’ll let the magic smoke out. This guide decodes every common flight controller pad and explains exactly where each wire goes.
## The Universal FC Layout
While every flight controller is different, they all follow the same logical layout. Understanding the categories makes any FC diagram readable in seconds.
| Pad Category | Typical Label | Function | Voltage |
|————-|—————|———-|———|
| Power input | BAT+, GND, VBAT | Main battery voltage in | 2S-6S (7.4-25.2V) |
| Regulated output | 5V, 9V, 3.3V | Power for accessories | Regulated |
| UART TX/RX | TX1, RX1, TX2, RX2… | Serial communication | 3.3V or 5V |
| Motor outputs | M1, M2, M3, M4 or S1-S4 | ESC signal pads | 3.3V signal |
| Receiver | SBUS, DSM, PPM, CRSF | RC receiver input | 3.3V/5V |
| Video | VTX, CAM, Video In/Out | Analog video path | Varies |
| Buzzer/LED | BZ+, BZ-, LED_STRIP | Buzzer and LED control | 5V |
| I2C | SDA, SCL | External sensors (magnetometer, barometer) | 3.3V |
| Misc | RSSI, CUR, TX5-ready | Analog/digital inputs | Varies |
## Power Section: Don’t Fry Your Board
### VBAT (Battery Voltage)
Direct battery voltage from your LiPo. This pad connects to the main power lead (XT60/XT30). It also powers the FC’s onboard voltage regulators.
**Critical**: Never connect anything to VBAT that can’t handle full battery voltage. A 6S LiPo at full charge is 25.2V. A component rated for 5V will be destroyed instantly.
### 5V Output
The 5V regulator powers most accessories: receiver, GPS, buzzer, analog camera. Check your FC’s 5V BEC (battery eliminator circuit) current rating:
| BEC Rating | Can Power |
|———–|———–|
| 1A | Receiver + buzzer only |
| 2A | Receiver + GPS + buzzer |
| 3A+ | Receiver + GPS + camera + buzzer + LED |
**If your accessories draw more than the BEC rating**: The 5V rail will brown out in flight, causing receiver dropouts and camera blackouts. Use an external BEC or power the camera from VBAT if it supports it.
### 9V / VCC / VTX Power
Some FCs include a dedicated filtered power output for the VTX, often switchable between 5V and 9V. If your VTX supports 7-26V input, power it from VBAT instead to reduce load on the 5V regulator and avoid LC filter limitations.
## UARTs: The Nervous System
UARTs (Universal Asynchronous Receiver/Transmitter) are how the FC talks to external devices. Each UART is a pair: TX (transmit) and RX (receive).
**The golden rule**: TX on one device connects to RX on the other, and vice versa.
“`
FC TX1 → Receiver RX
FC RX1 ← Receiver TX
“`
### Typical UART Assignments
| UART | Common Assignment | Notes |
|——|——————|——-|
| UART1 | Receiver (SBUS/CRSF) | Often the first UART, near the RX pads |
| UART2 | SmartAudio / IRC Tramp (VTX control) | Only needs TX — no RX connection required |
| UART3 | GPS | Needs both TX and RX |
| UART4 | ESC telemetry | Only needs RX on the FC side |
| UART5 | Spare / configurable | Camera control, additional GPS, etc. |
| USB VCP | Betaflight Configurator | Virtual COM port, always available |
### UART Voltage Levels
| FC Type | UART Voltage | Compatible With |
|———|————-|—————–|
| Most F4/F7 FCs | 3.3V | Almost everything |
| Some F7/H7 FCs | 5V tolerant | Check manual — 3.3V devices may need level shifting |
| Older F3 FCs | 5V | May damage 3.3V-only receivers |
**Warning**: Some receivers (especially FrSky R-XSR) run at 3.3V logic but can be powered by 5V. The signal wires operate at 3.3V — never connect a 5V UART TX to a 3.3V receiver without a level shifter or resistor divider.
## Receiver Wiring: Protocol by Protocol
| Protocol | Wires Needed | FC Pads | Notes |
|———-|————-|———|——-|
| CRSF (ELRS/Crossfire) | 4 wires: 5V, GND, TX1, RX1 | TX→RX on RX, RX→TX on RX | Both TX and RX required for telemetry |
| SBUS (FrSky, Futaba) | 3 wires: 5V, GND, SBUS | SBUS pad or any RX pad | Inverted signal on F4 — use dedicated SBUS pad |
| FPort (FrSky) | 3 wires: 5V, GND, TX | Any TX pad | Combines control + telemetry on one wire |
| Spektrum DSM | 3 wires: 3.3V, GND, RX | DSM pad or any RX pad | 3.3V power — not 5V! |
| PPM | 3 wires: 5V, GND, RX | PPM pad | Legacy — avoid, high latency |
### CRSF Wiring for ELRS
ExpressLRS and Crossfire both use the CRSF protocol. The wiring is identical:
“`
FC 5V ──────► Receiver VCC (red)
FC GND ──────► Receiver GND (black)
FC TX1 ──────► Receiver RX (yellow/white)
FC RX1 ◄────── Receiver TX (green/gray)
“`
In Betaflight Ports tab, enable `Serial Rx` on UART1. In Receiver tab, set mode to `Serial-based receiver` and provider to `CRSF`.
## Motor Output Wiring
### 4-in-1 ESC Wiring (Modern Standard)
Most builds use a 4-in-1 ESC connected via a single plug or direct-solder:
| ESC Wire | FC Pad | Signal |
|———-|——–|——–|
| VCC/VBAT | BAT+ | Motor power |
| GND | GND | Motor ground |
| S1 | M1 | Motor 1 signal |
| S2 | M2 | Motor 2 signal |
| S3 | M3 | Motor 3 signal |
| S4 | M4 | Motor 4 signal |
| TLM (optional) | RX on a UART | ESC telemetry |
| CUR (optional) | CUR | Current sensor |
### Motor Numbering and Direction
Betaflight uses the “Quad X” configuration. Viewed from above with the front of the quad pointing away from you:
“`
Front
M4 (CW) M2 (CCW)
\\ //
\\ //
=========
// \\
// \\
M3 (CCW) M1 (CW)
Back
“`
– **M1**: Rear right — Clockwise (CW)
– **M2**: Front right — Counter-clockwise (CCW)
– **M3**: Rear left — Counter-clockwise (CCW)
– **M4**: Front left — Clockwise (CW)
Motor direction can be reversed in BLHeli Suite or Bluejay Configurator — you don’t need to swap motor wires.
## Video System Wiring
### Analog VTX + Camera
“`
Camera 5V ◄── FC 5V (or camera-specific 5V pad)
Camera GND ──── FC GND (use CAM-GND if available)
Camera VID ──── FC CAM (Video In)
FC VTX (Video Out) ──── VTX Video In
FC 9V/VBAT ──────► VTX Power In
FC GND ──────────── VTX GND
SmartAudio wire from VTX ──── FC TX (on configured UART)
“`
The FC sits between the camera and VTX, injecting the OSD overlay. If you bypass the FC (camera directly to VTX), you lose OSD.
### Digital Systems (DJI O3/O4, Walksnail)
Digital VTX systems combine camera and VTX into one unit with a single UART connection:
“`
DJI O3 9V ◄── FC 9V (or VBAT if BEC is 2A+)
DJI O3 GND ──── FC GND
DJI O3 RX ◄── FC TX (on configured UART — OSD data)
DJI O3 TX ──── FC RX (on configured UART — telemetry/recording control)
“`
In Betaflight Ports tab, enable `MSP` (not Serial Rx) on the UART connected to the DJI/Walksnail air unit. Set baud rate to 115200.
## LED and Buzzer Wiring
| Pad | Connect To | Notes |
|—–|———–|——-|
| LED_STRIP | Data in on WS2812 LED strip | 5V signal, max ~32 LEDs without external power |
| BZ+ | Buzzer positive (red) | 5V output |
| BZ- | Buzzer negative (black) | Switched ground — FC activates buzzer by grounding this |
**Important**: Active buzzers (the loud kind) go to BZ+ and BZ-. Passive piezo buzzers (quiet) use different pads — check your FC manual.
## GPS Wiring
“`
FC 5V ──────► GPS VCC
FC GND ──────► GPS GND
FC TX3 ──────► GPS RX
FC RX3 ◄────── GPS TX
“`
In Betaflight Ports tab, enable `Sensor Input` → `GPS` at 115200 baud on the GPS UART. In Configuration tab, enable GPS and set protocol to UBLOX.
## Common Wiring Mistakes
| Mistake | Symptom | Fix |
|———|———|—–|
| TX→TX, RX→RX (crossed) | Device not detected | Swap — always TX→RX |
| 5V power to 3.3V device | Component burns out immediately | Check voltage ratings before connecting |
| VBAT to 5V accessory | Component destroyed on plug-in | Use regulated 5V/9V output |
| Motor order wrong (M1→M4 swapped) | Quad flips on arming | Verify motor map in Betaflight |
| CRSF wired to SBUS pad | No receiver input | CRSF needs full UART (TX+RX), not SBUS |
| Camera ground not connected | OSD flickering or missing | Use dedicated CAM-GND pad |
—
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