# Cable Wiring Basics

Cables are a common source of electromagnetic interference (EMI) (opens new window), which can cause problems including flyways, "toilet bowling" and generally poor flight. These problems can be avoided by using appropriate cabling in the UAV.

The following basic concepts should be kept in mind when designing drone cabling:

  • High-Power and signal cables should be separated as much as is practical
  • Cable lengths should be the minimum needed to enable easy handling of wired components. The wire tension should be adequate to survive possible airframe deformations even in a crash landing (wires must not be the first thing to break).
  • Cable loops to reduce excess length should be avoided - use shorter lengths!
  • For digital signals you can decrease the baudrate to reduce radiated energy and increase the robustness of data transfer. This means that you may be able to use longer cables when high data rates are not needed.

# Signal wiring

Signalling protocols have different characteristics, and therefore the cables used in each case require slightly different specifications.

This topic provides specific guidance of cabling for different signalling protocols, along with the colour coding used by a number different drone hardware vendors.

# I²C cables

The I2C bus (opens new window) is widely used for connecting sensors. Cable colors from several vendors are specified in following table.

Signal Pixhawk Colors ThunderFly colors CUAV colors (I2C/CAN)
+5V red Red red Red red Red
SCL black Black yellow Yellow white White
SDA black Black green Green yellow Yellow
GND black Black black Black black Black

The Dronecode standard (opens new window) assumes a 1.5k ohm pull-up resistor on SDA and SCL signals in autopilot.

# Cable twisting

I2C bus signal cross-talk and electromagnetic compatibility can be greatly improved by proper twisting of the cable wires. Twisted pairs (opens new window) is especially important for sensor wiring.

  • 10 turns for each pair SCL/+5V and SDA/GND per 30cm cable length. I²C JST-GH cable
  • 4 turns of both pairs together per 30cm cable length. I²C JST-GH connector detail

When using appropriate twisted pair cables, the I²C bus is generally suitable for submeter-scale airframes. For larger aircraft the use of CAN or other differential signaling based interface is generally more reliable.

注解

This turns/cable-length recommendation has been successfully used with I2C sensors including the ThunderFly TFSLOT airspeed sensor and TFRPM01 Revolution Counter.

# Pull-up resistors

Pull-up resistors are required for all ends of an I2C bus. This acts both as signal termination (opens new window) and as bus idle signal generator.

An oscilloscope measurement is sometimes required to check correct value of pull-up resistors. The signals on the I2C bus should have clear sharp rectangle-like edges and amplitude of few volts. In case the signal has a low amplitude, the value of pull-up resistors is too low and should be decreased. In the case of rounded signals, the value of pull-up resistors is too high.

# UAVCAN cables

Signal Pixhawk ThunderFly Zubax CUAV (I2C/CAN)
+5V red Red red Red red Red red Red
CAN_H black Black white White white White white White
CAN_L black Black yellow Yellow yellow Yellow yellow Yellow
GND black Black black Black black Black black Black

# Cable twisting

CAN cables should also be twisted, for exactly the same reason as I2C cables. For CAN the recommended twisting is:

  • 10 turns for each pair GND/+5V and CAN_L/CAN_H per 30cm cable length. CAN JST-GH cable

  • 4 turns of both pairs together per 30cm cable length.

# SPI

SPI (opens new window) is synchronous serial communication interface used for connecting faster sensors and devices. This protocol is commonly use is for connecting optical flow sensors or special telemetry modems.

Signal Pixhawk Color ThunderFly color
+5V red Red red Red
SCK black Black yellow Yellow
MISO black Black blue Blue
MOSI black Black green Green
CS! black Black white White
CS2 black Black blue Blue
GND black Black black Black

# UART

UART is used to connect peripherals to the autopilot. By default, UART does not support networking and therefore it directly connects two devices together. It is often used to connect an autopilot and a radio modem.

CTS and RTS are signals that are used to indicate that data is being transmitted on TX/RX pins. This hand-shake mechanism increases reliability of data transfer. CTS and RTS may remain loose when it is not used by the device.

The connecting cable is not crossed. Therefore, it is necessary to connect only the autopilot and peripherals with this straight cable. The device must cross the wiring internally by swapping RX/TX and RTS/CTS pins.

Signal Pixhawk Color ThunderFly color
+5V red Red red Red
TX black Black white White
RX black Black green Green
CTS black Black blue Blue
RTS black Black yellow Yellow
GND black Black black Black

UART signals are common sources of low frequency EMI, therefore the length of the cable should be minimized as much as possible. Cable twisting is not needed for UART cables.

# GPS(UART) & SAFETY

GPS receivers and magnetometers are generally very sensitive to EMI. Therefore thse should be mounted far away from RF sources (high-power cabling, ESCs, radio modems and its antenna). This may be insufficient if the cabling is badly designed.

Signal Pixhawk Color ThunderFly color
+5V red Red red Red
TX black Black white White
RX black Black green Green
SCL black Black yellow Yellow
SDA black Black green Green
SAFETY_SW black Black white White
SAFETY_SW_LED black Black blue Blue
+3v3 black Black red Red
BUZZER black Black blue Blue
GND black Black black Black

# GPS

Signal Pixhawk Color ThunderFly color
+5V red Red red Red
TX black Black white White
RX black Black green Green
SCL black Black yellow Yellow
SDA black Black green Green
GND black Black black Black

The GPS cable connects to both the UART and I2C bus. As twisting of UART is not applicable the length of the cable should be minimized as much as possible.

# Analog signal (power module)

Signal Pixhawk Color ThunderFly color CUAV color
VCC red Red red Red red Red
VCC black Black red Red red Red
CURRENT black Black white White white White
VOLTAGE black Black yellow Yellow yellow Yellow
GND black Black black Black black Black
GND black Black black Black black Black

This connector is example of mix of relatively high-power and low voltage signaling. Unfortunately, twisting is applicable for high-power GND and VCC wires only. That does not help much for noisy reading of analog signals by autopilot.

# SAFETY

Signal Pixhawk Color ThunderFly color
SAFE_VCC red Red red Red
SAFETY_SW_LED black Black blue Blue
SAFETY_SW black Black white White
BUZZER black Black blue Blue
+5V black Black red Red
GND black Black black Black

# High-power wiring

For high power wiring the most important design criteria is having an appropriate wire thickness, in order to allow sufficient current to flow. The general cross section requirement is area of 1 mm² per 8A of wire current.

While rarely practical, it is beneficial to have positive and negative wires twisted together.

EMI from high power cabling has a significant effect on magnetometers. For this reason a large seapration between high-power cables and navigation magnetometers is always required.

# Cable colour coding

Most manufacturers use red for the high voltage line and black for ground. Other colouring is at the manufacturer discretion. The Pixhawk connector standard (opens new window) requires only that the Voltage Common Collector (VCC) pin/cable be red.

Color coding signal wires can help identify particular cables, making it easier to assemble your drone.

A colour coding scheme designed for easy cable identification might follow the following rules:

  • The red and black colors are reserved for power.
  • The same signal type should have the same colour.
  • Color of the signal does not repeat in the connector for wires adjacent to each other.
  • Wiring harnesses of the same pin count have to have a unique color sequence. This determines the cable type. (This is especially useful on photographs used in a manual).

An example of a cable colouring designed to these rules is:

Color Name Preferred usage
red Red Power voltage
green Green General purpose signal
white White General purpose signal
yellow Yellow General purpose signal
blue Blue Power return, Open-collector control signal
black Black GND, Power return ground
:::note The above rules were provided by Thunderfly and are used in their cable design.

The cable colour-coding for Thunderfly and some other vendors are given in the sections below. The pin labels correspond to the pinout on the autopilot side. All cables are straight (1:1). If they require crossover (e.g. UART), this should be solved by internal connection of the devices. :::