Communication Overview

Introduction

A robot is a distributed system — sensors, controllers, actuators, and host computers all need reliable data exchange. The choice of communication protocol directly affects system real-time performance, reliability, and scalability.

Communication Layer Model

Robot communication can be divided into four layers:

graph TB
    subgraph Long-Range Communication
        A1[4G/5G]
        A2[LoRa]
        A3[RC Transmitter 2.4GHz]
    end

    subgraph Wireless LAN Communication
        B1[WiFi]
        B2[BLE Bluetooth]
        B3[ESP-NOW]
    end

    subgraph Inter-Board Communication
        C1[UART Serial]
        C2[CAN Bus]
        C3[EtherCAT]
        C4[RS-485]
    end

    subgraph Intra-Board Communication
        D1[SPI]
        D2[I2C]
        D3[GPIO/PWM]
    end

    A1 & A2 & A3 --> B1 & B2 & B3
    B1 & B2 & B3 --> C1 & C2 & C3 & C4
    C1 & C2 & C3 & C4 --> D1 & D2 & D3

    style A1 fill:#f9f,stroke:#333
    style B1 fill:#bbf,stroke:#333
    style C2 fill:#bfb,stroke:#333
    style D1 fill:#ffb,stroke:#333

Typical Protocols for Each Layer

Intra-Board Communication

Short-distance communication between chips and peripherals, typically on the same PCB.

Protocol	Speed	Wires	Topology	Typical Peripherals
SPI	1–100 MHz	4 (+CS)	Master-slave	IMU, display, Flash
I2C	100k–3.4M	2	Multi-master/multi-slave	Sensors, OLED, EEPROM
GPIO/PWM	—	1	Point-to-point	LED, servo, buzzer

Inter-Board Communication

Communication between different modules/boards within the robot.

Protocol	Speed	Distance	Topology	Typical Applications
UART	115.2k–1M bps	<15 m	Point-to-point	MCU-to-MCU, GPS, bus servos
RS-485	100k–10M bps	<1200 m	Multi-drop	Industrial sensors, Dynamixel
CAN	1M bps (CAN) / 8M (CAN FD)	<40 m @1M	Bus	Motor networks, automotive
EtherCAT	100M bps	100 m/segment	Ring	Industrial robot joints
USB	480M (2.0) / 5G (3.0)	<5 m	Star	Cameras, LiDAR

Wireless LAN Communication

Protocol	Speed	Distance	Power	Typical Applications
WiFi 2.4G	72–150 Mbps	~50 m	High	Image transmission, ROS2 communication
WiFi 5G	433–866 Mbps	~30 m	High	High-bandwidth sensors
BLE 5.0	2 Mbps	~100 m	Very low	Remote control, status reporting
ESP-NOW	1 Mbps	~200 m	Low	ESP32 peer-to-peer communication

Long-Range Communication

Protocol	Speed	Distance	Features
4G LTE	50–150 Mbps	Coverage area	High bandwidth, requires SIM card
5G	1–10 Gbps	Coverage area	Ultra-low latency
LoRa	0.3–50 kbps	2–15 km	Ultra-low power, small data volume
RC Transmitter (2.4G)	~100 kbps	1–2 km	Real-time control, low latency

Bandwidth vs. Latency vs. Reliability

Three-Way Trade-off

Different applications prioritize the three core communication metrics differently:

Application	Bandwidth Need	Latency Requirement	Reliability Requirement
Motor control	Low (tens of bytes)	Very high (<1 ms)	Very high
IMU data	Low-medium	High (<5 ms)	High
LiDAR	High (MB-level)	Medium (<50 ms)	High
Camera images	Very high (tens of MB/s)	Medium (<100 ms)	Medium
Remote commands	Very low	High (<20 ms)	High
Remote monitoring	Medium	Low (<1 s)	Medium
Map data	High	Low	High

Real-Time Classification

Level	Latency	Jitter	Protocols
Hard real-time	<1 ms	<10 us	EtherCAT, CAN
Soft real-time	<10 ms	<1 ms	CAN, RS-485, UART
Near real-time	<100 ms	<10 ms	WiFi, USB
Non-real-time	>100 ms	Unlimited	4G, LoRa, HTTP

Robot Communication Architecture Examples

Small Wheeled Robot

RC Transmitter (2.4G/BLE)
      ↓
  Main Controller (ESP32)
   ├── UART ──→ LiDAR
   ├── I2C ───→ IMU + OLED
   ├── PWM ───→ Servos × 2
   └── GPIO+PWM → Motor driver board → Motor+Encoder

Six-Axis Robotic Arm

Host PC (ROS2)
      │ EtherCAT / CAN
      ↓
  Controller (Embedded Linux)
   ├── CAN ──→ Joint 1 driver ──→ BLDC + Encoder
   ├── CAN ──→ Joint 2 driver ──→ BLDC + Encoder
   ├── CAN ──→ ... (Joints 3-6)
   ├── RS-485 → End effector
   └── Ethernet → Vision system

Quadruped Robot

WiFi/4G ──→ Host (Jetson)
               ├── Ethernet ──→ Depth camera
               ├── USB ────────→ LiDAR
               ├── SPI ────────→ IMU (high speed)
               └── CAN Bus
                    ├── Joint 1 (Hip) CAN ID=0x01
                    ├── Joint 2 (Knee) CAN ID=0x02
                    ├── Joint 3 (Ankle) CAN ID=0x03
                    └── ... (×4 legs = 12 joints)

Protocol Selection Decision

graph TD
    A[Communication Requirement] --> B{On the same board?}
    B -->|Yes| C{Speed requirement?}
    C -->|High >1MHz| D[SPI]
    C -->|Low-medium| E[I2C]
    B -->|No| F{Wired or wireless?}
    F -->|Wired| G{How many nodes?}
    G -->|2 point-to-point| H[UART]
    G -->|Multiple nodes| I{Real-time requirement?}
    I -->|Hard real-time| J[EtherCAT]
    I -->|Moderate| K[CAN Bus]
    F -->|Wireless| L{Distance?}
    L -->|<100m| M{Bandwidth need?}
    M -->|High| N[WiFi]
    M -->|Low| O[BLE/ESP-NOW]
    L -->|>1km| P[4G/LoRa]

    style D fill:#ffb,stroke:#333
    style E fill:#ffb,stroke:#333
    style H fill:#bfb,stroke:#333
    style J fill:#f9f,stroke:#333
    style K fill:#bfb,stroke:#333
    style N fill:#bbf,stroke:#333
    style O fill:#bbf,stroke:#333
    style P fill:#fbb,stroke:#333

Communication Safety Considerations

Common Issues

Issue	Impact	Countermeasure
Data loss	Command not executed	CRC check, retransmission mechanism
Latency jitter	Control instability	Real-time protocols, priority
EMI	Data errors	Differential signals (CAN/RS-485), shielded cables
Signal attenuation	Communication interruption	Appropriate transmission distance, signal boosting
Ground loop current	Signal offset	Optocoupler isolation, differential transmission

Error Handling Strategies

• CRC check: Detects transmission errors (built into CAN)
• Timeout retransmission: Automatic resend after packet loss
• Heartbeat mechanism: Periodic connection state checking
• Watchdog: Automatically enters safe state upon communication interruption

Summary

• Robot communication is divided into four layers: intra-board, inter-board, wireless LAN, and long-range
• Each layer has corresponding optimal protocol choices
• Motor control requires hard real-time (CAN/EtherCAT); sensor acquisition requires moderate real-time
• Bandwidth, latency, and reliability require trade-offs
• Communication architecture design must align with the overall robot architecture
• EMI resistance and error handling are critical for reliable operation in practical systems

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