UART vs USB vs Serial vs Ethernet

Understanding the differences between communication protocols is essential for hardware and embedded systems development.

Overview

UART (Universal Asynchronous Receiver-Transmitter)

Definition: A hardware communication protocol that transmits data serially, one bit at a time.

Characteristics

• Asynchronous: No shared clock signal; uses start/stop bits
• Point-to-point: Direct connection between two devices
• Simple: Easy to implement, low overhead
• Configurable: Baud rate, parity, stop bits

Common Baud Rates

9600, 19200, 38400, 57600, 115200 bps

USB (Universal Serial Bus)

Definition: A standardized connection interface for computers and peripherals.

USB Versions

Features

• Hot-pluggable: Connect/disconnect without rebooting
• Power delivery: Can provide up to 100W (USB-C PD)
• Device classes: HID, Mass Storage, Audio, Video
• Hub support: Connect up to 127 devices

How USB Works

USB Enumeration Process:

USB Packet Structure

Token Packet:
┌──────┬──────┬─────────┬──────────┬─────┐
│ SYNC │ PID  │ Address │ Endpoint │ CRC │
└──────┴──────┴─────────┴──────────┴─────┘

Data Packet:
┌──────┬──────┬──────────┬─────┐
│ SYNC │ PID  │ Data     │ CRC │
└──────┴──────┴──────────┴─────┘

Handshake Packet:
┌──────┬──────┐
│ SYNC │ PID  │
└──────┴──────┘

PID Types: OUT, IN, SOF, SETUP, DATA0, DATA1, ACK, NAK, STALL

USB Transfer Types

RS-232 (Serial Communication)

Definition: A standard for serial communication with voltage levels between -15V and +15V.

Signal Levels

Logic 1 (Mark): -3V to -15V
Logic 0 (Space): +3V to +15V

Applications

• Industrial automation (PLCs, sensors)
• Networking equipment (routers, switches)
• Scientific instruments
• Legacy systems

How RS-232 Works

Frame Format:

RS-232 Data Frame (8N1 configuration):
┌───────┬────┬────┬────┬────┬────┬────┬────┬────┬──────┐
│ Start │ D0 │ D1 │ D2 │ D3 │ D4 │ D5 │ D6 │ D7 │ Stop │
│  Bit  │    │    │    │    │    │    │    │    │ Bit  │
└───────┴────┴────┴────┴────┴────┴────┴────┴────┴──────┘
  (0)    LSB                              MSB    (1)

Timing: Each bit lasts 1/baud_rate seconds
Example at 9600 baud: 1 bit = 104 microseconds

RS-232 Signal Lines

Hardware Flow Control (RTS/CTS):

Ethernet

Definition: A family of networking technologies for local area networks (LANs).

Ethernet Standards

OSI Model Layers

• Physical Layer: Cables, connectors, signaling
• Data Link Layer: MAC addresses, frame formatting

How Ethernet Works

Ethernet Frame Structure (IEEE 802.3):

Ethernet II Frame:
┌──────────┬─────────┬──────────┬─────────┬──────────┬─────┬─────┐
│ Preamble │   SFD   │   Dest   │  Source │   Type   │Data │ FCS │
│  (7 B)   │  (1 B)  │ MAC (6B) │ MAC(6B) │  (2 B)   │     │(4B) │
└──────────┴─────────┴──────────┴─────────┴──────────┴─────┴─────┘

Preamble: 10101010... (synchronization)
SFD: 10101011 (Start Frame Delimiter)
Dest MAC: Destination hardware address
Source MAC: Source hardware address
Type: Protocol type (0x0800 = IPv4, 0x0806 = ARP)
Data: 46-1500 bytes
FCS: Frame Check Sequence (CRC-32)

CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

MAC Address Format

MAC Address: 48 bits (6 bytes)
Format: XX:XX:XX:XX:XX:XX (hexadecimal)

Example: 00:1A:2B:3C:4D:5E

┌─────────────────┬─────────────────┐
│ OUI (24 bits)   │ NIC (24 bits)   │
│ Manufacturer ID │ Device Specific │
└─────────────────┴─────────────────┘

Special Addresses:
FF:FF:FF:FF:FF:FF = Broadcast (all devices)
01:00:5E:XX:XX:XX = IPv4 Multicast
33:33:XX:XX:XX:XX = IPv6 Multicast

Ethernet Communication Flow

HTTP (HyperText Transfer Protocol)

Definition: Application-layer protocol for transmitting hypermedia documents and powering the World Wide Web.

Request-Response Model

HTTP Methods

HTTP Request Structure

GET /api/products/123 HTTP/1.1
Host: api.example.com
User-Agent: Mozilla/5.0
Accept: application/json
Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...

[Optional Request Body]

HTTP Response Structure

HTTP/1.1 200 OK
Content-Type: application/json
Content-Length: 145
Cache-Control: max-age=3600

{
  "id": 123,
  "name": "Laptop",
  "price": 999.99,
  "stock": 42
}

Status Code Categories

• 1xx (Informational): Request received, continuing process
• 2xx (Success): 200 OK, 201 Created, 204 No Content
• 3xx (Redirection): 301 Moved Permanently, 304 Not Modified
• 4xx (Client Error): 400 Bad Request, 401 Unauthorized, 404 Not Found
• 5xx (Server Error): 500 Internal Server Error, 503 Service Unavailable

HTTP Versions Comparison

MQTT (Message Queuing Telemetry Transport)

Definition: Lightweight publish-subscribe messaging protocol designed for IoT devices with limited bandwidth.

Publish-Subscribe Model

MQTT Topics

// Topic hierarchy
home/livingroom/temperature
home/livingroom/humidity
home/bedroom/temperature
home/bedroom/light/status

// Wildcards
home/+/temperature        // + matches single level
home/#                    // # matches multiple levels

// Example publish
mosquitto_pub -h broker.example.com -t "home/livingroom/temp" -m "22.5"

// Example subscribe
mosquitto_sub -h broker.example.com -t "home/#"

Quality of Service (QoS) Levels

MQTT Features

• Retained Messages: Broker stores last message for new subscribers
• Last Will and Testament (LWT): Automatic message on unexpected disconnect
• Clean Session: Control whether broker remembers subscriptions
• Keep Alive: Periodic ping to maintain connection

MQTT Connection Flow

Modbus

Definition: Industrial communication protocol for connecting PLCs, sensors, and other automation devices.

Modbus Variants

Master-Slave Architecture

Common Function Codes

Modbus RTU Frame Structure

Modbus RTU Frame:
┌──────────┬──────────┬─────────┬──────┬─────┐
│ Slave ID │ Function │ Data    │ CRC  │     │
│ (1 byte) │ (1 byte) │ (N bytes)│(2 B) │     │
└──────────┴──────────┴─────────┴──────┴─────┘

Example - Read 10 holding registers starting at address 100:
Slave ID: 01
Function: 03 (Read Holding Registers)
Start Address: 00 64 (100 in hex)
Quantity: 00 0A (10 registers)
CRC: XX XX (calculated)

Request: 01 03 00 64 00 0A [CRC]
Response: 01 03 14 [20 bytes of data] [CRC]

Modbus Communication Example

I2C (Inter-Integrated Circuit)

Definition: Two-wire serial communication protocol for short-distance communication between microcontrollers and peripherals.

I2C Bus Architecture

I2C Signals

• SDA (Serial Data): Bidirectional data line
• SCL (Serial Clock): Clock signal from master
• Pull-up Resistors: Required on both lines (typically 4.7kΩ)

I2C Addressing

I2C Communication Protocol

I2C Transaction:
┌───────┬─────────┬────┬──────┬────┬──────┬────┬──────┐
│ START │ Address │ R/W│ ACK  │Data│ ACK  │... │ STOP │
└───────┴─────────┴────┴──────┴────┴──────┴────┴──────┘

START: SDA falls while SCL is high
STOP: SDA rises while SCL is high
ACK: Receiver pulls SDA low
NACK: Receiver leaves SDA high

Example - Write to EEPROM (Address 0x50):
START | 0x50 (Write) | ACK | 0x00 (Mem Addr) | ACK | 0xFF (Data) | ACK | STOP

I2C Speed Modes

Arduino I2C Example

#include <Wire.h>

void setup() {
  Wire.begin();  // Join I2C bus as master
}

void loop() {
  // Write to device at address 0x48
  Wire.beginTransmission(0x48);
  Wire.write(0x01);  // Register address
  Wire.write(0xA0);  // Data to write
  Wire.endTransmission();
  
  // Read from device at address 0x48
  Wire.beginTransmission(0x48);
  Wire.write(0x00);  // Register to read
  Wire.endTransmission();
  
  Wire.requestFrom(0x48, 2);  // Request 2 bytes
  if (Wire.available() >= 2) {
    byte msb = Wire.read();
    byte lsb = Wire.read();
    int value = (msb << 8) | lsb;
  }
  
  delay(1000);
}

I2C Features

• Multi-Master: Multiple masters can control the bus (with arbitration)
• Clock Stretching: Slave can hold SCL low to slow down master
• Hot-Swapping: Some devices support connection/disconnection while powered
• Simple Wiring: Only 2 wires + ground needed

When to Use Each Protocol