Arduino UART Token-Based Star Network

A Deterministic Multi-Node Communication System using SoftwareSerial

Important Notes

• All comments, docstrings, and runtime messages in the source code are written in Portuguese
• This project was developed and validated in a simulated environment

---

Overview

This project implements a deterministic UART-based communication system between multiple Arduino nodes using a token-passing mechanism over a star topology.

A central node (ID 0) coordinates communication between peripheral routers:

• NORTE (NORTH)
• SUL (SOUTH)
• LESTE (EAST)
• OESTE (WEST)

Each router transmits only when it receives a TOKEN, ensuring collision-free communication.

All inter-router communication is mediated by the central node, meaning no direct router-to-router transmission occurs.

The project demonstrates several core embedded systems and networking concepts:

• Token-based medium access control over UART communication
• Star topology network with a centralized arbitrator
• Flit-based packet structure (ORIG / DEST / PAY)
• ACK-based reliable delivery with timeout and retransmission
• Buffer handling and flushing to prevent residual data corruption
• Multi-channel SoftwareSerial management on a single Arduino

---

Repository Structure

Arduino_UART_Token-Based_Star_Network/
│
├── images/
│   ├── funcionamento_central.png
│   ├── funcionamento_norte.png
│   ├── funcionamento_sul.png
│   └── topologia_estrela_uart_arduinos.png
|
├── src/
│   ├── uart_central.ino
│   ├── uart_north.ino
│   ├── uart_south.ino
│   ├── uart_east.ino
│   └── uart_west.ino
│
├── License
│
└── README.md

---

How It Works

1. Central sends a TOKEN to the current node
2. The node replies with a 3-flit packet or NADA if it has nothing to send
3. Central forwards the packet to the destination node
4. Destination receives the packet and replies with ACK
5. Central forwards the ACK back to the origin
6. Central advances the token only after successful delivery or when the node reports NADA
7. When all nodes reply NADA, the session ends

Packet Structure

Each message is split into 3 flits, sent sequentially over UART:

Flit	Format	Description
1	ORIG:<name>	Sender node name
2	DEST:<name>	Destination node name
3	PAY:<text>	Message payload

Control Messages

Message	Direction	Meaning
TOKEN	Central → Node	Node may now transmit
NADA	Node → Central	Node has nothing to send
ACK	Both directions	Packet received successfully

---

Example Communication Flow

NORTE → CENTRAL → SUL

Message flow from NORTE to SUL through the CENTRAL node.

Step-by-step

1. CENTRAL sends TOKEN to NORTE
2. NORTE transmits:

  ORIG:NORTE
  DEST:SUL
  PAY:<message>

4. CENTRAL receives the packet
5. CENTRAL forwards the packet to SUL
6. SUL sends ACK to CENTRAL
7. CENTRAL sends ACK back to NORTE
8. CENTRAL passes the TOKEN to the next router

---

Reliable Delivery (Timeout + Immediate Retransmission)

If the destination router does not send an ACK:

• CENTRAL detects a timeout
• CENTRAL does not advance the token
• The origin does not receive an ACK and automatically retransmits the packet within the same token cycle

This guarantees that:

• No packet is lost
• Delivery is confirmed before progressing
• Communication remains deterministic

---

Wiring

Each peripheral node uses one dedicated TX pin and one dedicated RX pin on the Central Arduino via SoftwareSerial.

Node	D2 (RX) Connected To	D3 (TX) Connected To
NORTH	Central TX (D3)	Central RX (D2)
SOUTH	Central TX (D5)	Central RX (D4)
EAST	Central TX (D7)	Central RX (D6)
WEST	Central TX (D9)	Central RX (D8)

Note: Always connect GND between all boards sharing UART lines.

---

Timing Configuration

All timing constants must be consistent across Central and peripheral nodes.

Configuration Constants:

Constant	Default	Description
BAUD_RATE	2400	Serial baud rate
DELAY_FLIT	600–800 ms	Pause between consecutive flits
TIMEOUT_ACK	12000 ms	Node timeout waiting for ACK from Central
TIMEOUT_ACK_DESTINO	8000 ms	Central timeout waiting for ACK from destination
ESPERA_APOS_TOKEN	2000 ms	Central wait after sending TOKEN
DELAY_APOS_ULTIMO_FLIT	1500 ms	Extra wait after last flit before listening ACK

---

Peripheral Node Configuration

To configure a peripheral node, edit the following section at the top of its .ino file:

const char MEU_NOME[]    = "NORTE";   // This node's name
const char MEU_DESTINO[] = "SUL";   // Valid: SOUTH, EAST, WEST

const char* payloads[] = {
  "Message 1",
  "Message 2",
  "Message 3"
};

int totalPacotes = 3; // Must match the number of entries in payloads[]

---

Features

• Centralized token arbitration in star topology
• Reliable delivery with ACK and automatic retransmission on timeout
• Buffer handling and flushing to prevent residual data corruption
• Flit misalignment detection and automatic reset
• Nodes marked as finished after replying NADA; token skips them
• Session terminates cleanly when all nodes are done
• Per-node serial monitor output for easy debugging

---

Future Improvements

• Priority-based token scheduling
• Error detection (e.g., CRC)
• Fault tolerance (node failure handling, scalable networks)

---

License

This project is open-source and available under the GNU General Public License v3.0 (GPLv3).

---

Author

Developed as an embedded systems and digital communication project.