---
title: "[아두이노#614] Arduino에 W5500을 연결해서 C#윈폼으로 모드버스(modbus) TCP의 프로토콜에 대해서 이해하고 통신하는 방법 알아보기!(녹칸다의 아두이노 시즌2)"
url: "https://maker.wiznet.io/irina/projects/614-arduino-w5500--c--modbus-tcp---------2/"
markdown_url: "https://maker.wiznet.io/irina/projects/614-arduino-w5500--c--modbus-tcp---------2/md"
type: "UCC: User Created Content"
author: "녹칸다"
author_url: "https://bota.tistory.com/2336"
editor: "WIZnet"
editor_url: "https://maker.wiznet.io/"
original_author: "녹칸다"
original_url: "https://bota.tistory.com/2336"
published: "2025-09-24"
language: "ko"
likes: 0
views: 1054
comments: 0
source: "WIZnet Makers (https://maker.wiznet.io/)"
---

# [아두이노#614] Arduino에 W5500을 연결해서 C#윈폼으로 모드버스(modbus) TCP의 프로토콜에 대해서 이해하고 통신하는 방법 알아보기!(녹칸다의 아두이노 시즌2)

> Arduino + W5500 runs Modbus TCP with C# WinForms UI, showing coil control, register read/write, and key RTU vs TCP differences.

Original author: 녹칸다 (source: https://bota.tistory.com/2336)

## Article

[Embedded media](https://www.youtube.com/embed/kBNqc-gWM-A)

## 1. Overview

By combining Arduino with the **W5500 Ethernet module**, you can easily implement **Modbus TCP communication** for industrial and IoT applications.
This post explains the differences between **Modbus RTU and TCP**, introduces common function codes, and demonstrates a **C# WinForms UI** that controls and monitors LEDs via Modbus TCP.

---

## 2. Modbus RTU vs TCP

- **Modbus RTU**
  - Requires **CRC16 checksum** for error detection.
  - Works over serial communication such as **RS-485/RS-232**.
  - Slave address is represented in 1 byte.

- **Modbus TCP**
  - No CRC required, since error handling is done by TCP/IP layers.
  - Adds a **6-byte MBAP header** (Transaction ID, Protocol ID, Length, Unit ID).
  - Still keeps the Unit ID field, but in practice devices are identified by **IP address**.

👉 Conclusion: Modbus TCP is **more scalable and suitable for network-based industrial control** compared to RTU.

---

## 3. Modbus Basics

- **Coils**: 1-bit storage (ON/OFF, usually mapped to GPIO pins).

- **Registers**: 16-bit storage (used for sensor values or internal variables).

- **Function Codes (FC):**
  - `0x05`: Write Single Coil
  - `0x01`: Read Coils
  - `0x03`: Read Holding Registers
  - `0x06`: Write Single Register
  - `0x10`: Write Multiple Registers

---

## 4. Arduino + W5500 Example

#### (1) Write Single Coil (FC = 0x05)

Control an LED connected to **digital pin 2**.

- ON Command

```plaintext
0x00 0x01 0x00 0x00 0x00 0x06 0x01 0x05 0x00 0x00 0xFF 0x00
```

- OFF Command

```plaintext
0x00 0x01 0x00 0x00 0x00 0x06 0x01 0x05 0x00 0x00 0x00 0x00
```

#### (2) Read Coils (FC = 0x01)

- Reads multiple coil states from Arduino.

- Example: LED status is displayed on the WinForms UI.

#### (3) Address Mapping

- Coil 0 → Arduino Pin 2

- Coil 1 → Arduino Pin 3

- Easily extendable to more GPIO pins and devices.

---

## 5. C# WinForms UI Integration

The C# WinForms application acts as the Modbus TCP Master:

- Input fields for Transaction ID, Unit ID, Coil Address.

- Buttons for LED ON/OFF control using Write Single Coil (FC=05).

- A timer periodically issues Read Coils (FC=01) to display LED states in real time.

- Communication handled over TCP sockets to Arduino (port 502).

---

## 6. Implementation Details

#### Arduino (Slave)

- Initialize network:

```plaintext
Ethernet.begin(mac, ip, gateway, subnet);
EthernetServer modbusServer(502);
```

- Wait for incoming TCP connection, parse MBAP header, extract **Function Code**.

- Map coil addresses to GPIO pins.

- Example: Coil 0 → Pin 2, Coil 1 → Pin 3.

- Process requests and return responses according to Modbus TCP framing.

#### C# WinForms (Master)

- Connect to Arduino:

```plaintext
TcpClient client = new TcpClient(ip, 502);
NetworkStream stream = client.GetStream();
```

- Build Modbus TCP frames: **MBAP Header + PDU (Function Code + Data)**.

- Send Write/Read requests and parse responses.

- Update the UI with LED state values.

#### Example (FC = 05: Write Single Coil)

- **Request**: Contains coil address + ON/OFF value (0xFF00 = ON, 0x0000 = OFF).

- **Response**: Echoes the same data back.

---

## 7. Debugging & Tips

- **Endian issues**: Modbus uses **Big-Endian** format, so check byte order.

- **Logging**: Print request/response frames in HEX for debugging.

- Use **Modbus Poll** or similar tools to test Arduino server before connecting to C#.

- Define a clear **Address Map** to expand easily to sensors, relays, and more actuators.

---

## 8. Conclusion

This project demonstrates how to implement **Modbus TCP with Arduino and W5500 Ethernet**, and how to connect it with a **C# WinForms UI**.
The setup can be applied to:

- Industrial control systems

- IoT gateways

- Remote monitoring applications

## ✅ Key SEO Keywords

- Arduino Modbus TCP

- W5500 Ethernet module

- Modbus RTU vs TCP

- C# WinForms Modbus example

- Arduino LED control

---

Source: https://maker.wiznet.io/irina/projects/614-arduino-w5500--c--modbus-tcp---------2/