Wiznet makers

1. Overview

In this post, we go beyond a simple Modbus TCP example and dive into practical implementation details using Arduino UNO + W5500 Ethernet and a C# WinForms master application.

You will learn how to:

• Control multiple coils with FC15 (Write Multiple Coils).
• Update variables with FC06 (Write Single Register).
• Read input registers with FC04 (Read Input Registers).
• Safely exchange 32-bit floating-point values.

We also explain the Modbus TCP frame structure, endianness, word-order pitfalls, and provide debugging tips from a chip engineer’s perspective.

2. Modbus TCP Frame Structure

• MBAP Header (7 bytes):
  Transaction ID (2) + Protocol ID (2, always 0x0000) + Length (2) + Unit ID (1)
• PDU: Function Code (1) + Data (N)

Coils & Registers

• Coils/Discrete Inputs: bit-addressable (mapped to Arduino GPIO).
• Holding/Input Registers: 16-bit word-based (mapped to variables or sensor values).

👉 Important: TCP removes CRC (unlike RTU) and adds MBAP for reliable transport.

3. Function Codes in Action

FC15 – Write Multiple Coils

• Request: Start Address + Quantity + Byte Count + Coil Bits (LSB = lowest address).
• Example: Write 10 coils starting at address 19 with pattern 11001101 00000001.
• Response: Echoes start address and quantity written.

FC06 – Write Single Register

• Writes one 16-bit register.
• Meaning (signed/unsigned, scaling) must be agreed upon by both master & slave.

FC04 – Read Input Registers

• Request: Start address + quantity (1–125).
• Response: Byte count + register data (2 bytes each).
• Floats: 1 float = 2 registers (4 bytes). Request 4 registers for 2 floats.

⚠️ Beware of word-order differences (ABCD, BADC, CDAB, DCBA). Always fix one convention.

4. Endianness & Word Order Pitfalls

• MBAP Header: Always big-endian (network order).
• Registers: Often big-endian by spec, but Arduino/W5500 code may use little-endian internally.
• Coil Packing: LSB of each byte = lowest coil address.
• Floats: Must align word order + byte order consistently between master & slave.

5. Arduino UNO + W5500 Implementation

1. Ethernet Init:
   Configure MAC, IP, and open Modbus TCP port 502.

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

2. Receive Loop:

• Read MBAP (7B), then payload according to Length.
• Parse Function Code → handle request.
• Return proper response frame.

3. Error Handling:

• Illegal Function (0x01), Illegal Data Address (0x02), Illegal Data Value (0x03).

4. Performance Notes:

• W5500 has 8 sockets, 32KB total buffer (default 2KB each).
• RX halts if free size < packet size → keep frames small and predictable.

6. C# Master (WinForms) Implementation

1. Option A – Raw Socket:
   Build MBAP + PDU manually, send via TcpClient, parse response.
2. Option B – NModbus4 Library:
   Simplifies frame building, parsing, and exception handling.
3. Tips:

• Always add timeout & retry logic.
• Parse exception frames (FC | 0x80).
• Display coil/register values in real time.

7. Example Address Map

• Coil 0 → Arduino D2
• Coil 1 → Arduino D3
• Coil 2 → Arduino D4
• Coil 3 → Arduino D5
• Coil 4 → Arduino D6
• Coil 5 → Arduino D7

👉 Use:

• FC15 to batch-control LEDs.
• FC06 to write integer values to Arduino variables.
• FC04 to read sensor registers.
• FC04 with 4 registers to read 32-bit float data.

8. Extended Examples (from C# WinForms Integration)

• Write Multiple Coils (FC15): Toggle multiple LEDs at once.
• Write Single Register (FC06): Update Arduino variable remotely.
• Read Input Registers (FC04): Fetch 32-bit float sensor values.

💡 Mapping rule: Arduino GPIO pins ↔ Modbus coil addresses (e.g., D2 → Coil0).
💡 Use bit operations for packing/unpacking coil data.

9. Troubleshooting Checklist

• Wrong endian/word order → garbage float values.
• Length mismatch → master rejects frame.
• Coil packing error → wrong LED toggles.
• W5500 buffer overflow → incomplete frame, fix with chunked reads.

10. Conclusion

By extending Modbus TCP beyond the basics, you can:

• Control multiple coils at once.
• Write signed/unsigned registers.
• Read input registers in bulk.
• Safely exchange 32-bit floating-point data.

With Arduino UNO + W5500 (slave) and C# WinForms (master), this setup becomes a practical solution for:

• Industrial control
• IoT gateways
• Embedded prototyping

✅ SEO Keywords

Arduino Modbus TCP
W5500 Ethernet module
FC15 multiple coil write
FC06 single register write
FC04 read input registers
32-bit float Modbus data
Modbus RTU vs TCP differences