--- title: "ModbusTCP_RU" url: "https://maker.wiznet.io/lawrence/projects/modbustcp-ru/" markdown_url: "https://maker.wiznet.io/lawrence/projects/modbustcp-ru/md" type: "UCC: User Created Content" author: "Daniil" author_url: "https://github.com/UterGrooll/ModbusTCP_RU" editor: "WIZnet" editor_url: "https://maker.wiznet.io/" original_author: "Daniil" original_url: "https://github.com/UterGrooll/ModbusTCP_RU" published: "2026-03-29" language: "en" tags: ["Modbus", "Arduino", "Arduino Ethernet Shield"] hardware: ["Arduino UNO", "Arduino Ethernet Shield"] likes: 0 views: 534 comments: 0 source: "WIZnet Makers (https://maker.wiznet.io/)" --- # ModbusTCP_RU > ModbusTCP_RU is an Arduino library for building Modbus TCP Master and Slave devices on boards equipped with W5100 or W5500 Ethernet controllers. Original author: Daniil (source: https://github.com/UterGrooll/ModbusTCP_RU) ## Components - **Arduino UNO** x 1 ([docs](https://www.farnell.com/datasheets/1682209.pdf)) - **Arduino Ethernet Shield** x 1 ([docs](https://docs.arduino.cc/retired/shields/arduino-ethernet-shield-without-poe-module)) ## Article ## ModbusTCP_RU: A Lightweight Modbus TCP Library for Arduino and W5500 ### Project Overview `ModbusTCP_RU` is an Arduino library for building **Modbus TCP Master and Slave devices** on boards equipped with **W5100 or W5500 Ethernet controllers**. The project is intended for direct device-to-device communication as well as SCADA integration, and the current public release is **v0.1.3**, documented for **Arduino IDE 2.3.5** and tested with **W5500**. ### Where It Fits This project is useful when a small embedded device needs to expose data or accept control commands over **industrial Ethernet using Modbus TCP**. Typical use cases include relay control nodes, simple sensor gateways, Modbus test benches, and Arduino-based field devices that need to communicate with PLCs, HMIs, or SCADA systems. The included examples cover relay control, HTU21D sensor data mapping, and direct Master-to-Slave communication without SCADA. ### Key Features The library supports both **Slave mode** and **Master mode**, which makes it flexible for small control and monitoring projects. It also supports the most common Modbus function codes: **FC1, FC2, FC3, FC4, FC5, FC6, FC15, and FC16**. Internally, Modbus data is stored in a shared `MbData[]` array, with a default length of **30 16-bit registers**, which also allows bit-level access across the same memory model. ### Technical Highlights The design is intentionally simple. In Slave mode, the library processes requests through `MbsRun()`, while in Master mode it sends requests with `Req()` and handles responses through `MbmRun()`. The current release also improved boundary checks, request/response buffer handling, automatic TCP server startup in Slave mode, and direct Master-to-Slave example support. ### Why Master Support Matters ![](https://maker.wiznet.io/upload/ckeditor5/221215851%5F1774869364%2Epng) (* created by AI) A notable point of `ModbusTCP_RU` is that it supports not only **Slave mode** but also **Master mode**. Along with `MbsRun()` for Slave operation, the library provides `Req()`, `MbmRun()`, and `remSlaveIP` for sending requests to remote Slave devices. The included direct **Master-to-Slave example without SCADA** shows that Arduino + W5500 can act not only as a passive Modbus TCP endpoint, but also as a lightweight polling, control, and test node. This expands the project beyond simple data serving into practical use cases such as sensor polling, relay control, and small device-to-device automation. - **Direct device-to-device communication** Master mode allows Arduino + W5500 to communicate directly with remote Slave devices, not just act as a PC-connected endpoint for SCADA or HMI. - **Sensor polling node** The Arduino can periodically read holding registers from remote Slave sensors and collect data such as temperature, humidity, or power values. - **Lightweight controller** It can read data from one Slave and send write commands to another, making simple automation and relay control possible. - **Test and validation node** Arduino + W5500 can also work as a small Modbus TCP test master, which is useful for checking register maps and communication flow without a full PLC or SCADA setup. ### Why W5500 Matters W5500 is a strong match for this project because it provides a **hardwired TCP/IP stack**, **embedded 10/100 Ethernet MAC/PHY**, **8 independent sockets**, and **32 KB of internal Tx/Rx buffer memory**, all connected to the MCU through SPI. That makes wired Ethernet implementation much easier on Arduino-class hardware. In the context of `ModbusTCP_RU`, W5500 helps turn a small MCU board into a practical **wired Modbus TCP node** with less software overhead than a fully software-driven Ethernet stack. #### QnA ##### 1. What is ModbusTCP_RU? ModbusTCP_RU is a lightweight Arduino library for implementing Modbus TCP communication over Ethernet. It supports both Master and Slave modes and is designed for boards using W5100 or W5500 Ethernet controllers. ##### 2. What does ModbusTCP_RU do? ModbusTCP_RU allows Arduino-based devices to communicate over Modbus TCP, either as a Slave that responds to requests or as a Master that actively polls and controls remote Modbus TCP devices. ##### 3. Which Ethernet hardware is supported by ModbusTCP_RU? ModbusTCP_RU is built for Ethernet-capable Arduino systems using W5100 or W5500. The project documentation specifically mentions testing with W5500. ##### 4. Does ModbusTCP_RU support both Master and Slave modes? Yes. One of the key features of ModbusTCP_RU is support for both Modbus TCP Master and Modbus TCP Slave operation in the same library. ##### 5. Why is Master support important in ModbusTCP_RU? Master support allows Arduino + W5500 to do more than just respond to SCADA or PLC requests. It can actively read sensor values, control remote Slave devices, and work as a small polling or automation node in a Modbus TCP network. --- Source: https://maker.wiznet.io/lawrence/projects/modbustcp-ru/