Arduino Nano MAX485: Build a Modbus RTU Slave | ShillehTek

July 29, 2026 2 views

Project

Build an Arduino Nano Modbus RTU slave using a MAX485 RS485 module, then read and write holding registers from a PC master for industrial-style control with ShillehTek.

1 hr Intermediate6 parts

Project Overview

Arduino Nano + MAX485 RS485 module Modbus RTU slave: In this guide, you build a Modbus RTU slave device on an Arduino Nano using a MAX485 TTL to RS485 converter module, then read and write its registers from a PC acting as the Modbus master, including toggling an LED by writing register 400007.

Modbus is a common protocol in industrial equipment like PLCs, meters, VFDs, and sensors, often used as Modbus RTU over an RS485 twisted pair. An Arduino can act as a Modbus slave once it has an RS485 interface.

Time: 1-2 hours
Skill level: Intermediate
What you will build: A Modbus RTU slave device you can poll and control from a Modbus master.

Arduino Nano acting as a Modbus RTU slave on an RS485 bus — Arduino as a Modbus slave on an RS485 bus.

Parts List

From ShillehTek

MAX485 TTL to RS485 Converter Module - adds an RS485 interface (A/B bus) to the Arduino UART
Arduino Nano V3 - UNO-compatible Arduino for the Modbus slave
Resistor Kit - 220 ohm for the LED, 10K for the button pull-down
Tactile Button Kit, breadboard, and jumper wires - for the LED/button demo I/O wiring

External

USB-RS485 adapter for the PC master side
Red LED
Twisted-pair wire for the A/B RS485 bus
Modbustester (free Windows utility)
SimpleModbus library: github.com/angeloc/simplemodbusng

Note: RS485 is a differential bus with two shared data lines (A and B). Use twisted pair for long runs and terminate only at the two physical ends of the bus.

Step-by-Step Guide

Step 1 - Gather the hardware and software

Goal: Get both the Modbus slave (Arduino) and the Modbus master (PC) ready.

What to do: Use an Arduino plus an RS485 interface as the slave. Use a PC with a USB-RS485 adapter as the master. Install the Arduino IDE, the SimpleModbus library, and Modbustester on the PC.

Arduino Modbus RTU project tools including Arduino board, MAX485-style RS485 interface, and cabling — Everything needed for both ends of the bus.

Expected result: You have a clear master device (PC) and slave device (Arduino) and the required software tools.

Step 2 - Wire the RS485 bus

Goal: Connect the slave to the RS485 A/B pair correctly.

What to do: Connect the interface A terminal to the bus A wire and B to B. On a MAX485 module, wire the TTL side as follows: DI to Arduino TX, RO to Arduino RX, DE and RE tied together to a control pin (or to auto-direction circuitry), VCC to 5V, and GND to GND.

RS485 A and B twisted-pair wiring diagram connecting PC adapter and Arduino RS485 interface — A to A, B to B. Use twisted pair for distance.

Expected result: The Arduino RS485 interface is physically connected to the same A/B bus as the PC USB-RS485 adapter.

Step 3 - Add the demo LED and button (optional)

Goal: Add a visible output and a readable input to test Modbus reads and writes.

What to do: Wire a red LED through a 220 ohm resistor to an Arduino output pin. Wire a push button with a 10K pull-down resistor to an Arduino input pin. These I/O points will be exposed through Modbus registers.

Arduino wired on a breadboard to a red LED with resistor and a push button for Modbus RTU demo — The demo I/O on a breadboard.

Schematic showing Arduino pin connected to LED with 220 ohm resistor and push button with 10K pull-down — LED and button schematic.

Expected result: You have one output (LED) and one input (button) to verify control and monitoring over RS485.

Step 4 - Configure the interface (shield users)

Goal: Set UART routing and termination correctly for reliable RS485 communication.

What to do: If you are using a full RS485 shield: set its DIP banks per the photos (bank 1: OFF-ON-ON-OFF; bank 2: OFF-OFF-ON-ON; bank 3: all OFF except switch 1, which is ON only if the shield is at a physical end of the bus, enabling the 120 ohm terminating resistor). Set the supply jumper to 5V for an UNO-class board, route UART pins 0/1 to the RS485 side, and leave RX/TX direction control on automatic. If you are using a MAX485 module instead of a shield, the same concepts apply: 5V supply, correct UART wiring, and termination only at the ends of the bus.

RS485 shield DIP switch settings for Modbus RTU mode on an Arduino — DIP settings for Modbus RTU.

RS485 shield power jumper set to 5V for an Arduino UNO or Nano — Supply jumper: 5V for UNO-class boards.

RS485 shield jumpers routing Arduino UART pins 0 and 1 to the RS485 transceiver — UART 0/1 routed to the RS485 transceiver.

Expected result: The interface is configured properly, with termination enabled only when the device is at a bus end.

Step 5 - Set up the PC master

Goal: Configure a Modbus master to communicate with the Arduino slave.

What to do: Plug in the USB-RS485 adapter. Unzip and open Modbustester, select the adapter COM port, and match the serial settings shown in the screenshot.

Modbustester configured with COM port and serial settings for Modbus RTU over USB-RS485 — Modbustester pointed at the USB-RS485 adapter.

Expected result: Your PC is ready to send Modbus RTU read and write requests over RS485.

Step 6 - Flash the slave sketch

Goal: Program the Arduino with a Modbus RTU slave register map.

What to do: Install the SimpleModbus library, then load the SimpleModbusSlave example for your board (UNO version for UNO/Nano) from the original project downloads and upload it. The sketch maps holding registers as follows: the six ADC ports are in registers 400001 to 400006, the LED is at 400007, and the button state is at 400008.

Expected result: The Arduino is serving Modbus registers on the RS485 bus.

Step 7 - Read and write registers

Goal: Verify end-to-end Modbus communication between the PC master and Arduino slave.

What to do: In Modbustester, click Read to fetch eight registers. You should see analog values in 400001 to 400006 and the button state in 400008 (press the button and read again). Then write a 1 to address 400007 to turn the LED on, and write 0 to turn it off.

Modbustester reading and writing Modbus holding registers to control an Arduino LED over RS485 — Register reads and writes flowing over the RS485 pair.

Expected result: The PC can read analog and button registers and can control the Arduino LED by writing register 400007.

Conclusion

You built a Modbus RTU slave using an Arduino with an RS485 interface (MAX485), exposing ADC inputs and a button as readable registers and controlling an LED by writing holding register 400007 from a PC Modbus master.

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