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In this tutorial, we will explain how to communicate between two Arduino boards with the help of the RS485 serial data communication protocol. Arduino doesn’t have any peripherals for RS485 communication. We will use an RS485-based sensor module called MAX485 for communication between two Arduino boards. We will follow the RS485 master-slave communication method. For demonstration purposes, we will create a project that will ON or OFF a LED connected to a Slave Arduino from Master Arduino by sending some commands through RS485 Module. You may also like reading: How to interface Modbus RTU (RS485) sensor with Arduino How to interface STM32 with RS485 (Modbus) sensors Introduction to RS232 Serial Communication Protocol How to interface Arduino with RS232 communication protocol How to interface esp32 with rs485 (Modbus) sensors with example code Multiple ESP32 Communication via RS485 What is RS485? There are different types of physical media and they are: RS232: serial communication protocol RS485: serial communication protocol RS422: serial communication protocol Ethernet: LAN (Local Area Network) Technology RS485 is an asynchronous half-duplex serial communication protocol that communicates over devices by using the master-salve method. It can connect a maximum of 32 devices on the same line. Figure: RS485 serial communication protocol Most of the industry uses RS485 protocol rather than RS232 because of its longer distances (up to 1200 meters), higher data transfer rate (Up to 30Mbps), and allow for multiple devices connection on a single network by using only two wires. MAX485 TTL to RS485 Module MAX485 is a low-power transceiver for TTL to RS485 communication. This module needs 5V to operate and uses a 5V logic level so that it can be easily interfaced with microcontrollers like Arduino. Key Features of MAX485: It can cover longer distances of up to 1200 meters Supports a higher data transfer rate of 10Mbit/s MAX485 can connect a maximum of 32 devices A low power consumption Slew-rate limited transceiver Onboard 5.08mm pitch 2P terminal for RS-485 communication wiring Figure: MAX485 - TTL to RS485 MAX485 Module Pinout Figure: MAX485 pin-out RS485 communication between two Arduino boards by using MAX485 module (The project) In this part of the article, We will explain how the RS485 communication works by doing of a simple project. We will use two Arduino Uno boards for this project. One is used as a master and another is as a slave. We will send a command “1” or “0” from the master to the slave. Slave will receive and check the value. If the value is “1” then the slave turns on a LED and prints “LED ON” on the serial monitor of Arduino IDE or if the value is “0” the slave turns off the led and prints “LED OFF” in the serial monitor of Arduino IDE. We use a 5V Li-ion battery to power the master Arduino Board and for power the slave Arduino we use a computer 5V USB power supply. Component List To follow the project you will need several components and they are: Component NameQuantityPurchase LinkArduino development board2Amazon | AliExpressPush button1Amazon | AliExpressMAX485 TTL to RS485 converter module2Amazon | AliExpressBreadboard1Amazon | AliExpressJumper Wire pack1Amazon | AliExpressLED1Amazon | AliExpress100 ohm Resistor1Amazon | AliExpress For troubleshooting, some extremely useful test equipment Equipment NamePurchase LinkBest Oscilloscope for ProfessionalsAmazon | AliExpressBest Oscilloscope for BeginnersAmazon | AliExpressLogic AnalyzerAmazon | AliExpressBest Budget MultimeterAmazon | AliExpress Affiliate Disclosure: When you click on links to make a purchase, this can result in this website earning a commission. Affiliate programs and affiliations include, but are not limited to Amazon.com Figure: Arduino RS485 real circuit connection Circuit Schematics In this article section, We will show you how to connect MAX485 module to Arduino Uno with the help of the connection diagram and pin description table. Figure: Arduino RS485 circuit diagram Connection description between Arduino and MAX485 ( master) Figure: Arduino RS485 master connection pin-out We also connect a push button to pin 2 of Arduino. Connection description between Arduino and MAX485 ( slave) Figure: Arduino RS485 slave connection pin-out We also connect a red led to pin 8 of Arduino with a 100 Ohm resistor in series. Arduino and RS485 communication Code We have written two Arduino sketch. One is for the master and another is for the slave. The Master will send a command and the slave will react according to the master's command. Arduino Code for Master Open your Arduini IDE and go to File > new and create a new sketch. Copy the code below and paste it to the Arduino sketch and save it. In this sketch, we will send a command “0” or “1” to the slave Arduino Board. We connect Arduino pin 2 to a push button. We configure an internal pull-up (normally high) for the push button. .ino file for Arduino IDE // Initialize the button state to 0 bool button_state=0; void setup() { // put your setup code here, to run once: // arduino default baud rate for serial communication Serial.begin(9600); // DE,RE Controling pin of RS-485 pinMode(7, HIGH); // pin 2 set as input with PULL-UP for button pinMode(2, INPUT_PULLUP); // DE,RE = HIGH, RS485 Transmit Enabled digitalWrite(7,HIGH); } void loop() { // put your main code here, to run repeatedly: // check if button is prasssed or not. //If digitalRead() function return 0 than the button is pressed- //because we use input pull-up for pin 2 if(digitalRead(2) == 0){ // wait 300ms to prevent debouncing delay(300); // evry time when button pressed button state will reverse it value from 0 to 1 or 1 to 0 button_state = !button_state; // print the value in serial monitor Serial.print(button_state); } } Arduino Code for Slave Again, open your Arduini IDE and go to File > new and create a new sketch. Copy the code below and paste it to the Arduino sketch and save it. In this sketch, the slave Arduino receives the command from the master through the MAx485 module by using the RS485 protocol. We connect Arduino pin 8 to a LED. If the slave receives command “1” then LED will Turn ON and print “LED ON” to the Arduino IDE serial monitor or, if slave receives command “0” then LED will turn OFF and print “LED OFF” to the serial monitor. .ino file for Arduino IDE // Initialize the LED status bool led_status; void setup() { // arduino default baud rate for serial communication Serial.begin(9600); //DE,RE Controling pin of RS-485 pinMode(7, OUTPUT); //pin 8 set as output for LED pinMode(8, OUTPUT); //DE,RE = 0, RS485 Receive Enabled digitalWrite(7,LOW); } void loop() { // wait untill all the bytes receive from data buffer while(Serial.available() > 0){ // take the integer value from buffer (0 or 1) led_status = Serial.parseInt(); // delay for 10 milliseconds delay(10); // check if command is 1 if(led_status == 1){ // LED pin high means LED ON digitalWrite(8,HIGH); Serial.println("LED ON"); } // check if command "0" if(led_status == 0){ // LED pin low means LED OFF digitalWrite(8,LOW); Serial.println("LED OFF"); } } } Conclusion RS485 serial data communication protocol is widely used in industries. In this tutorial, I expelled the basics of RS485 protocol and also interface it with Arduino through a TTL to RS485 module. I will write more articles about this topic soon. Video https://www.youtube.com/watch?v=3tUQhvR_K1c