ESP32 with MODBUS RTU RS485 Protocol Using Arduino IDE | microdigisoft.com (2024)

This tutorial, we will learn how to handle Modbus RTU RS485 Protocol with ESP32 board using Arduino IDE. In this user guide, we will explore some basic concept of Modbus protocol, Interfacing of RS485 Module with ESP32 board, MAX485 TTL To RS485 Module and finally will prepare simple code to read the data over ESP32 serial port using SoftwareSerial Library.

Let us start with some basics understanding of Modbus Protocol:

What is Modbus Protocol?

Modbus is a serial communication protocol developed by Modicon in 1979. In simple terms, it is a method used for transmitting information over serial lines between electronic devices. Modbus is an open protocol, meaning that it’s free for manufacturers to communicate between electronics devices. Modbus is used in monitoring; to communicate between intelligent devices, sensors or instruments and to monitor field devices using computer and HMI.

It has become a standard communications protocol in industry, and is now the most commonly available means of connecting industrial electronic devices. It is used widely by many manufacturers throughout many industries.Modbus is typically used to transmit signals from instrumentation and control devices back to a main controller or data gathering system, for example a system that measures temperature and humidity and communicates the results to a computer. Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. Versions of the Modbus protocol exist for serial lines (Modbus RTU and Modbus ASCII) and for Ethernet (Modbus TCP).

HOW DOES IT WORK?

Modbus is transmitted over serial lines between devices. The simplest setup would be a single serial cable connecting the serial ports on two devices, a Master and a Slave.

In a standard Modbus network, there is one Master and up to 247 Slaves, each with a unique Slave Address from 1 to 247. The Master can also write information to the Slaves. The official Modbus specification can be found athttp://www.modbus.org/. There are basically three types or variations of the Modbus protocol:

• Modbus ASCII
• Modbus RTU
• Modbus TCP and IP

Modbus ASCII:

Modbus was originally developed using ASCII character using messages.

Modbus RTU:

Modbus-RTU (Remote Terminal Unit) means that the Modbus protocol is used on top of a serial line with an RS-232, RS-485 or similar physical interface. It is point to point communication which means it has only one master and slave. Modbus RTU is using binary coding and CRC error checking for transmission implementation. The two modes are incompatible when they do not use the same communication protocol for example if we are using binary or ascii at the transmission side than at the receiving end we will also use the same protocols, so a device is configured for ASCII mode cannot communicate with one using RTU. Modbus RTU devices typically use one of three electrical interfaces which are:

• RS232
• RS485
• RS422

RS232:

RS232 was first introduced in 1962. This is the most common type of serial interface, it was the standard communication before the PS2 and USB became popular in the computer industry. RS232 is simple point to point arrangement which will send one bit at a time therefore it is known as serial communication. If we need to connect only one device to another and the distance between the two devices is less than 50 feet or 15 meters, then RS232 will do the job.

RS422:

RS422 is an improved version of RS232 which used twisted pair cables to reduce the noise and interference; it uses signalling balancing to transmit data. The signal becomes balanced it uses a voltage difference between the two lines as an indication of the signal value, with this method the data is able to transmit for a longer distance with faster data rates.

With RS422 the data can transmit up to 10 Mbps at 50 feet or 100Kbps or 4000 feet. RS422 is capable of multiple drop capability which it limits up to 10 slaves in the data line.

RS485:

RS485 is used to send data and receive serial data. If more than two devices connected serially then we can communicate devices with one another. To connect more than two devices on the same line and have a distance greater than 50 feet than we will use RS485 or RS422. RS485 can travel signal up to 1 KM. For master communicating with multiple slave devices RS485 is by far the most popular method. RS485 was developed by as a replacement or upgrade of RS432 as well as RS422.

After above discussion about basics of Modbus RTU protocol let us discuss about the RS485 Module which we need interface with ESP32 board to receive data serially over serial port.

About MAX485 TTL To RS485 Module

The MAX485 TTL To RS485 Module is a level shifter module which maintain full compliance to RS485 and RS422 standards when operating from a 3.3V supply. Competing parts sometimes drive a reduced V_ODwhen powered by 3.3V.

TheMAX485 TTL To RS485 Module converts TTL signal to RS485 forlong range, high data rate error prone differential communication.Digital communications networks implementing the EIA-485 standard can be used effectively over long distances and in electrically noisy environments.

Multiple receivers may be connected to such a network in a linear, multi-dropconfiguration. These characteristics make such networks useful in industrial environments and similar applications.

This converter lets you send and receive data using theRS485network from your Arduino / microcontroller. Similar to Serial TTL andRS232interfaces, the RS485 lets you transfer data between microcontrollers and devices but with extra features! RS485 is an industry standard protocol for data transfer and provides numerous advantages.

It let’s you transfer data between up to 32 devices, through the same data line over a cable length of up to 1.2km (4000ft) with a maximum data rate of 10Mbit/s. This converter is designed for office and industrial applications (non-isolated) and provides superior characteristics/features normally found only on more expensive units.

MAX485 TTL To RS485 Schematic

Features :

1. On-board MAX485 chip, A low power consumption for the RS-485 communication, slew-rate limited transceiver.
2. A low power consumption for the RS-485 communication
3. Onboard then 5.08 (mm) pitch 2P terminal to facilitate RS-485 communication wiring
4. Slew-rate limited transceiver
5. All pins of the chip have been lead to can be controlled through the microcontroller
6. Convenient RS-485 communication wiring
7. Multiple Units can be connected to the same RS-485 bus wiring
8. All Chip pins are brought out for proper controls

Concept Of Project

In this project , we will read the RS485 serial data using SoftwareSerial port using RS485 TTL module.

Components Required:

• ESP32 Development Board
• MAX485 TTL To RS485
• PLC or HMI supporting RS485 protocol
• Jumper Wires
• Breadboard
• Cable (USB 2.0)

Pinout and Schematic

Please refer below pinout for ESP32 with 38 Pin to build the schematic for this project. Refer our previous tutorial forInstalling the ESP32 Board in Arduino IDE?. Below schematic is for how to connect the MAX485 TTL To RS485 Module with ESP32 board. The connection for MAX485 TTL To RS485 with ESP32 should be as below:

• MAX485 TTL To RS485 >> VCC >> +5V of ESP32
• MAX485 TTL To RS485 >> GND >> GND of ESP32
• MAX485 TTL To RS485 >> RO >> GPIO26 of ESP32 (SoftwareSerial RX)
• MAX485 TTL To RS485 >> DI >> GPIO27 of ESP32 (SoftwareSerial TX)
• MAX485 TTL To RS485 >> RE >> GPIO32 of ESP32
• MAX485 TTL To RS485 >> DE >> GPIO33 of ESP32

ESP32 Programming using Arduino IDE:

Copy the below code to Arduino IDE, Then upload the following example code to your ESP32 board using theArduino IDE.

#include <SoftwareSerial.h>// RS485 setup with ESp32#define RE 32 // Connect RE terminal with 32 of ESP#define DE 33 // Connect DE terminal with 33 of ESP const byte ModReadBuffer[] = {0x01, 0x03, 0x00, 0x1e, 0x00, 0x01, 0xe4, 0x0c};byte BufferValue[8];SoftwareSerial mod(26, 27); // RX=26 , TX =27void setup() { Serial.begin(115200); mod.begin(9600);// modbus configuration pinMode(RE, OUTPUT); pinMode(DE, OUTPUT);}void loop() { byte val1; while (mod.write(ModReadBuffer, sizeof(ModReadBuffer)) == 8) { val1 = ModbusData(); delay(5000); }}byte ModbusData(){ byte i; digitalWrite(DE,HIGH); digitalWrite(RE,HIGH); delay(10); if(mod.write(ModReadBuffer,sizeof(ModReadBuffer))==8){ digitalWrite(DE,LOW); digitalWrite(RE,LOW); for(i=0;i<4;i++){ //Serial.print(mod.read(),HEX); BufferValue[i] = mod.read(); } if(BufferValue[1]!=255 && BufferValue[2]!=255 && BufferValue[2]!=255){ Serial.println("Serial Data received On:"); Serial.print("Modbus Buffer[1]="); Serial.println(BufferValue[1],DEC); Serial.print("Modbus Buffer[2]="); Serial.println(BufferValue[2],DEC); Serial.print("Modbus Buffer[3]="); Serial.println(BufferValue[3],DEC); Serial.println("");} // } } return BufferValue[8];}

How the Code Works?

Importing Libraries

In this example, we are going to use the ESPSoftwareSerial Library to read the Modbus RTU RS485 Data on serial port. To read this data we need to include SoftwaresSerial library at the start of code.

 #include <SoftwareSerial.h> 

Next, we need declare a variable for interfacing MAX485 TTL To RS4485 module RE and DE terminal with ESP32 board.

RE is the ‘Receiver Enable’ pin and must be pulled low whenever you want to be able to receive data. DE is the ‘Driver Enable’ pin and must be pulled high while you’re transmitting data. You’ll notice that the DE and RE pins have opposite polarity. DE is active-high and RE is active-low. So you can tie them together and control them from one pin if you want to – high means you want to transmit (DE active, RE inactive), and low means you want to receive (RE active, DE inactive).

// RS485 setup with ESp32
#define RE 32 // Connect RE terminal with 32 of ESP
#define DE 33 // Connect DE terminal with 33 of ESP

Now, let us create a buffer to to store the data, when Modbus is active and stated sending a data will be stored in BufferValue[8].

const byte ModReadBuffer[] = {0x01, 0x03, 0x00, 0x1e, 0x00, 0x01, 0xe4, 0x0c};byte BufferValue[8];

Below is the SoftwareSerial Port declaration at ESP32 GPIO26 and GPIO27 to read the data from MAX485 TTL To RS4485 module .

SoftwareSerial mod(26, 27); // RX=26 , TX =27

In the setup() function, First we will initialize the ESP32 Serial monitor at Baud rate 115200. After initializing ESP32 serial, we need start SoftwareSerial port at Baud rate of 9600.

Note: The baud rate of Softwareserial port should be same as Transmitting device like PLC or HMI etc.

In the next line we need define direction of RE and DE terminal which we have to connect with ESP32 board at GPIO32 and GPIO33.

void setup() { Serial.begin(115200); mod.begin(9600);// modbus configuration pinMode(RE, OUTPUT); pinMode(DE, OUTPUT);}

In the Infinite loop() function, we need to call SoftwareSerial to write the data from PLC device which will buffered in the ModReadBuffer variable.

void loop() { byte val1; while (mod.write(ModReadBuffer, sizeof(ModReadBuffer)) == 8) { val1 = ModbusData(); delay(5000); }}

Below is ModbusData read function to read the data from Modbus. Here, when buffer is loaded with 8 byte of data, we have make make logic level High to low at the RE and DE terminal to get receipt of data to BufferValue variable to make data readable and can be printed over Serial Monitor of the ESP32 board. This serial data can be printed in Hex or Decimal format with Serial,Print(); with given augments.

byte ModbusData(){ byte i; digitalWrite(DE,HIGH); digitalWrite(RE,HIGH); delay(10); if(mod.write(ModReadBuffer,sizeof(ModReadBuffer))==8){ digitalWrite(DE,LOW); digitalWrite(RE,LOW); for(i=0;i<4;i++){ //Serial.print(mod.read(),HEX); BufferValue[i] = mod.read(); } if(BufferValue[1]!=255 && BufferValue[2]!=255 && BufferValue[2]!=255){ Serial.println("Serial Data received On:"); Serial.print("Modbus Buffer[1]="); Serial.println(BufferValue[1],DEC); Serial.print("Modbus Buffer[2]="); Serial.println(BufferValue[2],DEC); Serial.print("Modbus Buffer[3]="); Serial.println(BufferValue[3],DEC); Serial.println("");} // } } return BufferValue[8];}

Result With Video:

After uploading the Arduino Sketch to ESP32 Board, open the Serial Monitor Window and Monitor Serial data received from Modbus device.

Conclusion:

In this user guide for ESP32 board with Arduino IDE, we have demonstrated how we can read the data from Modbus RTU device on ESP32 using SoftwareSerial libarary. Hope this tutorial is useful to understand the Basic concept of using Modbus RTU protocol with ESP32 board. Any doubts please share your feedback in the comment box.. Thanks!..