How to interface ESP32 with RS485 (Modbus) sensors with example code

In this tutorial, we will discuss how we can interface ESP32 (as master) to any RS485 (Modbus RTU) based sensor (as slave) using Arduino IDE. We will create an example project by interfacing a RS485 Modbus protocol based energy meter (DDM18SD) using MAX485 (TTL to RS485) module and collect current, voltage, and frequency data from the energy meter.

Table of Contents

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DDM18SD RS485 based Energy meter

The DDM18SD is a type of energy meter, specifically an electronic watt-hour meter used to measure the electric energy consumed by residential or commercial customers. The DDM18SD displays the energy consumption in kilowatt-hours (kWh) and can be read remotely using a handheld device or connected to a data collection system for an automated meter reading. Energy meters like this can play an important role in accurately measuring and billing customers for their electricity consumption.

DDM18SD Energy Meter Features

Display and reading: current, voltage, frequency, power factor, active power, reactive power, etc
Automatically detect the flow direction and command of the load current
One-way measurement of single-phase two-wire active power
RS485 data communication enabled
Voltage Specification: 220V
Frequency: 50Hz
Communication protocol: MODBUS-RTU, DL/T645-1997, DL/T645-2007
Screen Display: Multi-function Cycle Display

DDM18SD Pinout

The pinout of the DDM18SD energy meter can vary depending on the manufacturer and the specific model. However, in general, most energy meters have several terminal pins that connect the meter to the electrical wiring of the building.

In our case, the DDM18SD pinout is given below

MAX485 – TTL to RS485 converter

The MAX485 is a popular RS-485 transceiver integrated circuit (IC) that allows communication between devices using the RS-485 communication protocol. The MAX485 is designed to convert a standard TTL (Transistor-Transistor Logic) voltage level to the differential voltage levels required by the RS-485 standard, and vice versa.

MAX485 Features

Supply Voltage: 3.3 to 5.5 V
Data Rate: Up to 10 Mbps
Driver Output Voltage: ±7V (min)
Receiver Input Voltage: ±200 mV (min)
Driver Output Current: ±120 mA (min)
Driver Short-Circuit Current Limit: ±500 mA
Operating Temperature: -40°C to +85°C

MAX485 Pinout

RS485 serial data communication between esp32 and DDM18SD energy meter (The Project)

In this section of the tutorial, we will first draw the circuit diagram of the rs485 communication between esp32 (as master) and DDM18SD (as a slave sensor). After that, we will code an example project which is to collect the voltage (in Volt), current (in Amps), and frequency (in Hz) data from DDM18SD energy meter vai MAX485 IC to esp32. Then we will show the collected data to Arduino IDE serial monitor

Component List for the Project

Component Name	Quantity	Purchase Link
ESP32 development board	1	Amazon \| AliExpress
MAX485 TTL to RS485 converter module	1	Amazon \| AliExpress
Breadboard	1	Amazon \| AliExpress
Jumper Wire pack	1	Amazon \| AliExpress
RS485 based Energy Meter	1	Amazon \| AliExpress

For troubleshooting, some extremely useful test equipment

Equipment Name	Purchase Link
Best Oscilloscope for Professionals	Amazon
Best Oscilloscope for Beginners	Amazon \| AliExpress
Logic Analyzer	Amazon \| AliExpress
Best Budget Multimeter	Amazon \| AliExpress
Adjustable Bench Power Supply	Amazon \| AliExpress

Affiliate Disclosure: When you click on links to make a purchase, this can result in this website earning a commission.

Circuit Connection between ESP32, MAX485 and DDM18SD

Circuit Diagram

Preparing Arduino IDE

Step 1: Just open your Arduino IDE and go to File > New and open a new file.

Step 2: Then go to Tools > manage Libraries and search for ModbusMaster by Doc walker in the search bar the hit Install.

Modbus Library Functions

The following Modbus functions are available in this library:

Discrete Coils/Flags functions:

0x01 – Read Coils

readCoils(uint16_t u16ReadAddress, uint16_t u16BitQty);

0x02 – Read Discrete Inputs

readDiscreteInputs(uint16_t u16ReadAddress, uint16_t u16BitQty);

0x05 – Write Single Coil

writeSingleCoil(uint16_t u16WriteAddress, uint8_t u8State);

0x0F – Write Multiple Coils

writeMultipleCoils(uint16_t u16WriteAddress, uint16_t u16BitQty);

Registers functions:

0x03 – Read Holding Registers

readHoldingRegisters(uint16_t u16ReadAddress, uint16_t u16ReadQty);

0x04 – Read Input Registers

readInputRegisters(uint16_t u16ReadAddress, uint8_t u16ReadQty);

0x06 – Write Single Register

writeSingleRegister(uint16_t u16WriteAddress, uint16_t u16WriteValue);

0x10 – Write Multiple Registers

writeMultipleRegisters(uint16_t u16WriteAddress, uint16_t u16WriteQty);

0x16 – Mask Write Register

maskWriteRegister(uint16_t u16WriteAddress, uint16_t u16AndMask, uint16_t u16OrMask);

0x17 – Read Write Multiple Registers

readWriteMultipleRegisters(uint16_t u16ReadAddress, uint16_t u16ReadQty, uint16_t u16WriteAddress, uint16_t u16WriteQty);

Please Note: All these functions return 0 on success; exception number on failure. If you want to read more about the library, please visit its GitHub page.

Full Code

.ino file for Arduino IDE

// github link: https://github.com/4-20ma/ModbusMaster
#include <ModbusMaster.h>

/* Modbus stuff */
#define MODBUS_DIR_PIN  4 // connect DR, RE pin of MAX485 to gpio 4
#define MODBUS_RX_PIN 18 // Rx pin  
#define MODBUS_TX_PIN 19 // Tx pin 
#define MODBUS_SERIAL_BAUD 9600 // Baud rate for esp32 and max485 communication

// voltage, current and frequency data register of DDM18SD
uint16_t data_register[3] = {0x0000, 0x0008, 0x002A};

//Initialize the ModbusMaster object as node
ModbusMaster node;

// Pin 4 made high for Modbus transmision mode
void modbusPreTransmission()
{
  delay(500);
  digitalWrite(MODBUS_DIR_PIN, HIGH);
}
// Pin 4 made low for Modbus receive mode
void modbusPostTransmission()
{
  digitalWrite(MODBUS_DIR_PIN, LOW);
  delay(500);
}


void setup()
{
  //  esp serial communication
  Serial.begin(115200);
  pinMode(MODBUS_DIR_PIN, OUTPUT);
  digitalWrite(MODBUS_DIR_PIN, LOW);

  //Serial2.begin(baud-rate, protocol, RX pin, TX pin);.
  Serial2.begin(MODBUS_SERIAL_BAUD, SERIAL_8E1, MODBUS_RX_PIN, MODBUS_TX_PIN);
  Serial2.setTimeout(200);
  //modbus slave ID 14
  node.begin(14, Serial2);

//  callbacks allow us to configure the RS485 transceiver correctly
   node.preTransmission(modbusPreTransmission);
   node.postTransmission(modbusPostTransmission);
  
  }

void loop()
{
    uint8_t result;
    uint16_t data[2];
    int i;
    float reading;
    for(i=0; i<=2; i++){
      //Modbus function 0x04 Read Input Registers according to energy meter datasheet
      result = node.readInputRegisters(data_register[i], 1);
        if (result == node.ku8MBSuccess) {
          Serial.println("Success, Received data: ");
          
          //Retrieve the data from getResponseBuffer(uint8_t u8Index) function.
          //that is return 16-bit data. our energy meter return 32-bit data everytime.
          //that's why, we take the value to a array called data
           data[0]=node.getResponseBuffer(0x00);
           data[1]=node.getResponseBuffer(0x01);
           
           //read voltage
           if(data_register[i] == 0x0000){
            Serial.print("Volatge: ");
            // we just convert the uint16_t type data array to float type using type casting
            reading = *((float *)data);
            Serial.print(reading);
            Serial.println(" Volt");
           }
           //read current
           if(data_register[i] == 0x0008){
            Serial.print("Current: ");
             // we just convert the uint16_t type data array to float type using type casting
            reading = *((float *)data);
            Serial.print(reading);
            Serial.println(" Amps");
           }
           //read Frequency
           if(data_register[i] == 0x002A){
            Serial.print("Frequency: ");
             // we just convert the uint16_t type data array to float type using type casting
            reading = *((float *)data);
            Serial.print(reading);
            Serial.println(" Hz");
           }
        } else {
          Serial.print("Failed, Response Code: ");
          Serial.print(result, HEX);
          Serial.println("");
          delay(5000); 
        }
      
    }

    delay(1000);
  }

Output of the Code

Output of the code in Arduino IDE serial monitor