ESP32 Flowmeter - RS485 Modbus

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As described in the first article of this series, my first goal was to read out a TUF-2000M Ultrasonic Flow Meter via an ESP32. For this purpose I found an example for an ESP8266 on the internet: Reading a TUF-2000M Ultrasonic Flow Meter with an Arduino or ESP8266 and I described the setup of the TUF-2000M in the following article: Field Report: Ultrasonic Flow Meter TUF-2000M. To be able to read the TUF-2000M via RS485, it must first be connected to the ESP32 via the RS485 converter:

Connect hardware

RS485 and ESP32

RS485 to TTL 5V converter with MAX13487 chip for Raspberry Pi Arduino and other MCU:

The supplied cable can be connected to the ESP32 as follows:

Connect TUF-2000M

After we have everything connected, we can take care of the software:


In preparation, the sketch requires the packages in Arduion: ModbusMaster and EspSoftwareSerial:

Modbus Master

Software Serial

Error 226

At the first attempt I used the two PINs RX and TX for the communication to the RS485 board out of ignorance, which ended in an error 226. The display mirrored the output of the serial monitor to the TUF-2000M: Menu 49, see: Serial Port Traffic


After connecting the RS485 board to PINS 16 and 17 of the ESP32, I was able to read out the flowmeter:

RX_PIN 16 

WLAN, MQTT and Flow

Combined with ESP32 WiFi and ESP32 MQTT - Send Data, I was able to write the current value to the MQTT broker and thus into HomeAssistant with the following sketch:

#include <WiFi.h>
#include <PubSubClient.h>
#include <SoftwareSerial.h>
#include <ModbusMaster.h>

#define RX_PIN 17 // connect to converter's RX wire
#define TX_PIN 16 // connect to converter's TX wire
SoftwareSerial swSerial(RX_PIN, TX_PIN);
ModbusMaster sensor;

const char* ssid = "home";
const char* password = "???";
const char* mqttServer = "";
const int mqttPort = 1883;
const char* mqttUser = "mqtt";
const char* mqttPassword = "???";

WiFiClient espClient;
PubSubClient client(espClient);

void setup() {
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    Serial.println("Connecting to WiFi..");
  Serial.println("Connected to the WiFi network");
  sensor.begin(MODBUS_DEVICE_ID, swSerial);
  client.setServer(mqttServer, mqttPort);
  while (!client.connected()) {
    Serial.println("Connecting to MQTT...");    
    if (client.connect("ESP32Client", mqttUser, mqttPassword )) {    
    } else {    
      Serial.print("failed with state ");

void loop() {
  if (!client.connected()) {
    if (!client.connected()) {

void readFlow() {
  uint8_t result;
  uint16_t buf[2];
  float flow;
  result = sensor.readHoldingRegisters(1, 2);
  if (result == sensor.ku8MBSuccess)
    buf[1] = sensor.getResponseBuffer(0);
    buf[0] = sensor.getResponseBuffer(1);
    memcpy(&flow, &buf, sizeof(float));
    client.publish("flowmeter/flow",  String(flow).c_str());

For integration into Home-Assistant, see: Home Assistant MQTT

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