Read a flow sensor with modbus RTU

[noir235]

Hello togehter,

First of all, I'm a newbie in programming ESP32 with Arduino IDE. My target is to read the air flow from the a sensor with the modbus protocol.

I found out the following details for Modbus-configuration:

Transmission speed: 9600 BAUD Start bit: 1 Data bits: 8 Stop bit: 2 parity: no adress: 247 (no LED reaction on sensor fo communcation, I used 0) Function: 03 (read holding register) register: 0x0000 - 0x0001 value-format: float32

I use this code with Arduino IDE on M5Stack-Core with PLC-module:

include

uint32_t res = 0; ModbusRTU mb;

void setup() { Serial.begin(115200); Serial2.begin(9600, SERIAL_8N2, 16, 17); mb.begin(&Serial2); mb.master(); }

void loop() { mb.readHreg(0, 0, (uint16_t*)&res, 2, nullptr); mb.task(); delay(100); res = (res>>16) | (res<<16);
Serial.println(res); }

Can you help me with that issue?

[emelianov]

Hello,

I suggest to write something like below code. From one hand it allows to see request result (timeout or any kind of modbus error - see Modbus.h)

#include <ModbusRTU.h>
uint32_t res = 0;
bool flag = false;
ModbusRTU mb;

bool cb(Modbus::ResultCode event, uint16_t transactionId, void* data) {
  Serial.printf("Result code: %02X\n", event);
  flag = true;
  return true;
}

void setup() {
  Serial.begin(115200);
  Serial2.begin(9600, SERIAL_8N2, 16, 17);
  mb.begin(&Serial2);
  mb.master();
}

void loop() {
  mb.readHreg(0, 0, (uint16_t*)&res, 2, cb);
  while (!flag) {
    delay(100);
    mb.task();
  }
  res = (res>>16) | (res<<16);
  Serial.println(res);
  delay(10000);
}

From other hand mb.task() in your code called to early to get any responce from slave. You may try at least

mb.readHreg(0, 0, (uint16_t*)&res, 2, nullptr);
delay(100);
mb.task();