Tag Archives: SKU101990881

Arduino RS485 Temperature, Dewpoint & Humidity Sensor

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As part of this series of samples comparing Arduino to nanoFramework to .NET IoT Device “Proof of Concept (PoC) applications, and a couple of posts use a SenseCAP Temperature dewpoint, and humidity Sensor SKU101990882.

I cut up a spare Industrial IP68 Modbus RS485 1-to-4 Splitter/Hub to connect the sensor to the breakout board as I find this much easier than soldering connectors

This sensor has an operating voltage of 3.6-30V/DC so it can be powered by the 5V output of a RS485 Breakout Board for Seeed Studio XIAO (SKU 113991354). The red wire is for powering the breakout and device with a 12V power supply so was tied back so it didn’t touch any of the other electronics.

HardwareSerial RS485Serial(1);
ModbusMaster node;

// -----------------------------
// RS485 Pin Assignments (Corrected)
// -----------------------------
const int RS485_RX = 6;  // UART1 RX
const int RS485_TX = 5;  // UART1 TX
const int RS485_EN = D2;

// Sensor/Modbus parameters (from datasheet)
#define MODBUS_SLAVE_ID 0x2A
#define REG_TEMPERATURE 0x0000
#define REG_HUMIDITY 0x0001
#define REG_DEWPOINT 0x0002

// Forward declarations for ModbusMaster callbacks
void preTransmission();
void postTransmission();

void setup() {
  Serial.begin(9600);
  delay(5000);

  Serial.println("ModbusMaster: Seeed SKU101990882 starting");

  // Wait for the hardware serial to be ready
  while (!Serial)
    ;
  Serial.println("Serial done");

  pinMode(RS485_EN, OUTPUT);
  digitalWrite(RS485_EN, LOW);  // Start in RX mode

  // Datasheet: 9600 baud, 8N1
  RS485Serial.begin(9600, SERIAL_8N1, RS485_RX, RS485_TX);
  while (!RS485Serial)
    ;
  Serial.println("RS485 done");

  // Tie ModbusMaster to the UART we just configured
  node.begin(MODBUS_SLAVE_ID, RS485Serial);

  // Register callbacks for half-duplex direction control
  node.preTransmission(preTransmission);
  node.postTransmission(postTransmission);
}
...
void loop() {
  float temperature;
  uint16_t humidity;
  uint16_t dewPoint;

  uint8_t result = node.readInputRegisters(0x0000, 3);

  if (result == node.ku8MBSuccess) {
    // --- Read Temperature ---
    uint16_t rawTemperature = node.getResponseBuffer(REG_TEMPERATURE);
    temperature = (int16_t)rawTemperature / 100.0;

    // --- Read Humidity ---
    humidity = node.getResponseBuffer(REG_HUMIDITY);
    humidity = humidity / 100;

    // --- Read DewPoint ---
    dewPoint = node.getResponseBuffer(REG_DEWPOINT);
    dewPoint = dewPoint / 100;

    Serial.printf("Temperature: %.1f°C Humidity: %u%%RH Dewpoint: %u°C\n", temperature, humidity, dewPoint);
  } else {
    Serial.printf("Modbus error: %d\n", result);
  }

  delay(60000);
}

The Arduino ModbusMaster based application worked first time I forgot to scale the dewpoint.

I have order an Industrial Soil Moisture & Temperature Sensor MODBUS-RS485 sensor from Mouser which will be my next project.