How to Use SHT20 MODBUS: Examples, Pinouts, and Specs

The SHT20 MODBUS is a digital temperature and humidity sensor designed for high-accuracy environmental measurements. It communicates using the MODBUS protocol, making it suitable for industrial and commercial applications requiring robust and reliable data transmission. The sensor integrates a capacitive humidity sensor and a band-gap temperature sensor, ensuring precise and stable readings over time.

• HVAC systems for monitoring and controlling environmental conditions
• Industrial automation and process control
• Weather stations and environmental monitoring
• Smart agriculture and greenhouse management
• Data logging and IoT applications

The SHT20 MODBUS sensor is built to deliver reliable performance under a wide range of conditions. Below are its key technical details:

The SHT20 MODBUS sensor typically comes with a 4-pin interface for easy integration into systems. Below is the pinout:

1. Power Supply: Connect the VCC pin to a 3.3V or 5V power source and the GND pin to the ground of your circuit.
2. RS485 Communication: Connect the A (D+) and B (D-) pins to the corresponding RS485 interface of your microcontroller or RS485-to-USB converter.
3. Termination Resistor: If the sensor is at the end of an RS485 bus, add a 120-ohm termination resistor between A and B to prevent signal reflections.
4. MODBUS Configuration: Configure your microcontroller or software to communicate using the MODBUS RTU protocol. The default baud rate is typically 9600 bps, with 8 data bits, no parity, and 1 stop bit (8N1).

• Wiring: Use twisted-pair cables for the RS485 lines to minimize noise and signal degradation.
• Addressing: Each SHT20 MODBUS sensor has a unique slave address. Ensure no two devices on the same bus share the same address.
• Environmental Protection: If used in harsh environments, consider housing the sensor in a protective enclosure with proper ventilation.
• Warm-Up Time: Allow the sensor to stabilize for a few seconds after powering up before taking measurements.

Below is an example of how to interface the SHT20 MODBUS with an Arduino UNO using an RS485 module:

#include <ModbusMaster.h>

// Instantiate ModbusMaster object
ModbusMaster node;

// RS485 control pin
const int DE_RE_PIN = 2;

void preTransmission() {
  digitalWrite(DE_RE_PIN, HIGH); // Enable RS485 transmitter
}

void postTransmission() {
  digitalWrite(DE_RE_PIN, LOW);  // Disable RS485 transmitter
}

void setup() {
  Serial.begin(9600);            // Initialize serial communication
  pinMode(DE_RE_PIN, OUTPUT);    // Set RS485 control pin as output
  digitalWrite(DE_RE_PIN, LOW);  // Set RS485 to receive mode

  node.begin(1, Serial);         // Set MODBUS slave ID to 1
  node.preTransmission(preTransmission);
  node.postTransmission(postTransmission);
}

void loop() {
  uint8_t result;
  uint16_t data[2];

  // Read temperature and humidity registers (example addresses: 0x0001 and 0x0002)
  result = node.readInputRegisters(0x0001, 2);

  if (result == node.ku8MBSuccess) {
    data[0] = node.getResponseBuffer(0); // Temperature data
    data[1] = node.getResponseBuffer(1); // Humidity data

    float temperature = data[0] / 10.0;  // Convert to Celsius
    float humidity = data[1] / 10.0;     // Convert to %RH

    Serial.print("Temperature: ");
    Serial.print(temperature);
    Serial.println(" °C");

    Serial.print("Humidity: ");
    Serial.print(humidity);
    Serial.println(" %RH");
  } else {
    Serial.println("Failed to read from sensor.");
  }

  delay(2000); // Wait 2 seconds before next reading
}

1. No Data Received

   • Cause: Incorrect wiring or loose connections.
   • Solution: Verify all connections, especially the RS485 A and B lines.

2. MODBUS Communication Fails

   • Cause: Incorrect slave address or baud rate settings.
   • Solution: Ensure the slave address and communication parameters match the sensor's configuration.

3. Inaccurate Readings

   • Cause: Sensor exposed to condensation or contaminants.
   • Solution: Allow the sensor to dry and ensure it is used in a clean environment.

4. Interference on RS485 Bus

   • Cause: Long cable runs or improper termination.
   • Solution: Use shielded cables and add termination resistors at both ends of the bus.

Q: Can I use the SHT20 MODBUS with a 3.3V microcontroller?
A: Yes, the sensor supports a supply voltage range of 3.3V to 5.5V, making it compatible with 3.3V systems.

Q: How many sensors can I connect to a single RS485 bus?
A: You can connect up to 32 devices on a single RS485 bus, provided each has a unique slave address.

Q: What is the default slave address of the SHT20 MODBUS?
A: The default slave address is typically 0x01, but refer to the sensor's datasheet for confirmation.

Q: Can the sensor operate in high-humidity environments?
A: Yes, the SHT20 MODBUS is designed to operate in 0% to 100% RH, but prolonged exposure to condensation should be avoided.