Example 45: Modbus Temp/Humidity Sensor

Experiment Overview

Reads RS485 temperature and humidity transmitter data (e.g., SHT20/SHT30 RS485 type) via Modbus RTU protocol. This is the most common sensor integration method in industrial environments, suitable for greenhouses, server rooms, warehouses, and similar monitoring scenarios.

Hardware Wiring

RS485 Communication Wiring

ESP32 Pin	TTL-to-RS485 Module	Description
GPIO 16	TXD	Serial transmit
GPIO 17	RXD	Serial receive
GPIO 4 (optional)	DE/RE	Direction control
3.3V	VCC	Power
GND	GND	Ground

RS485 Bus to Sensor

RS485 Module	Temp/Humidity Transmitter	Description
A+	A+	Differential signal positive
B-	B-	Differential signal negative

A 120Ω termination resistor is recommended at the end of the bus; multiple devices can be connected to the same bus with different slave addresses.

JSON Configuration Example

{
  "id": "modbus_th",
  "name": "Modbus Temp/Humidity Sensor",
  "type": 1,
  "enabled": false,
  "pins": [16, 17],
  "params": {
    "baudRate": 9600,
    "dataBits": 8,
    "parity": "none",
    "stopBits": 1,
    "dePin": 4,
    "protocol": "modbus_rtu",
    "slaveId": 1,
    "function": 3,
    "startRegister": 0,
    "registerCount": 2,
    "interval": 5000,
    "dataMap": [
      { "register": 0, "name": "temperature", "scale": 0.1, "unit": "°C" },
      { "register": 1, "name": "humidity", "scale": 0.1, "unit": "%" }
    ]
  }
}

Parameter Description

Parameter	Description
slaveId	Slave address (factory default is usually 1; can be modified via DIP switch or command)
function	Modbus function code; 3 for reading holding registers
startRegister	Starting register address (varies by manufacturer; check manual)
registerCount	Number of registers to read
interval	Polling interval (ms)
dataMap	Mapping from registers to data fields; scale is the scaling factor

Data Reporting

Sensor data is reported via MQTT:

[{"id": "modbus_th", "value": "temperature:25.3,humidity:60.5"}]

Peripheral Execution Linkage

Scenario 1: High Temperature Alarm (Polling Trigger)

Feature: Trigger buzzer alarm when temperature exceeds 35°C

Web UI Configuration Steps

Step 1: Ensure Buzzer Peripheral is Configured

• Peripheral ID: buzzer_gpio
• Type: GPIO Output

Step 2: Create Rule

1. Click Peripheral Config in the left menu → Switch to Peripheral Execution tab
2. Click New Rule button
3. Fill in basic configuration:
   • Rule Name: Temp/Humidity Alarm
   • Report Data: ✅ Enabled
   • Enable: ✅ Enabled

Step 3: Configure Trigger (Polling Trigger)

1. Click Add Trigger button

2. Fill in trigger configuration:

   Field	Value	Description
   Trigger Type	Select Polling Trigger	Periodic condition check
   Target Peripheral	Select modbus_th	Modbus temp/humidity
   Data Field	temperature	Temperature field
   Operator	>	Greater than
   Threshold	35	35°C
   Polling Interval	5000	5 seconds

Step 4: Configure Action

1. Click Add Action button

2. Fill in:

   • Action Type: Select HIGH
   • Target Peripheral: Select buzzer_gpio

3. Click Save button

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Scenario 2: OLED Display Linkage (Timer Trigger)

Feature: Periodically display temperature and humidity data on OLED

Configuration Steps:

1. Create rule, name: Display Temp/Humidity
2. Trigger configuration:
   • Trigger Type: Select Timer Trigger
   • Interval: 5000 (5 seconds)
3. Action configuration (2 actions required):
   • Action 1: Select Sensor Read, target modbus_th
   • Action 2: Select Display Control, target oled1, template Temp:{modbus_th.temperature}C\nHumi:{modbus_th.humidity}%
4. Click Save

Notes

1. Address Confirmation: Use the system Modbus scan feature to confirm slave address; factory address may not be 1
2. Register Format: Temperature register may be signed integer (requires scale=0.1) or float, depending on manufacturer
3. Baud Rate Matching: Sensor and ESP32 baud rates must match; common values are 4800/9600
4. Wiring Polarity: A+/B- must not be reversed, otherwise communication will fail
5. Power Requirements: RS485 sensors typically require 12-24V DC power; independent power supply needed
6. Bus Length: Longer bus = worse interference resistance; use shielded twisted pair for distances over 50m