Example 47: Modbus Relay Module Control

Experiment Overview

Controls RS485 multi-channel relay modules via Modbus RTU protocol, enabling remote on/off switching. Suitable for smart home, industrial control, agricultural irrigation, and other scenarios requiring remote switching.

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 to Relay Module

RS485 Module	Relay Board	Description
A+	A	Differential signal positive
B-	B	Differential signal negative

Relay modules typically require 12V/24V DC power; contacts support AC 250V/10A or DC 30V/10A.

JSON Configuration Example

4-Channel Relay Control

{
  "id": "modbus_relay",
  "name": "4-CH Modbus Relay",
  "type": 1,
  "enabled": false,
  "pins": [16, 17],
  "params": {
    "baudRate": 9600,
    "dataBits": 8,
    "parity": "none",
    "stopBits": 1,
    "dePin": 4,
    "protocol": "modbus_rtu",
    "slaveId": 1,
    "function": 5,
    "startRegister": 0,
    "registerCount": 4,
    "interval": 0,
    "controlMap": [
      { "register": 0, "name": "ch1", "onValue": 65280, "offValue": 0 },
      { "register": 1, "name": "ch2", "onValue": 65280, "offValue": 0 },
      { "register": 2, "name": "ch3", "onValue": 65280, "offValue": 0 },
      { "register": 3, "name": "ch4", "onValue": 65280, "offValue": 0 }
    ]
  }
}

Relay Status Readback

{
  "id": "modbus_relay_status",
  "name": "Relay Status",
  "type": 1,
  "enabled": false,
  "pins": [16, 17],
  "params": {
    "protocol": "modbus_rtu",
    "slaveId": 1,
    "function": 1,
    "startRegister": 0,
    "registerCount": 4,
    "interval": 2000,
    "dataMap": [
      { "register": 0, "name": "ch1_state", "scale": 1 },
      { "register": 1, "name": "ch2_state", "scale": 1 },
      { "register": 2, "name": "ch3_state", "scale": 1 },
      { "register": 3, "name": "ch4_state", "scale": 1 }
    ]
  }
}

Parameter Description

Parameter	Description
function 5	Write Single Coil
function 1	Read Coils
onValue	Coil ON value: 0xFF00 = 65280
offValue	Coil OFF value: 0x0000 = 0

Control Commands

Control relays via MQTT:

[{"id": "modbus_relay", "value": "ch1:1,ch2:0,ch3:1,ch4:0"}]

• 1 = Close (ON)
• 0 = Open (OFF)

Peripheral Execution Linkage

Scenario 1: Temperature-Controlled Fan (Polling Trigger)

Feature: Turn on fan (relay CH1) when temperature exceeds 30°C

Web UI Configuration Steps

Step 1: Ensure Peripherals are Configured

• Modbus temp/humidity: modbus_th
• Modbus relay: modbus_relay

Step 2: Create Rule

1. Click Peripheral Execution in the left menu
2. Click New Rule button
3. Fill in basic configuration:
   • Rule Name: High Temp Fan ON
   • 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	30	30°C
   Polling Interval	5000	5 seconds

Step 4: Configure Action (Modbus Write)

1. Click Add Action button

2. Fill in:

   • Action Type: Select Modbus Write
   • Target Peripheral: Select modbus_relay
   • Register: 0 (CH1)
   • Value: 65280 (Close)

3. Click Save button

---

Scenario 2: Scheduled Irrigation (Timer Trigger)

Feature: Open irrigation valve at 6:00 AM daily for 5 minutes

Configuration Steps:

1. Create rule, name: Scheduled Irrigation
2. Trigger configuration:
   • Trigger Type: Select Timer Trigger
   • Timer Mode: Select Cron Expression
   • Cron: 0 6 * * * (daily at 6:00)
3. Action configuration (3 actions required):
   • Action 1: Select Modbus Write, register 1 (CH2), value 65280 (Open)
   • Action 2: Select Delay, time 300000 (5 minutes)
   • Action 3: Select Modbus Write, register 1 (CH2), value 0 (Close)
4. Click Save

Typical Application Scenarios

Scenario	Wiring	Description
Light Control	NO contact in series with light	Close = light ON
Fan Control	NO contact in series with fan	Automatic temperature control
Solenoid Valve	NO contact in series with valve	Irrigation/ventilation
Heater	NO contact in series with heater	Low-temperature heating

Notes

1. Contact Rating: Relay contacts have maximum current limits; high-power loads require a contactor
2. Back-EMF: When controlling inductive loads (motors/solenoids), add a flyback diode on the load side
3. Power-off State: Relay defaults to open after power loss; select latching type if state retention is needed
4. Operating Frequency: Mechanical relay lifespan is ~100,000 cycles; use solid-state relay for frequent switching
5. Slave Address: When multiple relay modules share the same bus, ensure addresses do not conflict
6. Safety Distance: Maintain safe distance between high voltage (220V AC) and low voltage (RS485) wiring