Example 46: Modbus Stepper Motor Control

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Example 46: Modbus Stepper Motor Control

Experiment Overview

Controls an RS485 stepper motor driver via Modbus RTU protocol, enabling remote precision positioning, speed adjustment, and direction control. Suitable for industrial automation, smart agriculture (roller blinds/irrigation valves), precision positioning, and similar 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 to Stepper Motor Driver

RS485 Module	Stepper Motor Driver	Description
A+	RS485-A	Differential signal positive
B-	RS485-B	Differential signal negative

Stepper Motor Driver to Motor

Driver	Stepper Motor	Description
A+	A phase +	Motor coil A
A-	A phase -	Motor coil A
B+	B phase +	Motor coil B
B-	B phase -	Motor coil B
VCC	External Power	12-48V DC

Stepper motor drivers (e.g., DM542, TB6600 RS485 version) require independent power supply; voltage depends on motor model.

JSON Configuration Example

Status Read Configuration

{
  "id": "modbus_stepper",
  "name": "Modbus Stepper Motor",
  "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": 4,
    "interval": 2000,
    "dataMap": [
      { "register": 0, "name": "position", "scale": 1, "unit": "steps" },
      { "register": 1, "name": "speed", "scale": 1, "unit": "RPM" },
      { "register": 2, "name": "status", "scale": 1, "unit": "" },
      { "register": 3, "name": "error", "scale": 1, "unit": "" }
    ]
  }
}

Control Write Configuration

{
  "id": "modbus_stepper_ctrl",
  "name": "Stepper Motor Control",
  "type": 1,
  "enabled": false,
  "pins": [16, 17],
  "params": {
    "protocol": "modbus_rtu",
    "slaveId": 1,
    "function": 6,
    "startRegister": 100,
    "registerCount": 3,
    "interval": 0,
    "controlMap": [
      { "register": 100, "name": "target_position", "min": -32768, "max": 32767 },
      { "register": 101, "name": "target_speed", "min": 0, "max": 3000 },
      { "register": 102, "name": "command", "values": { "start": 1, "stop": 2, "home": 3, "reset": 4 } }
    ]
  }
}

Parameter Description

Parameter	Description
slaveId	Driver slave address (DIP switch setting)
function	3=read registers, 6=write single register, 16=write multiple registers
controlMap	Control register mapping, defines writable fields
target_position	Target position (steps, signed)
target_speed	Target speed (RPM)
command	Motion command: start/stop/home/reset

Control Commands

Send control via MQTT:

[{"id": "modbus_stepper_ctrl", "value": "target_position:2048,target_speed:100,command:1"}]

target_position: Target steps (positive=forward, negative=reverse)
target_speed: Running speed in RPM
command: 1=start, 2=stop, 3=home, 4=reset alarm

Slide Table Boundary Protection

When using a Modbus stepper motor for a linear slide table, it is recommended to enable soft limits in the Modbus sub-device configuration:

{
  "name": "Slide Stepper Motor",
  "sensorId": "slide_motor",
  "deviceType": "motor",
  "slaveAddress": 8,
  "motorRegs": [0, 1, 2, 5, 7],
  "motorMinPosition": 0,
  "motorMaxPosition": 10000,
  "motorCurrentPosition": 0,
  "motorMoveStep": 1600,
  "motorLastPulse": 1600,
  "enabled": true
}

motorMinPosition: Left boundary, in steps.
motorMaxPosition: Right boundary, in steps.
motorCurrentPosition: Current estimated position; typically set to 0 after homing.
motorMoveStep: Default steps per forward/reverse command.
motorLastPulse: Most recent setPulse count; used as fallback step size when rule does not specify pulses.

When soft limits are enabled, the system calculates the next position before sending forward/reverse commands. If it would exceed the left or right boundary, the pulse count is automatically reduced to stay within bounds; if the current position is already at a boundary, the action is rejected. Hardware limit switches and emergency stop must still be retained; software limits cannot replace physical safety protection.

Peripheral Execution Linkage

Periodic Back-and-Forth Motion

{
  "name": "Stepper Motor Back-and-Forth",
  "enabled": true,
  "triggers": [
    { "type": "timer", "params": { "interval": 10000 } }
  ],
  "actions": [
    {
      "type": "modbus_write",
      "params": {
        "periphId": "modbus_stepper_ctrl",
        "register": 100,
        "value": 2048
      }
    },
    {
      "type": "modbus_write",
      "params": {
        "periphId": "modbus_stepper_ctrl",
        "register": 102,
        "value": 1
      }
    }
  ]
}

Button-Triggered Homing

{
  "name": "Button Homing",
  "enabled": true,
  "triggers": [
    {
      "type": "event",
      "params": { "periphId": "btn1", "event": "pressed" }
    }
  ],
  "actions": [
    {
      "type": "modbus_write",
      "params": {
        "periphId": "modbus_stepper_ctrl",
        "register": 102,
        "value": 3
      }
    }
  ]
}

{
  "name": "High Temp Open Blind",
  "enabled": true,
  "triggers": [
    {
      "type": "poll",
      "params": {
        "periphId": "modbus_th",
        "field": "temperature",
        "operator": ">",
        "threshold": 32
      }
    }
  ],
  "actions": [
    {
      "type": "modbus_write",
      "params": { "periphId": "modbus_stepper_ctrl", "register": 100, "value": 5000 }
    },
    {
      "type": "modbus_write",
      "params": { "periphId": "modbus_stepper_ctrl", "register": 102, "value": 1 }
    }
  ]
}

Notes

Driver Selection: Ensure the driver supports RS485 Modbus RTU protocol (not pulse/direction type)
Register Addresses: Different brands define registers differently; always consult the corresponding manual
Motion Safety: Limit switches are recommended to prevent mechanical damage from over-travel
Emergency Stop: Hardware emergency stop button is essential; cannot rely on software stop alone
Communication Timeout: Stepper motor execution takes time; polling interval should be ≥ 2000ms
Independent Power: Motor driver power supply must be completely isolated from ESP32 power; share ground only