How to Send RS485 Commands Using Python?

Python is a commonly used programming language. Sending Modbus commands via Python allows for more convenient control of motor operations. Below is a simple code example for sending Modbus commands using Python. The Modbus commands are sourced from the article A6 Servo Motor RS485 Operation Instruction, specifically from section 2.3 "Absolute Position Movement of 5000 pulses (Default setting: 10000 pulses/rev, C00.02=10000)". This code is applicable to StepperOnline's A6-RS series servo drives and can control the motor to run in absolute position mode for 5000 pulses.

Here is the Python code:

import serial
import time
import binascii
from crcmod import crcmod

# Serial port configuration - modify according to actual device
PORT = 'COM3'          # Serial port (Windows: 'COM3', Linux: '/dev/ttyUSB0')
BAUDRATE = 9600        # Baud rate (common values: 9600, 19200, 38400, 57600, 115200)
PARITY = serial.PARITY_NONE
STOPBITS = serial.STOPBITS_ONE
BYTESIZE = serial.EIGHTBITS
TIMEOUT = 0.5          # Read timeout in seconds

# Create Modbus CRC16 function
modbus_crc = crcmod.mkCrcFun(0x18005, rev=True, initCrc=0xFFFF, xorOut=0x0000)

# Command definitions (without CRC)
commands_without_crc = [
    "010600000000",  # Set drive control mode to position mode C00.00=0
    "010611010000",  # Set position profile command to absolute positioning C11.01=0
    "010603000002",  # Set position command source to step increment C03.00=2
    "0106030D0001",  # Set running speed to 1 rpm C03.0D=1
    "0106030C1388",  # Set displacement to 5000 pulses C03.0C=5000
    "010604000022",  # Configure DI1 as position step increment enable C04.00=34 (0x22)
    "010604110001",  # Enable motor and start operation C04.11=1
    "010604010001",  # Trigger DI1 enable C04.01=1
    "010604110000",  # Emergency stop command, disable motor C04.11=0
    "010604010000"   # Disable DI1 C04.01=0
]

# Command descriptions
descriptions = [
    "Set drive control mode to position mode (C00.00=0)",
    "Set position profile to absolute positioning (C11.01=0)",
    "Set position command source to step increment (C03.00=2)",
    "Set running speed to 1 rpm (C03.0D=1)",
    "Set displacement to 5000 pulses (C03.0C=5000)",
    "Configure DI1 as position step increment enable (C04.00=34)",
    "Enable motor and start operation (C04.11=1)",
    "Trigger DI1 enable (C04.01=1)",
    "Emergency stop command, disable motor (C04.11=0)",
    "Disable DI1 (C04.01=0)"
]

def calculate_crc(hex_str):
    """Calculate Modbus CRC16 for hex string"""
    data = binascii.unhexlify(hex_str)
    crc = modbus_crc(data)
    # Return CRC in little-endian format (low byte first)
    return bytes([crc & 0xFF, (crc >> 8)  & 0xFF])

def format_hex(data):
    """Format byte data to hex string"""
    return ' '.join([f"{b:02X}" for b in data])

def send_modbus_command(ser, hex_cmd, description):
    """Send single Modbus command and handle response"""
    try:
        # Calculate CRC and append to command
        crc_bytes = calculate_crc(hex_cmd)
        full_cmd = binascii.unhexlify(hex_cmd) + crc_bytes
        
        # Send command
        ser.write(full_cmd)
        print(f"\nSending command: {description}")
        print(f"Data: {format_hex(full_cmd)}")
        
        # Wait for device processing
        time.sleep(0.3)
        
        # Read response (Modbus RTU response typically 8 bytes)
        response = ser.read 8)         
        if response:
            print(f"Device response: {format_hex(response)}")
            return True
        else:
            print("Error: No response received!")
            return False
            
    except Exception as e:
        print(f"Error sending command: {str(e)}")
        return False

def main():
    # Create serial connection
    try:
        ser = serial.Serial(
            port=PORT,
            baudrate=BAUDRATE,
            parity=PARITY,
            stopbits=STOPBITS,
            bytesize=BYTESIZE,
            timeout=TIMEOUT
        )
        print(f"Successfully connected to serial port: {ser.name}\n")
    except serial.SerialException as e:
        print(f"Failed to open serial port: {str(e)}")
        return
    except Exception as e:
        print(f"Error occurred: {str(e)}")
        return

    # Send all commands
    for i, (cmd, desc) in enumerate(zip(commands_without_crc, descriptions), 1):
        print(f"Executing command {i}/{len(commands_without_crc)}")
        success = send_modbus_command(ser, cmd, desc)
        
        if not success:
            print("Command execution failed, terminating program")
            break
            
        # Special delay for critical commands
        if "Enable" in desc or "Trigger" in desc or "Emergency" in desc:
            time.sleep(1.0)  # Longer delay for motion-related commands
        else:
            time.sleep(0.5)  # Standard delay for configuration commands
    
    # Close serial port
    ser.close()
    print("\nAll commands processed, serial port closed")

if __name__ == "__main__":
    main()

Note:

a. This code relies on the crcmod library to perform CRC verification on Modbus commands. Therefore, you need to install the crcmod library to run the code and add CRC verification codes to each Modbus command.

pip install pyserial crcmod

b. Please set the correct COM port (PORT) and baud rate (BAUDRATE) according to your device's actual specifications. Adjust the delay parameters (time.sleep) based on the device's response time.

c. The Modbus commands in this example are only applicable to StepperOnline's A6-RS series servo drives. If you are using other drives, such as StepperOnline's DSY-RS series servo drives, T6 series servo drives, RS series stepper drives, etc., please modify the Modbus commands according to the respective drive's manual.

Updated on: 30/07/2025