8 Channel RS485 MODBUS RTU relay board type R421A08

This Python project can control up to 64 individual 8 channel R421A08 relay boards from the command line and GUI. It uses a universal USB - RS485 dongle.

Project features:

• Python packages for easy application integration.
• Simple command line interface, useful for scripting.
• wxPython GUI tested with Python 3.5 only (Windows 10 and Ubuntu 16.04)
• Unit tests with coverage.

Hardware

The following hardware is required for this project:

• One or more R421A08 relay boards.
• RS485 - USB dongle.
• Optional: A second RS485 - USB dongle to monitor MODBUS frames.

R421A08 relay board

• RS485 (binary) interface.
• 8 x 12V Relays: 10A 125VAC / 10A 28VDC.
• 8 status LED's.
• 6 DIP switches for 64 board addresses.
• Board power: 12V DC.
• Current board idle: ~11mA.
• Current one relay: ~26mA.
• Current all relays on: ~220mA.
• Length: 90mm, width: 60mm, height: 20mm.

WARNING: DO NOT USE THIS RELAY BOARD WITH 230V AC!
The distance between relay traces on the PCB are < 2mm without holes for isolation. This is dangerous when using it with high voltages. See the picture above.

R421B16 relay board

The 16 relay board version works by changing NUM_RELAYS = 16 in R421A08.py as reported by a user in issue #1.

RS485 - USB dongle

This project requires a RS485 - USB dongle which is widely available, for example:

• On Windows, open the Device Manager | Ports (COM & LPT) to find the USB-SERIAL CH340 (COMxx) serial port.
• On Linux, use the dmesg command to find the serial port, such as /dev/ttyUSB0.

Software

Screenshot R421A08 relay GUI

Screenshot MODBUS GUI

Screenshot commandline

Screenshot relay toggle example

Usage relay_boards Python package

getting_started.py

import time
import relay_boards


if __name__ == '__main__':
    print('Getting started R421A08 relay board\n')

    # Create MODBUS object
    _modbus = relay_modbus.Modbus(serial_port=SERIAL_PORT)

    # Open serial port
    try:
        _modbus.open()
    except relay_modbus.SerialOpenException as err:
        print(err)
        sys.exit(1)

    # Create relay board object
    board = relay_boards.R421A08(_modbus, address=1)

    print('Status all relays:')
    check(board.print_status_all())
    time.sleep(1)

    print('Turn relay 1 on')
    check(board.on(1))
    time.sleep(1)

    print('Turn relay 1 off')
    check(board.off(1))
    time.sleep(1)

    print('Toggle relay 8')
    check(board.toggle(8))
    time.sleep(1)

    print('Latch relays 6 on, all other relays off')
    check(board.latch(6))
    time.sleep(1)

    print('Turn relay 4 on for 5 seconds, all other relays off')
    check(board.delay(4, delay=5))
    time.sleep(6)

    print('Turn relays 3, 7 and 8 on')
    check(board.toggle_multi([3, 7, 8]))
    time.sleep(1)

    print('Turn all relays on')
    check(board.on_all())
    time.sleep(1)

    print('Turn all relays off')
    check(board.off_all())
    time.sleep(1)

Documentation

Please refer to the Wiki page for installation and usage.

MODBUS Application Protocol Specification v1.1.b

The following Python packages are available in this project:

• R421A08 relay board Python package:
  • Read state relay(s).
  • Turn relay(s) on.
  • Turn relay(s) off.
  • Toggle relay(s).
  • Latch relay (One relay on, rest off).
  • Momentary (Turn relay on for one second).
  • Delay (Turn relay on for 1..255 seconds).
  • Single or multiple relay boards.
• Serial MODBUS Python package:
  • MODBUS monitor frames on the RS485 bus
  • Send raw ASCII MODBUS frames

RS485 RTU MODBUS communication protocol

MODBUS is an open binary serial communication protocol, mainly used for PLCs.

• Serial port at 9600 baud, 8 databits, 1 stop, no parity.
• This board supports MODBUS RTU protocol only.
• This board supports 64 addresses, configurable with 6 DIP switches.
• This board does not support ASCII mode, but can be send with the Python--send option.
• This board does not support AT commands.
• This board does not support broadcasts.
• All relays are off after power-on.