cheap_modbus_rtu
Lightweight control of cheap Modbus RTU components using Python

Lightweight control of cheap Modbus RTU components using Python

Currently supported hardware

"bestep" brand:

  • 2-CH Digital IN + Relay module 2-CH Relay PCB
  • 4-CH Digital IN + Relay module 4-CH Relay PCB
  • (untested) 8-CH Digital IN + Relay module
  • PWM8A04 3-CH PWM OUT PCB PWM8A04 PCB
  • R4DIF08 8-CH Digital IN module R4DIF08 PCB
  • N4AIA04 4-CH 0..5V / 0..10V / 0..20mA analog input module N4AIA04 PCB
  • N4DAC02 2-CH 0..5V / 0..10V analog output module N4DAC02
# This requires Python >= 3.9
# and recent version of setuptools and pip
user@linux:~/mysrc$ python -m pip install --upgrade pip
# Install from local git sources:
user@linux:~/mysrc$ git clone https://github.com/ul-gh/cheap_modbus_rtu.git
user@linux:~/mysrc$ cd cheap_modbus_rtu
user@linux:~/mysrc$ pip install .

R4DIF08, 8-Channel digital input PCB: Read all eight inputs

from cheap_modbus_rtu import R4DIF08
# Slave ID for these modules is pre-set to 1
input_module = R4DIF08(slave_id=1, serial_device_name="/dev/ttyUSB0")
inputs = input_module.get_inputs()
print(inputs)

Relay IO PCB: Toggle one relay on-off in a 1-second cycle

import time
# Also works with the other multi-channel variants
from cheap_modbus_rtu import Relay2Ch
# Slave ID for these modules is pre-set to 255
modbus_relay = Relay2Ch(slave_id=255, serial_device_name="/dev/ttyUSB0")
while True:
modbus_relay.set_output(1, True)
time.sleep(0.5)
modbus_relay.set_output(1, False)
time.sleep(0.5)

PWM8A04, 3-Channel PWM output PCB: Set output 1 to 20 kHz, 33 % duty cycle

from cheap_modbus_rtu import PWM8A04
# Slave ID for these PWM modules is pre-set to 1
pwm = PWM8A04(slave_id=1, serial_device_name="/dev/ttyUSB0")
pwm.set_output_frequency(1, 20000)
pwm.set_output_duty(1, 33)

Relay IO PCB: Read both inputs of the 2-Channel variant and write them directly to the relay outputs

while True:
input_1 = modbus_relay.get_input(1)
input_2 = modbus_relay.get_input(2)
modbus_relay.set_output(1, input_1)
modbus_relay.set_output(2, input_2)
print(f"\rInput 1: {input_1} Input 2: {input_2} ", end="")
time.sleep(0.1)
Input 1: False  Input 2: False  

Relay IO PCB: Set Slave ID from 255 to 1

This must only be done once, not at every device start, otherwise flash wear might be an issue. Once a different Slave ID is configured, invoke the constructor of this library with the new ID as an argument.

modbus_relay.get_broadcast_slave_id()
>>> 255
modbus_relay.set_slave_id(1)
modbus_relay.get_broadcast_slave_id()
>>> 1

Nice documentation of the Modbus-RTU protocol (English): https://ipc2u.com Modbus Protocol Description and Examples

(Same content in German) Beschreibung des Modbus-RTU Protokolls: https://ipc2u.de

Please also see:
API Documentation (Github Link)