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:

  • **_NOTE:_**
    While the 2-CH and the 4-CH relay PCB seem to come without the 120-Ohms termination resistor, the 1-CH relay does have the resistor fitted, this is marked R14 on the 1-CH-PCB. For multiple devices on the bus, this resistor must be removed.
  • **_NOTE:_**
    The relay PCBs require an external, biased RS-485 termination:
    RS-485 biased termination
    The on-board bias network is not sufficient when the bus is properly terminated with 120-Ohms on each end
  • ⚠️ **_WARNING:_**
    Even though the 2-CH and the 4-CH module come with optocouplers fitted on the digital inputs, the clearance and creepage distances on the PCB are /not/ sufficient for basic or reinforced insulation of 115/230V mains voltage. Use an external optocoupler when connecting the inputs to mains voltage circuits!
  • 1-CH Digital IN + Relay module (non-isolated input)
    1-CH Relay PCB
  • 2-CH Digital IN + Relay module (input with low-voltage isolation)
    2-CH Relay PCB
  • 4-CH Digital IN + Relay module (input with low-voltage isolation) 4-CH Relay PCB
  • (untested) 8-CH Digital IN + Relay module
    (no image)

"eletechsup" brand:

  • 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 .

Documentation Link:

API Documentation (Github Link)

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