cheap-modbus-rtu

cheap_modbus_rtu

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:
  
  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)
  

• 2-CH Digital IN + Relay module (input with low-voltage isolation)
  

• 4-CH Digital IN + Relay module (input with low-voltage isolation)

• (untested) 8-CH Digital IN + Relay module
  (no image)

"eletechsup" brand:

• PWM8A04 3-CH PWM OUT PCB
  

• R4DIF08 8-CH Digital IN module
  

• N4AIA04 4-CH 0..5V / 0..10V / 0..20mA analog input module
  

• N4DAC02 2-CH 0..5V / 0..10V analog output module
  

# 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