Hello World for OpenPLC and Modbus on Windows SoftPLC
My explorations into PLC runtimes has taken me to discover OpenPLC. The goal of this post is to explain how I was able to deploy a “Hello World” project running on the Windows SoftPLC and connected to a Modbus client.
To get started, I followed the Creating You First Project and deployed to the SoftPLC running on my machine. However, when I created the project, I used a different mapping for the IO so that I would be able to control them from Python.
| Name | Location | Alternative Location |
|---|---|---|
PB1 |
%IX0.0 |
%QX0.1 |
PB2 |
%IX0.1 |
%QX0.2 |
LED |
%IX0.0 |
%QX0.0 (unchanged) |
As above, this maps all items to coils that support both read and write.
The next step is to connect to it via Modbus. To do this, I write a simple Modbus client using pymodbus.
from pymodbus.client.sync import ModbusTcpClient as ModbusClient
import time
client = ModbusClient('localhost', port=502)
unit=0x01
# Get the initial state
rr = client.read_coils(0, 1, unit=unit)
print("Initial LED output: " + str(rr.getBit(0)))
time.sleep(1)
client.write_coil(1, True, unit=unit)
print("Pressed PB1 - turn on LED")
time.sleep(1)
client.write_coil(1, False, unit=unit)
print("Unpressed PB1")
time.sleep(1)
rr = client.read_coils(0, 1, unit=unit)
print("LED output after pressing PB1: " + str(rr.getBit(0)))
time.sleep(1)
client.write_coil(2, True, unit=unit)
print("Pressed PB2 - reset")
time.sleep(1)
client.write_coil(2, False, unit=unit)
print("Unpressed PB2")
time.sleep(1)
rr = client.read_coils(0, 1, unit=unit)
print("LED output after pressing PB2 (reset): " + str(rr.getBit(0)))
time.sleep(1)
client.close()
Running that with Python produces the expected output:
python .\mobus-client.py
Initial LED output: False
Pressed PB1 - turn on LED
Unpressed PB1
LED output after pressing PB1: True
Pressed PB2 - reset
Unpressed PB2
LED output after pressing PB2 (reset): False