UNO-Q Modbus TCP Coil Controller

A process control demonstration using the Arduino UNO Q dual-processor development board to monitor a temperature sensor and remotely control coils on an Arduino Opta PLC over Modbus TCP. A Python mock server is included for development and testing without hardware.

Architecture

Arduino UNO Q
┌──────────────────────────────────────────────┐
│  MCU (STM32)                                 │
│  sketch.ino (C++)                            │
│  - Simulates temperature sensor              │
│  - Broadcasts samples via Bridge             │
│           │ Bridge.notify("sample_temperature")
│  MPU (Qualcomm / Debian Linux)               │
│  main.py (Python)                            │
│  - Receives temperature via Bridge           │
│  - Connects to Opta as Modbus TCP client     │
│  - Threshold: 8000 (80 °C)                  │
│    ≥ 80 °C → coil_1=OFF, coil_2=ON          │
│    < 80 °C → coil_1=ON,  coil_2=OFF         │
└──────────────────────┬───────────────────────┘
                       │ Modbus TCP (port 502)
              ┌────────┴────────┐
              │  Arduino Opta   │
              │  (PLC server)   │
              │  4 coils        │
              └─────────────────┘

For development without an Opta, replace the PLC with the mock server (port 5020).

Repository Layout

Directory	Description
unoq-modbustcp-coil-controller/	Main UNO Q application — C++ MCU sketch + Python MPU client
mock-opta-modbus-tcp-server/	Python async Modbus TCP server simulating the Opta PLC
opta-modbustcp-server/	Arduino Opta firmware (AlPlc library + WiFi)
articles/	Tutorial article, images, and demo video

Components

unoq-modbustcp-coil-controller/

The UNO Q application split across two processors:

sketch/sketch.ino (C++ — MCU side)
Simulates a temperature sensor by ramping an integer value between 5000 (50 °C) and 10000 (100 °C) in steps, then back down, broadcasting each sample every second via Bridge.notify("sample_temperature", celsius).

python/main.py (Python — MPU side)
Receives temperature values via the Bridge callback, then writes to 4 coils on the Opta over Modbus TCP:

Constant	Default	Description
HOST	"opta"	Opta hostname (use IP or "YOUR HOST")
PORT	502	Modbus TCP port (5020 for mock server)
DEVICE	255	Modbus unit ID
THRESHOLD	8000	80 °C threshold
COIL_ADDRESS	0	Starting coil address
COIL_COUNT	4	Number of coils

Change detection skips redundant writes; up to 3 retries with automatic reconnection on failure.

python/requirements.txt

pymodbus

---

mock-opta-modbus-tcp-server/

An async Python Modbus TCP server that simulates an Arduino Opta with 4 coils (device ID 255). Useful for development and testing without physical hardware.

python3 mock_opta.py          # listens on port 5020
python3 mock_opta.py 5021     # custom port

Supports function codes FC01 (Read Coils), FC05 (Write Single Coil), and FC15 (Write Multiple Coils). PDU tracing is enabled for debugging.

To target the mock server from the UNO Q app, set HOST and PORT in main.py:

HOST = "YOUR HOST"  # IP address of the machine running the mock server
PORT = 5020

---

opta-modbustcp-server/

Arduino Opta PLC firmware acting as the Modbus TCP server. Built with the AlPlc library and WiFi connectivity.

File	Description
LLSketch/LLSketch.ino	Main sketch — WiFi setup, AlPlc initialisation
LLSketch/sketch.yaml	Board/library configuration for Opta 1.0.3–1.3.0
Opta_ModbusTCP_Server.imgx	Pre-built firmware image
Opta_ModbusTCP_Server.plcprj	Arduino Create project file

Update the WiFi credentials in LLSketch.ino before flashing.

---

articles/

A full tutorial article (uno-q-modbus-controller.md) covering board setup, Arduino App Lab IDE installation, WiFi onboarding, source code walkthrough, and testing with both the mock server and real Opta hardware. Supporting images (img/) and a demo video (video/uno-q-to-opta.mp4) are included.

Getting Started

1. Development (mock server, no hardware required)

# Terminal 1 — start the mock Modbus TCP server
cd mock-opta-modbus-tcp-server
python3 mock_opta.py

# In main.py, set:
# HOST = "<your machine IP>"
# PORT = 5020

Then deploy the UNO Q app via Arduino App Lab IDE.

2. With real hardware (Arduino Opta)

1. Update WiFi credentials in opta-modbustcp-server/LLSketch/LLSketch.ino
2. Flash the Opta using Arduino App Lab or the pre-built Opta_ModbusTCP_Server.imgx
3. In unoq-modbustcp-coil-controller/python/main.py set HOST = "opta" and PORT = 502
4. Deploy the UNO Q application via Arduino App Lab IDE

Hardware

• Arduino UNO Q (2 GB or 4 GB)
• Arduino Opta (optional — mock server can substitute)
• DIN rail power supply
• Indicator LEDs (e.g. Schneider Electric green/red)

Dependencies

• pymodbus — Modbus TCP client and server
• Arduino App Lab IDE — unified IDE for UNO Q dual-processor development
• Arduino platform: arduino:mbed_opta (≥ 4.5.0) for Opta firmware
• AlPlc_Opta library (1.3.2) for PLC functionality