BOLT — Block-Oriented Lightweight Tasker

BOLT is a lightweight IEC 61499-inspired runtime that executes Function Blocks (FBs) connected by event and data connections. It runs on a standard PC (Python 3) and communicates with embedded devices over MQTT.

The included example drives an ESP32 running MicroPython: the PC sends engine angle commands over MQTT and the ESP32 acknowledges them back.

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Repository structure

BOLT/
├── core/               # Runtime engine (device, resource, FB loader, MQTT client)
│   └── FBs/            # Built-in function blocks (START, E_CYCLE, MQTT_*, ...)
├── BOLT_mp/BOLT_mp/    # MicroPython firmware for the ESP32
├── docker/             # Mosquitto broker config
├── config.py           # Host-side configuration (broker host/port)
├── main.py             # Example application
├── requirements.txt    # Python dependencies
├── scripts/
│   ├── setup_venv.sh       # Venv creation + dependency installation script
│   └── deploy_mp_script.sh # Upload MicroPython firmware to the ESP32

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Prerequisites

• Python 3.11+
• Docker & Docker Compose (for the MQTT broker)
• An ESP32 flashed with MicroPython (for the embedded side)

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Host-side setup

1. Create the virtual environment and install dependencies

source ./scripts/setup_venv.sh

This creates .venv/, activates it, upgrades pip, and installs everything in requirements.txt (including mpremote).

Alternatively, do it manually:

python3 -m venv .venv
source .venv/bin/activate      # Windows: .venv\Scripts\activate
pip install -r requirements.txt

2. Configure the broker

Edit config.py in the project root:

BROKER_HOST = "localhost"   # or your broker's IP
BROKER_PORT = 1883

3. Start the MQTT broker with auto-discovery

To enable ESP32 devices to auto-discover the broker via mDNS on the local network, run:

./setup.sh

This script:

• Installs avahi-daemon if needed (required for mDNS on Linux)
• Registers the broker as a discoverable service (mqtt.local)
• Starts the Docker Compose broker

Alternatively, start manually without auto-discovery:

docker compose up -d

4. Run BOLT

python main.py

As the example, BOLT connects to the broker, starts the cyclic engine, and begins publishing ENGINE_DEGREES every 500 ms while listening for ENGINE_STATUS acknowledgements.

Performance tip: BOLT uses one thread per event chain. For maximum throughput on CPython, disable the GIL (Python 3.13+ free-threaded build).

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ESP32 / MicroPython setup

1. Configure the device

Edit configurations in BOLT_mp/BOLT_mp/config.py:

WIFI_SSID           = "your-wifi-ssid"
WIFI_PASSWORD       = "your-wifi-password"

# Enable auto-discovery of the broker via mDNS
BROKER_DISCOVERY_ENABLED = True
BROKER_MDNS_NAME = "mqtt"          # resolves as mqtt.local via mDNS
BROKER_DISCOVERY_TIMEOUT_MS = 3000 # ms to wait for each mDNS query attempt
BROKER_DISCOVERY_RETRIES = 3       # attempts before falling back

# Fallback if discovery fails or is disabled
BROKER_HOST         = "192.168.x.x"   # fallback IP if discovery is disabled or fails
BROKER_PORT         = 1883
CLIENT_ID           = "esp32-engine-01"

Auto-discovery notes:

• The broker must be running on a machine with mDNS support (Linux with avahi-daemon)
• Use ./setup.sh to start the broker with mDNS advertised
• If BROKER_DISCOVERY_ENABLED = False, the ESP32 will connect directly to BROKER_HOST
• If discovery fails, the device automatically falls back to the hardcoded BROKER_HOST

And in BOLT_mp/.micropythonrc

{
  "upload": {
    "port": "/dev/ttyACM0",
    "baud": 460800
  },
  "serial": {
    "port": "/dev/ttyACM0",
    "baud": 460800
  },
  "ignore": {
    "extensions": [
      ".micropythonrc"
    ],
    "directories": [
      ".vscode"
    ]
  },
  "tools": {
    "ampy": "/home/user/.local/bin/ampy",
    "rshell": "/home/user/.local/bin/rshell"
  }
}

2. Flash the firmware to the ESP32

Connect the ESP32 via USB, then run:

bash scripts/deploy_mp_script.sh

This copies boot.py, main.py, mqtt_client.py, and config.py to the device and resets it. The default port is /dev/ttyACM0 — edit the PORT variable in the script if yours differs.

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Built-in Function Blocks

FB	Description
START	Fires a single START event on resource initialisation
E_CYCLE	Periodic timer — fires EO every DT milliseconds
MQTT_PUBLISH	Publishes a value to an MQTT topic
MQTT_SUBSCRIBE	Subscribes to an MQTT topic and fires IND on each message
PRINT_CONSOLE	Prints the IN data variable to stdout
ADD_2	Adds two integers
INT2INT	Integer passthrough / type cast
STRING2STRING	String passthrough

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License

See LICENSE.