Reader Thread

The Reader thread is the primary data acquisition engine. It continuously pulls RS-422 serial packets from the MCU, validates their integrity via CRC-32, and distributes samples to downstream consumers.

Architecture

The Reader acts as a Single Producer, feeding four Concurrent Consumers via thread-safe queues.

• Input: RS-422 Serial Stream (250,000 bps)
• Outputs: - msed_writer_queue (Archival)
  • websocket_queue (Live Dashboard)
  • trigger_queue (Earthquake Detection)
  • notifier_queue (Alerts)

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Initialization

The thread is initialized with the global settings and a reference to the shared shutdown_event.

Constructor Parameters

Parameter	Type	Description
settings	Settings	Pydantic model containing serial port/baudrate.
queues	list[Queue]	The four downstream distribution queues.
shutdown_event	Event	Global signal to stop processing.

Internal State

Variable	Type	Purpose
port	str	e.g., /dev/ttyUSB0
last_heartbeat	float	Tracks the 500ms heartbeat timer.
buffer	bytearray	Sliding window for packet alignment.

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The Handshake Protocol

Before streaming begins, the Reader must synchronize settings with the MCU. If this fails, the daemon aborts to prevent data with incorrect sample rates or gains from being recorded.

1. Serial Connection: Opens the port. Note that opening the serial port often triggers a hardware reset on the MCU.
2. Boot Delay: Waits 2 seconds for the MCU to initialize its internal registers.
3. Transmit Settings: Sends a 6-byte MCUSettingsFrame containing the requested sampling_rate, gain, and adc_sample_rate.
4. Echo Verification: Waits up to 10 seconds for the MCU to echo the exact same 6 bytes back.
5. Validation: If the echo matches, streaming starts; otherwise, the daemon raises MCUNoResponse and exits.

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Main Loop Logic

The thread runs a high-frequency loop that performs three primary tasks:

1. Heartbeat Generation

To prevent the MCU from streaming data into a “dead” serial port, the MCU pauses if it doesn’t see a heartbeat.

• Frequency: Every 500ms.
• Payload: A single byte 0x01.

2. Packet Alignment

If the serial stream is interrupted, the Reader may find itself in the middle of a packet. It uses a sliding window to find the next valid header.

# Sliding window logic
if buffer[0] == 0xAA and buffer[1] == 0xBB:
    # Potential packet found
else:
    del buffer[0] # Shift 1 byte and try again