Data Format

mag-usb prints one JSON object per line to OUTPUT_PRINT. Each line represents a single sample of the magnetometer vector (and optionally other values in the future).

Baseline schema

{ "ts": "DD Mon YYYY HH:MM:SS", "rt": <float>, "x": <float>, "y": <float>, "z": <float> }

  • ts (string): UTC timestamp formatted like 25 Oct 2025 14:02:33 (RFC‑2822‑like time portion without timezone offset).

  • rt (number): Value of temperature measured in degree C of the sensor at its ‘remote’ location.

  • x, y, z (number): Field components in nanoTesla (nT), with 3 decimal places printed.

Example:

{ "ts":"26 Oct 2025 14:20:00", "rt":23.125, "x":12345.678, "y":-234.500, "z":987.001 }

Units and scaling

  • Raw RM3100 counts are converted using configured gains and NOS (number‑of‑samples) register value.

  • Outputs are provided in nanoTesla (nT). Internally the computation converts microTesla to nanoTesla by multiplying by 1000.

Orientation translations

If configured, 90° increment rotations are applied to (x,y,z) before printing. See docs/Orientation-and-Axes.md and [mag_orientation] in docs/Configuration.md.

Precision and rounding

  • Values are printed with %.3f (three digits after decimal). This does not imply instrument accuracy; it is a display choice.

Sampling cadence

  • In POLL mode, each call to formatOutput() prints one sample.

  • In CMM mode (continuous), the sample rate is controlled by cmm_sample_rate; see docs/Configuration.md.

Errors and diagnostics output

  • Informational and error messages (e.g., adapter checks) are printed to OUTPUT_PRINT/ around the JSON lines. If you need a clean stream of JSON only, redirect and/or prefilter lines not starting with {.

Logging and pipes

  • If you enable logging or named pipes in the configuration, the same JSON lines are written to files or pipes.

  • Named pipes (FIFOs) allow real-time IPC with local monitor/dashboard programs.

  • Default pipe paths are /run/mag-usb/magctl.fifo (input) and /run/mag-usb/magdata.fifo (output).

  • See docs/Configuration.md (output section) for details.