Serial interface

Description

Reads line-delimited data off a configurable serial port (baud rate configurable) and parses each line into named fields, publishing them as a Record — for custom robot sensors/boards that talk over a UART/USB-serial link and never reach a ROS topic on their own.

The port is opened lazily, on the first poll after activation, and never on onConfigure() — an unplugged or not-yet-connected device does not fail activation. Disconnection is detected two ways: a hard read error (e.g. EIO on some USB-serial removal paths) closes the port immediately; a hangup with no error (the more common case on a raw-mode, non-blocking read, where read() returning 0 alone can't be told apart from "no data yet") is caught via poll()'s POLLHUP/POLLERR. Either way the Measurement logs a warning, closes the file descriptor, and keeps polling; the next poll after the device reappears reopens and resumes normally, with no operator action needed and no busy-looping in between (opening only happens once per polling_interval tick).

Only line-delimited framing (\n, with an optional trailing \r stripped) is implemented today; framing is still a configuration knob for future framing modes. Only the most recently completed line in a given poll is parsed — if several lines arrive within one polling_interval, earlier ones are dropped, the same lossy-between-polls behavior other subscription/poll-driven Measurements (e.g. cmd_vel, diagnostics) already have.

Two parsing modes are supported, both producing the same fields object shape:

delimiter: splits the line on delimiter and assigns tokens to fields in order.
regex: matches the line against regex and assigns capture groups to fields in order. std::regex/ECMAScript has no native named-capture-group syntax, so "named groups" here means pairing each positional capture group with a name from fields, in capture order.

If the token/capture count doesn't match the configured fields count, a warning is logged and whatever fields do line up are still published — a malformed line degrades rather than drops.

Parameters

Parameter	Description	Type	Default
port	Serial device path (e.g. `/dev/ttyUSB0`)	str	"" (required)
baud_rate	Baud rate: one of 1200/2400/4800/9600/19200/38400/57600/115200/230400	int	9600
framing	Line framing mode; only `"line"` is implemented	str	"line"
parsing_type	`"delimiter"` or `"regex"`	str	"delimiter"
delimiter	Delimiter string used when `parsing_type: delimiter`	str	","
regex	ECMAScript regex (with capture groups) used when `parsing_type: regex`	str	""
fields	Ordered field names paired with delimiter tokens or regex capture groups	list[str]	[] (Optional)

Schema

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "SerialInterface",
  "description": "A line parsed off a configured serial port",
  "properties": {
    "raw": {
      "description": "The raw line read from the serial port, with any trailing CR/LF stripped",
      "type": "string"
    },
    "fields": {
      "description": "Named fields extracted from 'raw' per the configured parsing (delimiter split or regex capture groups)",
      "type": "object"
    }
  },
  "required": ["raw", "fields"],
  "type": "object"
}

Configuration

Delimiter split, e.g. a board emitting 23.5,60\n (temperature, humidity):

...
serial_sensor:
  plugin: "dc_measurements/SerialInterface"
  topic_output: "/dc/measurement/serial_sensor"
  port: "/dev/ttyUSB0"
  baud_rate: 9600
  parsing_type: "delimiter"
  delimiter: ","
  fields: ["temperature", "humidity"]

Regex capture groups, e.g. a board emitting T:23.5 H:60\n:

...
serial_sensor:
  plugin: "dc_measurements/SerialInterface"
  topic_output: "/dc/measurement/serial_sensor"
  port: "/dev/ttyUSB0"
  baud_rate: 9600
  parsing_type: "regex"
  regex: "^T:(\\d+\\.\\d+) H:(\\d+)$"
  fields: ["temperature", "humidity"]

Example output

Derived from the delimiter example above (23.5,60 split on , into fields: ["temperature", "humidity"]):

{
  "raw": "23.5,60",
  "fields": {
    "temperature": "23.5",
    "humidity": "60"
  }
}

Testing without hardware

The gtest suite (test/test_measurement_serial_interface.cpp) verifies this Measurement against a virtual serial pair created with socat, no hardware required:

socat -d -d pty,raw,echo=0,link=/tmp/dc_serial_dev pty,raw,echo=0,link=/tmp/dc_serial_peer

The Measurement is pointed at /tmp/dc_serial_dev; the test writes fixture lines to /tmp/dc_serial_peer. Killing and restarting socat against the same link= paths simulates an unplug/replug cycle and exercises the reconnect path.

Data Collection for ROS 2