Returns a specification to start this module under a supervisor.

See Supervisor.

@spec close(pid()) :: :ok

Closes the UART port. Returns :ok and clears any pending command.

@spec configure(pid(), keyword()) :: :ok

Updates the master's connection parameters (:tty, :active, :timeout, :uart_opts) at runtime, reopening the UART port.

@spec open(pid()) :: :ok

Opens the UART port for the configured TTY device.

@spec read(pid()) :: nil | :ok | {:ok, list()} | {:error, term()}

Re-reads the UART for the last sent command in passive mode. Returns nil if no command has been sent yet, or :error in active mode.

@spec request(pid(), tuple()) :: :ok | {:ok, list()} | {:error, term()}

Packs and sends a Modbus frame over RTU. In passive mode blocks until the slave responds; in active mode returns :ok and delivers the result to parent_pid.

@spec start_link(keyword(), keyword()) :: {:ok, pid()} | {:error, term()}

Starts the RTU master GenServer. params must include :tty and may include :active, :timeout, and :uart_opts.

@spec state(pid()) :: %Modbuv.Rtu.Master{
  active: term(),
  cmd: term(),
  last_cmd: term(),
  parent_pid: term(),
  status: term(),
  timeout: term(),
  tty: term(),
  uart_opts: term(),
  uart_pid: term()
}

Returns the current internal state struct of the RTU master.

@spec stop(pid()) :: :ok

Stops the RTU master GenServer process.