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## 12. Operand Syntax

## 12.1 Driver Specification

A braille display or speech synthesizer driver must be specified via its two-letter driver identification code.

A comma-delimited list of drivers may be specified. If this is done then autodetection is performed using each listed driver in sequence. You may need to experiment in order to determine the most reliable order since some drivers autodetect better than others.

If the single word `auto` is specified then autodetection is performed using only those drivers which are known to be reliable for this purpose.

## 12.2 Braille Device Specification

The general form of a braille device specification (see the -d command line option, the braille-device configuration file directive, and the --with-braille-device build option) is `qualifier:`data. For backward compatibility with earlier releases, if the qualifier is omitted then `serial:` is assumed.

The following device types are supported:

Bluetooth

  For a bluetooth device, specify `bluetooth:`address (`bt:` and `bluez:` may also be used). The address must be six two-digit hexadecimal numbers separated by colons, e.g. `01:23:45:67:89:AB`.

Serial

  For a serial device, specify `serial:`/path/to/device. The `serial:` qualifier is optional (for backward compatibility). If a relative path is given then it's anchored at `/dev` (the usual location where devices are defined on a Unix-like system). The following device specifications all refer to the first serial device on Linux:

  - `serial:/dev/ttyS0`
  - `serial:ttyS0`
  - `/dev/ttyS0`
  - `ttyS0`
USB

  For a USB device, specify `usb:`. BRLTTY will search for the first USB device which matches the braille display driver being used. If this is inadequate, e.g. if you have more than one USB braille display which requires the same driver, then you can refine the device specification by appending the serial number of the display to it, e.g. `usb:12345`. N.B.: The "identification by serial number" feature doesn't work for some models because some manufacturers either don't set the USB serial number descriptor at all or do set it but not to a unique value.

A comma-delimited list of braille devices may be specified. If this is done then autodetection is performed on each listed device in sequence. This feature is particularly useful if you have a braille display with more than one interface, e.g. both a serial and a USB port. In this case it's usually better to list the USB port first, e.g. `usb:,serial:/dev/ttyS0`, since the former tends to autodetect more reliably than the latter.

## 12.3 PCM Device Specification

In most cases the PCM device is the full path to an appropriate system device. Exceptions are:

ALSA

  The name of and arguments for the physical or logical device, i.e. name[`:`argument`,`...].

The default PCM device is:

Platform | Device
FreeBSD | /dev/dsp
Linux/ALSA | hw:0,0
Linux/OSS | /dev/dsp
NetBSD | /dev/audio
OpenBSD | /dev/audio
Qnx | preferred PCM output device
Solaris | /dev/audio

## 12.4 MIDI Device Specification

In most cases the MIDI device is the full path to an appropriate system device. Exceptions are:

ALSA

  The client and port separated by a colon, i.e. client`:`port. Each may be specified either as a number or as a case-sensitive substring of its name.

The default MIDI device is:

Platform | Device
Linux/ALSA | the first available MIDI output port
Linux/OSS | /dev/sequencer

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