TB(4)

NAME
tb - line discipline for digitizing devices

SYNOPSIS
pseudo-device tb

DESCRIPTION
This line discipline provides a polled interface to many common digitizing devices which are connected to a host through a serial line. When these devices stream data at high speed, the use of the line discipline is critical in minimizing the number of samples that would otherwise be lost due to buffer exhaustion in the tty(4) handler.

The line discipline is enabled by a sequence:

    #include <sys/tablet.h>
    int ldisc = TABLDISC, fildes; ...
    ioctl(fildes, TIOCSETD, &ldisc);

A typical application program then polls the digitizing device by reading a binary data structure which contains: the current X and Y positions (in the device coordinate space), up-down status of the buttons or pen stylus, proximity information (when available), and a count of the number of samples received from the input device since it was opened. In addition, devices such as the GTCO append tilt and pressure information to the end of the aforementioned structure. For the Polhemus 3-D digitizer the structure read is completely different. Refer to the include file for a complete description.

While in tablet mode, normal teletype input and output functions take place. Thus, if an 8 bit output data path is desired, it is necessary to prepare the output line by putting it into RAW mode using ioctl(2). This must be done before changing the discipline with TIOCSETD, as most ioctl(2) calls are disabled while in tablet line-discipline mode.

The line discipline supports ioctl(2) requests to get/set the operating mode, and to get/set the tablet type and operating mode by or-ing the two values together.

The line discipline supports digitizing devices which are compatible with Hitachi, GTCO, or Polhemus protocol formats. For Hitachi there are several formats with that used in the newer model HDG-1111B the most common.

DIAGNOSTICS
None.

SEE ALSO
tty(4)

HISTORY
The tb interface appeared in 4.3BSD.