TB(4) BSD/vax Kernel Interfaces Manual TB(4)NAMEtb — line discipline for digitizing devices
This line discipline provides a polled interface to many common digitiz‐
ing 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:
int ldisc = TBLDISC, 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 sty‐
lus, 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
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
The line discipline supports digitizing devices which are compatible with
Hitachi, GTCO, or Polhemus protocol formats. For Hitachi there are sev‐
eral formats with that used in the newer model HDG-1111B the most common.
The tb interface appeared in 4.3BSD.
4.3 Berkeley Distribution June 5, 1993 4.3 Berkeley Distribution