11/3/71 /DEV/TTY0 ... TTY5 (IV)
NAME tty0 ... tty5 -- communications interfaces
SYNOPSIS --
DESCRIPTION These files refer to DC11 asynchronous communica-
tions interfaces. At the moment there are six of
them, but the number is subject to change. Names
for up to four others will be constructed by an
obvious algorithm.
When one of these files is opened, it causes the
process to wait until a connection is estab-
lished. (In practice, however, user's programs
seldom open these files; they are opened by init
and become a user's standard input and output
file.) The very first typewriter file open in a
process becomes the control typewriter for that
process. The control typewriter plays a special
role in the handling quit or interrupt signals,
as discussed below. The control typewriter is
inherited by a child process during a fork.
A terminal associated with one of these files or-
dinarily operates in full-duplex mode. Charac-
ters may be typed at any time, even while output
is occurring, and are only lost when the system's
character input buffers become completely choked,
which is rare.
When first opened, the interface expects the ter-
minal to use 15 odd-parity, 10-bit ASCII charac-
ters per second and to have the new-line func-
tion. Finally, the system calculates delays af-
ter sending the code for certain functions (e.g.,
new-line, tab) on the assumption that the termi-
nal is a Teletype model 37. All this is merely a
long way of saying that the system expects to be
used by a TTY 37. However, most of these assump-
tions can be changed by a special system call: in
particular, the expected parity can be changed;
the speed, character size, and stop bits can be
changed (speeds available are 134.5, 150, 300,
1200 baud; see the DC11 manual); the new-line
function can be simulated by a combination of the
carriage-return and line-feed functions; carriage
return can be translated into new-line on input;
upper case letters can be mapped into lower case
letters; echoing can be turned off so the termi-
nal operates in half duplex. See the system call
stty. (Also see init for the way 300-baud termi-
nals are detected.)
Normally, a typewriter operates in units of
lines. This means that a program attempting to
read will be suspended until an entire line has
been typed. Also, no matter how many characters
are requested in the read call, at most one line
will be returned. It is not however necessary to
read a whole line at once; any number of charac-
ters may be requested in a read, even one, with-
out losing information.
The EOT character may be used to generate an end
of file from a typewriter. When an EOT is re-
ceived, all the characters waiting to be read are
immediately passed to the program, without wait-
ing for a new-line. Thus if there are no charac-
ters waiting, which is to say the EOT occurred at
the beginning of a line, zero characters will be
passed back, and this is the standard end-of-file
signal.
When the carrier signal from the dataset drops
(usually because the user has hung up his termi-
nal) any read returns with an end-of-file indica-
tion. Thus programs which read a typewriter and
are sensitive to end-of-file on their inputs
(which all programs would be) will terminate ap-
propriately when hung up on.
Two characters have a special meaning when typed.
The ASCII DEL character (sometimes called "rub-
out") is the interrupt signal. When this charac-
ter is received from a given typewriter, a search
is made for all processes which have this type-
writer as their control typewriter, and which
have not informed the system that they wish to
ignore interrupts. If there is more than one
such process, one of these is selected, for prac-
tical purposes at random. Then either the pro-
cess is forced to exit or a trap is simulated to
an agreed-upon location in the process. See sys
intr for more information.
The ASCII character FS is the quit signal. Its
treatment is identical to the interrupt signal
except that unless the receiving process has made
other arrangements it will not only be terminated
but a core image file will be written. (See sys
quit for more information.)
During input, erase and kill processing is nor-
mally done. The character "#" erases the last
character typed, except that it will not erase
beyond the beginning of a line or an EOT. The
character "@" kills the entire line up to the
point where it was typed, but not beyond an EOT.
Both these characters operate on a keystroke ba-
sis independently of any backspacing or tabbing
that may have been done. Either "@" or "#" may
be entered literally by preceding it by "\"; the
erase or kill character remains, but the "\" dis-
appears.
It is also possible (again by sys stty) to put
the typewriter into raw mode. In this mode, the
program reading is awakened on each character,
and when a program reads, it waits only until at
least one character has been typed. In raw mode,
no erase or kill processing is done; and the EOT,
quit and interrupt characters are not treated
specially.
Output is prosaic compared to input. It should
be noted, however, that when one or more charac-
ters are written, they are actually transmitted
to the terminal as soon as previously-written
characters have finished typing. When a program
produces characters too rapidly to be typed, as
is very common, it may be suspended for a time.
Odd parity is always generated on output, except
that the characters EOT and NAK have the wrong
parity. Thus the 37 TTY will not hang up (EOT)
or lock its keyboard (NAK) if a program acciden-
tally prints these characters.
FILES --
SEE ALSO tty
DIAGNOSTICS --
BUGS As has been suggested, UNIX has a heavy predispo-
sition towards 37 Teletype terminals. However,
it is quite possible to use 300-baud terminals
such as the GE TermiNet 300. (See init for the
procedure.) The main difficulty in practice is
37-oriented delay calculations.
Terminals such as the IBM 2741 would theoreti-
cally be very desirable but there are many diffi-
culties related to its inadequate and non-ASCII
character sets (the 2741 has two, count 'em) and
the inherently half-duplex nature of the termi-
nal. It is possible to produce output on a 2741;
cf type.
OWNER ken, dmr