6/12/72 TTY0 (IV)
NAME tty0 ... tty7 -- communications interfaces
SYNOPSIS --
DESCRIPTION These files refer to DC11 asynchronous communica-
tions interfaces. At the moment there are eight
of them, but the number is subject to change.
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 handling quit or interrupt signals, as
discussed below. The control typewriter is in-
herited 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, or when the user has accumulated
the maximum allowed number of input characters
which have not yet been read by some program.
Currently this limit is 150 characters. When
this is happening the character "#" is echoed for
every lost input character.
When first opened, the standard interface mode
assumed includes: ASCII characters; 150 baud;
even parity accepted; 10 bits/character (one stop
bit); and newline action character. The system
delays transmission after sending certain func-
tion characters. Delays for horizontal tab, new-
line, and form feed are calculated for the Tele-
type Model 37; the delay for carriage return is
calculated for the GE TermiNet 300. Most of
these operating states can be changed by using
the system call stty(II). In particular the fol-
lowing hardware states are program settable inde-
pendently for input and output (see DC11 manual):
110, 134.5, 150, 300, 600, or 1200 baud; one or
two stop bits on output; and 5, 6, 7, or 8
bits/character. In addition, the following soft-
ware modes can be invoked: acceptance of even
parity, odd parity, or both; a raw mode in which
all characters may be read one at a time; a car-
riage return (CR) mode in which CR is mapped into
newline on input and either CR or line feed (LF)
cause echoing of the sequence LF-CR; mapping of
upper case letters into lower case; suppression
of echoing; suppression of delays after function
characters; the echoing of input tabs as spaces;
and setting the system to handle IBM 2741s. See
getty(VII) for the way that terminal speed and
type are detected.
Normally, typewriter input is processed 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 charac-
ters are requested in the read call, at most one
line will be returned. It is not however neces-
sary to read a whole line at once; any number of
characters may be requested in a read, even one,
without losing 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 possible to use raw mode in which the pro-
gram reading is awakened on each character. The
program 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 inter-
rupt characters are not treated specially.
The ASCII EOT character may be used to generate
an end of file from a typewriter. When an EOT is
received, all the characters waiting to be read
are immediately passed to the program, without
waiting for a new-line. Thus if there are no
characters waiting, which is to say the EOT oc-
curred at the beginning of a line, zero charac-
ters will be passed back, and this is the stan-
dard 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
test for end-of-file on their input can terminate
appropriately 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. If interrupts aren't
being ignored, the process is either forced to
exit or a trap is simulated to an agreed-upon lo-
cation in the process. See intr(II).
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 generated. See
quit(II).
Output is prosaic compared to input. When one or
more characters are written, they are actually
transmitted to the terminal as soon as
previously-written characters have finished typ-
ing. Input characters are echoed by putting them
in the output queue as they arrive. When a pro-
cess produces characters more rapidly than they
can be typed, it will be suspended when its out-
put queue exceeds some limit. When the queue has
drained down to some threshold the program is re-
sumed. Even-parity is always generated on out-
put. The EOT character is not transmitted to
prevent terminals which respond to it from being
hung up.
The system will handle IBM 2741 terminals. See
getty(VII) for the way that 2741s are detected.
In 2741 mode, the hardware state is: 134.5 baud;
one output stop bit; and 7 bits/character. Be-
cause the 2741 is inherently half-duplex, input
is not echoed. Proper function delays are pro-
vided. For 2741s without a feature known as
"transmit interrupt" it is not possible to col-
lect input ahead of the time that a program reads
the typewriter, because once the keyboard has
been enabled there is no way to send further out-
put to the 2741. It is currently assumed that
the feature is absent; thus the keyboard is un-
locked only when some program reads. The inter-
rupt signal (normally ASCII DEL) is simulated
when the 2741 "attention" key is pushed to gener-
ate either a 2741 style EOT or a break. It is
not possible to generate anything corresponding
to the end-of-file EOT or the quit signal. Cur-
rently IBM EBCDIC is default for input and out-
put; correspondence code output is settable (see
stty(I)). The full ASCII character set is not
available: "[", "]", "{", "}", "~", are missing
on input and are printed as blank on output; "¢"
is used for "\"; "¬" for "^"; " " for both "'"
and"`"on output; and " " maps into "'" on input.
Similar mappings occur with correspondence code
output.
FILES found in /dev
SEE ALSO tty(I), getty(VII)
DIAGNOSTICS --
BUGS The primarily Model 37 oriented delays may not be
appropriate for all other ASCII terminals.
OWNER ken, dmr, jfo