DC(IV) 8/22/73 DC(IV)
NAME
dc - DC-11 communications interface
DESCRIPTION
The special files /dev/tty0, /dev/tty1, ... refer to the
DC11 asynchronous communications interfaces. At the moment
there are 12 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 established. In practice user's
programs seldom open these files; they are opened by init
and become a user's 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 inherited by a
child process during a fork.
A terminal associated with one of these files ordinarily
operates in full-duplex mode. Characters 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 256
characters. When the input limit is reached all the saved
characters are thrown away without notice.
When first opened, the interface mode is 150 baud; either
parity accepted; 10 bits/character (one stop bit); and
newline action character. The system delays transmission
after sending certain function characters. Delays for
horizontal tab, newline, and form feed are calculated for
the Teletype 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 following hardware states are program
settable independently for input and output (see DC11
manual): 134.5, 150, 300, or 1200 baud; one or two stop bits
on output; and 5, 6, 7, or 8 data bits/character. In
addition, the following software 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
carriage 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; and the printing of tabs as spaces.
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 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
characters may be requested in a read, even one, without
losing information.
During input, erase and kill processing is normally 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 basis 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 `\' disappears.
In upper-case mode, all upper-case letters are mapped into
the corresponding lower-case letter. The upper-case letter
may be generated by preceding it by `\'. In addition, the
following escape sequences are generated on output and
accepted on input:
for use
` \'
| \!
~ \^
{ \(
} \)
It is possible to use raw mode in which the program reading
is awakened on each character. In raw mode, no erase or
kill processing is done; and the EOT, quit and interrupt
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 occurred at
the beginning of a line, zero characters will be passed
back, and this is the standard end-of-file signal. The EOT
is not passed on except in raw mode.
When the carrier signal from the dataset drops (usually
because the user has hung up his terminal) a hangup signal
is sent to all processes with the typewriter as control
typewriter. Unless other arrangements have been made, this
signal causes the processes to terminate. If the hangup
signal is ignored, any read returns with an end-of-file
indication. 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 `rubout') is not passed to a
program but generates an interrupt signal which is sent to
all processes with the associated control typewriter.
Normally each such process is forced to terminate, but
arrangements may be made either to ignore the signal or to
reveive a simulated trap to an agreed-upon location. See
signal (II).
The ASCII character FS generates the quit signal. Its
treatment is identical to the interrupt signal except that
unless a receiving process has made other arrangements it
will not only be terminated but a core image file will be
generated. See signal (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 typing. Input characters are echoed by putting
them in the output queue as they arrive. When a process
produces characters more rapidly than they can be typed, it
will be suspended when its output queue exceeds some limit.
When the queue has drained down to some threshold the
program is resumed. Even-parity is always generated on
output. The EOT character is not transmitted (except in raw
mode) to prevent terminals which respond to it from hanging
up.
FILES
/dev/tty[01234567abcd] 113B Dataphones
SEE ALSO
kl (IV), getty (VII), stty (I, II), gtty (I, II), signal
(II)
BUGS