DC (IV)                      6/12/72                      DC (IV)

NAME            dc -- DC-11 communications interfaces


        The special files /dev/tty0, /dev/tty1, ...  refer to the

        DC11 asynchronous communications interfaces.  At the mo-

        ment there are ten of them, but the number is subject to


        When one of these files is opened, it causes the process

        to wait until a connection is established.  (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 con-

        trol typewriter is inherited by a child process during a


        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 charac-

        ters which have not yet been read by some program.  Cur-

        rently this limit is 150 characters.  When this is hap-

        pening the character "#" is echoed for every lost input


        When first opened, the interface mode is ASCII charac-

        ters; 150 baud; even parity only accepted; 10

        bits/character (one stop bit); and newline action charac-

        ter.  The system delays transmission after sending cer-

        tain 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 partic-

        ular 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 addi-

        tion, the following software modes can be invoked: accep-

        tance 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 printing of tabs as

        spaces; and setting the system to handle IBM 2741s.  See

        getty(VII) for the way that terminal speed and type are


        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 how-

        ever necessary to read a whole line at once; any number

        of characters may be requested in a read, even one, with-

        out 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 inde-

        pendently 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.

        It is possible to use raw mode in which the program read-

        ing is awakened on each character.  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

        occurred at the beginning of a line, zero characters will

        be passed back, and this is the standard end-of-file sig-

        nal.  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) any read re-

        turns 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 the

        interrupt signal.  When this character is received from a

        given typewriter, a search is made for all processes

        which have this typewriter as their control typewriter,

        and which have not informed the system that they wish to

        ignore interrupts.  If there is more than one such pro-

        cess, one of these is selected, for practical purposes at

        random.  The process is either forced to exit or a trap

        is simulated to an agreed-upon location 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 gen-

        erated.  See quit(II).  The character is not passed on

        except in raw mode.

        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 transmit-

        ted 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.  Because the 2741 is inher-

        ently half-duplex, input is not echoed.  Proper function

        delays are provided.  For 2741s without a feature known

        as "transmit interrupt" it is not possible to collect in-

        put ahead of the time that a program reads the type-

        writer, because once the keyboard has been enabled there

        is no way to send further output to the 2741.  It is cur-

        rently assumed that the feature is absent; thus the key-

        board is unlocked only when some program reads.  The in-

        terrupt signal (normally ASCII DEL) is simulated when the

        2741 "attention" key is pushed to generate 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.  Currently IBM EBCDIC is default for input and

        output; 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           /dev/tty[01234567ab]    113B dataphones

                /dev/ttyc               display data set

                /dev/ttyd               113B with /dev/dn1

SEE ALSO        kl(IV), getty(VII)

BUGS            The primarily Model 37 oriented delays may not be

                appropriate for all other ASCII terminals.