VT (IV) 2/11/73 VT (IV)
NAME vt -- 11/20 (vt01) interface
DESCRIPTION
The file vt0 provides the interface to a PDP 11/20 which
runs both a VT01A-controlled Tektronix 611 storage dis-
play, and a Federal Screw Works (Vocal Interface Divi-
sion) voice synthesizer. The inter-computer interface is
a pair of DR-11C word interfaces.
Although the display has essentially only two commands,
namely "erase screen" and "display point", the 11/20 pro-
gram will draw points, lines, and arcs, and print text on
the screen. The 11/20 can also type information on the
attached 33 TTY and generate utterances via the voice
synthesizer.
This special file operates in two basic modes, selected
by bit 2 (octal 04) on the 11/20's console switches. If
this bit is on at the opening of the file, all bytes
written on the file are interpreted as ASCII characters
and written on the screen. The screen has 33 lines (1/2
a standard page). The file simulates a 37 TTY: the con-
trol characters NL, CR, BS, and TAB are interpreted cor-
rectly. It also interprets the usual escape sequences
for forward and reverse half-line motion and for full-
line reverse. Greek is not available yet. Normally,
when the screen is full (i.e. the 34th line is started)
the screen is erased before starting a new page. To al-
low perusal of the displayed text, it is usual to assert
bit 0 of the console switches (octal 01). As explained
below, this causes the program to pause before erasing
until one of the attached pushbuttons is depressed.
If bit 2 of the switches is down, the display is in
graphic mode. In this case bytes written on the file are
interpreted as display and vocal commands. Each command
consists of a single byte usually followed by parameter
bytes. Often the parameter bytes represent points in the
plotting area. Each point coordinate consists of 2 bytes
interpreted as a 2's complement 16-bit number. The plot-
ting area itself measures (+03777)X(+03777) (numbers in
octal); that is, 12 bits of precision. Attempts to plot
points outside the screen limits are ignored.
The graphic and sonic commands are:
order (1); 1 parameter byte
The parameter indicates a subcommand, possibly fol-
lowed by subparameter bytes, as follows:
erase (1)
The screen is erased. This action may be de-
layed, as explained below, until a pushbutton
is depressed.
label (2); several subparameter bytes
The following bytes up to a null character are
taken as a label and typed on the console TTY.
One of the console switches gives labels a spe-
cial interpretation, as explained below.
display label (3); several subparameter bytes
The following bytes up to a null byte are
printed as ASCII text on the screen. The ori-
gin of the text is the last previous point
plotted; or the upper left hand of the screen
if there were none.
point (2); 4 parameter bytes
The 4 parameter bytes are taken as a pair of coordi-
nates representing a point to be plotted.
line (3); 8 parameter bytes
The parameter bytes are taken as 2 pairs of coordi-
nates representing the ends of a line segment which
is plotted. Only the portion lying within the
screen is displayed.
frame (4); 1 parameter byte
The parameter byte is taken as a number of sixtieths
of a second; an externally-available lead is as-
serted for that time. Typically the lead is con-
nected to an automatic camera which advances its
film and opens the shutter for the specified time.
circle (5); 6 parameter bytes
The parameter bytes are taken as a coordinate pair
representing the origin, and a word representing the
radius of a circle. That portion of the circle
which lies within the screen is plotted.
arc (6); 12 parameter bytes
The first 4 parameter bytes are taken to be a
coordinate-pair representing the center of a circle.
The next 4 represent a coordinate-pair specifying a
point on this circle. The last 4 should represent
another point on the circle. An arc is drawn
counter-clockwise from the first circle point to the
second. If the two points are the same, the whole
circle is drawn. For the second point, only the
smaller in magnitude of its two coordinates is sig-
nificant; the other is used only to find the quad-
rant of the end of the arc. In any event only
points within the screen limits are plotted.
dot-line (7); at least 6 parameter bytes
The first 4 parameter bytes are taken as a
coordinate-pair representing the origin of a dot-
line. The next byte is taken as a signed x-
increment. The next byte is an unsigned word-count,
with "0" meaning "256". The indicated number of
words is picked up. For each bit in each word a
point is plotted which is visible if the bit is "1",
invisible if not. High-order bits are plotted
first. Each successive point (or non-point) is off-
set rightward by the given x-increment.
speak(8); several parameter bytes
The following bytes up to a null byte are taken to
represent phonemes which are fed to the voice syn-
thesizer. vsp(VII) gives the encoding.
The 3 low-order console switches of the 11/20 modify the
operation of the display as follows.
Bit 2 (octal 04) is examined at the time the display file
is opened (more precisely, when the first byte is written
after an open); as indicated, when on it selects charac-
ter mode, otherwise graphic mode.
Bit 1 (octal 02) determines whether TTY labels are to be
interpreted. Unless this bit is on, labels are ignored.
(except to terminate skip mode, see below).
Bit 0 (octal 01) determines whether the display will
pause before erasing the screen; if off there will be no
pause. If bit 0 is on, the erase will occur and display-
ing will resume only when one of the 16 pushbuttons is
depressed.
There is a box with 16 pushbuttons connected to the
11/20. Their state is at all times available in the
11/45 by executing the csw system call (II). They are
used by the 11/20 when it is pausing before an erase. 14
of the buttons merely serve to allow the display to con-
tinue. If, however, button 7 is pushed, the display will
ignore commands up to the next erase command, then ring
the TTY console's bell, thereby skipping an entire pic-
ture.
If button 8 is depressed, the display will ignore com-
mands up to the next TTY label (whether or not its typing
is suppressed) before resuming the displays. Thus a se-
quence of frames may be skipped.
FILES /dev/vt0
SEE ALSO csw(II), vsp(VII)
BUGS Two users using vt0 simultaneously can interfere
with each other, e.g. plot phonemes or speak dis-
play coordinates.