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The Plessey PT-100G is a high-resolution (1024 x 768) low-cost ($1395)
VT52/VT100/VT102/Tektronix-4014 compatible terminal. Its lower cost,
higher resolution, and better features and performance make it the best
choice today for low-budget graphics, replacing the Visual 500.
VT100-compatible terminals often have difficulty keeping up in
alphanumeric mode without XON/XOFF flow control enabled. The PT-100G
has a 256-character input buffer, and was usable for EMACS editing with
flow control disabled, although the screen occasionally lost a line or
two. When flow control is enabled, it becomes unusable with EMACS,
because the XOFF (CTL-S) character sent initiates a string search.
Severe data loss occurs with graphical output unless flow control is
selected. Unfortunately, this is only alterable in setup mode; it
cannot be changed by a command sequence from the host.
The screen displays either 24 or 48 lines, with 80 or 132 columns. In
48V x 132H mode, the characters are small, but still readable. The
monitor has a 14-inch diagonal (standard terminal size) and uses a P39
phosphor; this has a long persistence, causing objectionable ghosting
of scrolling text. This problem can be minimized by switching the
display to inverse video, which is possible with the Visual 500, but
not with the PT-100G.
This terminal has 4 additional keys for control of zoom and pan.
Unlike most others with these features, which implement them in the
hardware video generator, the PT-100G stores the vectors locally, then
redraws the display when the zoom is selected or deselected. When the
Zoom Out key is touched, a rectangle covering the zoomed area appears
on the screen. Its size can be adjusted up or down with two dedicated
keys marked by large and small rectangles; it can be moved in any of 8
directions with the arrow keys on the numeric keypad. Pressing the
shift key increases the speed of both these operations. When the Zoom
In key is touched, the display is cleared and the selected area is
redrawn in the entire screen area. Pressing the Zoom Out key clears
the display and redraws the original picture.
This software zoom is considerably slower than hardware zoom, but has
the advantage that the scaling is continuous and preserves single-pixel
lines, instead of being restricted to pixel replication factors of 1 ..
8 or 16.
The standard model has space for about 6500 vector vertices; any number
of vectors can be drawn, but if the zoom feature is activated, the last
part of the display will be lost when it is redrawn. Memory extensions
are available to increase the space to about 98000 vertices, and I
would strongly recommend their purchase.
The terminal setup allows setting of both the X and Y scale factors and
screen origin for converting Tektronix 4010 (0 .. 1023) and 4014 (0 ..
4095) coordinates to screen coordinates.
Vector drawing performance of any graphics terminal is very important,
so several tests were made comparing the PT-100G with the Encore
HostStation 110, which so far has been the fastest graphics terminal I
have found. The first set drew 1025 horizontal, vertical, and diagonal
lines on a square filling the shortest extent of the screen; this tests
performance of the vector drawing for long vectors. If special case
code has been developed for the case of dots falling into consecutive
memory locations, one would expect that at least one of these should
run 16 to 32 times faster. Neither the PT-100G nor the HS-110 have
this optimization. The second set of tests are Sierpinski curves from
DEM81 in Cartesian, plane polar, and elliptical coordinates; these
emphasis short vector performance, with about equal numbers of
horizontal, vertical, and diagonal vectors.
-------------------------------------
Test PT-100G HS-110
(sec) (sec)
-------------------------------------
horizontal 17 12
vectors
vertical 15 12
vectors
diagonal 31 14
vectors
Sierpinski 63 60
Cartesian
Sierpinski 101 101
plane polar
Sierpinski 102 102
elliptical
-------------------------------------
The short vector performance of the PT-100G is almost identical to the
HS-110, while the long vector performance indicates room for
improvement, particularly for diagonal lines. For typical plots, the
two can be considered comparable in speed, which speaks well for the
PT-100G. When the zoom feature was used for the third test, the 31 sec
was reduced to 24 sec when the plot was redrawn from the internal
display list.