of Microprocessors

342 MUSICAL ApPLICATIONS OF MICROPROCESSORS
and Y deflection of the beam. Unfortunately, home TV sets or standard video
monitors do not fall into this category. Oscilloscopes, however, do provide
random positioning capability, but the screen size is fairly small. A large
screen random deflection display monitor is currently a specialized, expensive
device. Another problem is that without an adjunct character generator
circuit and small-angle deflection circuit the amount of text that can be
displayed is limited.
A Simple Vector Display
Perhaps the best way to become acquainted with the characteristics of
vector displays is to describe a simple unit that is easily built, is inexpensive,
and gives surprisingly good performance. It may be interfaced to any computer
that has two 8-bit output ports. The output consists of X, Y, and Z
(beam on-off control) voltages that can use any oscilloscope having dccoupled
amplifiers as a display monitor. The resolution is 256x 256 raster
units, but the quality is equivalent to much larger dot-type displays.
One fact of life is that the image on the tube must be rewritten
continuously. This is called refreshing the display and is necessary to prevent
it from fading away within a fraction of a second. For most display monitors,
the image must be refreshed at least 40 times/sec to avoid flicker. Maximum
image complexity is directly proportional to the number of lines that may be
drawn in one refresh interval. Very detailed, complex drawings are possible,
however, if some flicker is allowed. It is also sometimes possible to obtain an
oscilloscope with a long-persistence phosphor in which the image does not fade
so quickly. These are usually yellow or orange in color as opposed to green
and are commonly used in medical applications. The display to be described
requires about 50 p,sec to draw a line. Thus, about 500 lines may be drawn in
the 25-msec interval allowed for a 40-Hz refresh rate.
Two 8-bit output ports are used to control the display. Port one is used
for specifying X and Y coordinates. The coordinates are unsigned numbers in
the range of 0 to 255 wi th zero corresponding to the bottom and left edges of
the screen. Only 4 bits of the other port are used. The "save X" bit when a
logic one causes the content of port one to be interpreted as an X coordinate
and stored in an internal register. The "move X" bit causes the previously
saved X to be sent to the X DAC, which then immediately moves the CRT
beam to the new X position. "Move Y" immediately transfers port 1 to the Y
DAC and moves the beam. The fourth bit is called "draw," which turns the
beam on for 50 p,sec and sets up for controlled movement so that straight lines
are drawn. For proper operation of draw, move X and move Y should be set
simultaneously with draw.
A typical sequence for drawing a line between two arbitrary endpoints,
Xl,Yl and X2,Y2 would be as follows:
1. Initially port 2 (the control port) is zeros.