of Microprocessors

CONTROL SEQUENCE DISPLAY AND EDITING 349
the filter about 20-fold. The switches themselves are inexpensive CMOS
transmission gates. Since the voltages being switched are between +2.5 V
and +7.5 V plus some margin for overshoot, the CMOS is powered between
+ 10 V and ground. This also allows simple open-collector TTL gates to
drive the switches.
Adjustment of the circuit is fairly simple. First the gain and offset pots
on each DAC must be adjusted so that 00 gives +2.5 V output and FF gives
+ 7.5 V. Next, using an oscilloscope, the low-pass filters in the vector
generator should be adjusted for identical step response shape and time to
first final value crossing. If the capacitors are initially matched to within 1%
or so, then shape matching should not be a problem and the two pots in the
X axis circuit can be adjusted for time matching. Finally, using a test pattern
consisting of a diamond (all lines at 45°), the end match pot should be
adjusted so that the lines just meet. If one line passes the other before
meeting, then the step response balance should be touched up a bit. Any
curvature of the lines due to the vector generator can be ascertained by
displaying a 45° angle line and then a series of points (lines with zero
length) along the path that should have been traversed by the line.
Note that very little additional expense is involved in improving the
resolution of the display. For example, the number of addressable points may
be increased 16-fold merely by using lO-bit DACs and a couple of extra latch
packages. With 12-bit DACs, the taster unit size becomes so small that X
and Y coordinates become essentially "continuous" quantities. The significance
of this is seen by considering the display of, say, an arbitrary number of
equally spaced lines in a fixed area of, say, one-half the screen width. With
256 raster units, one-half of the screen would be 128 raster units. If 47 lines
need to be displayed, the space between lines should be 128/47 = 2.723
raster units. On the screen some lines would actually be three units apart
while others are only two, obviously not equally spaced. With a 4096
raster-unit display, however, the lines would be 43 and 44 units apart and
thus appear quite equally spaced on the screen.
Raster Scan Displays
The other major CRT display type is commonly called a raster scan
display. This is because the deflection circuits in the CRT monitor constantly
scan a rectangular area in a set pattern consisting of numerous parallel
horizontal lines. As the beam scans, it may be turned on and off at controlled
times to show a dot pattern representing the desired image. Since the deflection
amplifiers always handle the same waveform (normally a sawtooth) at a
constant frequency, they may be made quite inexpensively even for the high
power levels required for large-screen magnetic deflection tubes. This and
the fact that our national television system works on the same principle are
key advantages of raster scan displays. Note that since time is the only
variable required to control dot position, the raster scan display is inherently
a digital device.