of Microprocessors

CONTROL SEQUENCE DISPLAY AND EDITING 347
OUTPUT
INPUT 51,52
1.0 5T\ /CL05ED
OUTPUT
51,52
OPEN
0.8
0.6
0,4
0.2
0'--------------------------
TIME
Fig. 11-3. Step response of vector generator
a convenient interface to the vector generator about to be described and also
includes a timer that can be used to conrrol the refresh rate.
Vector Generator Circuit
Figures 11-2A and B show a schematic of the vector display generator.
The digital portion of the interface is quite simple, consisting mostly of type
74LS75 quad transparent latches. The latches are necessary because the same
port is used for both X and Y coordinates and to insure that the X and Y
DACs are updated simultaneously when a line is to be drawn. The term
transparent means that as long as the clock input is high, the input data
passes straight through to the output. Thus, a move X operation, which calls
for setting both "save X" and "move X" conrrol bits high, enables the clocks
on both sets of X latches allowing data to pass through the leftmost set and
be latched into the rightmost set. The single shot is used to carefully control
the beam on time independent of program timing when a draw is executed.
This prevents bright dots at the ends of lines. The Z axis output is boosted to
15 V, since most oscilloscopes require high-level drive to control the beam.
The heart of the analog section is the X and Y DACs. Type MC140SLS
DACs are used in a circuit similar to that in Fig. 7-IS. The offset circuit has
been modified, however, to provide a voltage output (from current-to-voltage
converter op-amp) of + 2.5 for a digital input of 0 and + 7.5 for an input of
255. The type TLOOs4 is a very convenient quad FET input op-amp selected
here for its speed.
Unfortunately, generating the X and Y voltage contours necessary for
drawing a smooth, uniformly bright line between arbitrary endpoints on a
CRT is not as simple as it sounds. The task is called vector generation and
circuits that do it are called vector generators. An ideal vector generator would
move the beam at a constant, uniform velocity regardless of the line length or
orientation, would be free of wiggles and other distortions at the endpoints,
and would have good matching of lines sharing the same endpoint. To make
matters worse, the necessary analog computations must be done accurately at
high speeds. The vector generator used in this simple display sacrifices