computers program determines which mode is
initiated in the symbol generator.
Line Mode. When a line is to be drawn on the
CRT, the line mode signal goes to the input/output
buffer of the symbol generator. This signal starts a
counter that controls all operations of the generator.
The first operation is to transfer the X 1 data from
memory to the X channel deflection register. It is
transported through the adder and input/output buffer
and control. The X 1 data is the digital equivalent of
the analog voltage that drives the beam to the start
point (in the horizontal axis) of the line draw.
The second operation is to transfer the Y1 data
from memory to the Y channel deflection register.
The Y1 data is the digital equivalent of the analog
voltage that drives the beam to the start point (in the
vertical axis) of the line draw. The digital data in each
of the deflection registers immediately converts to an
equivalent analog voltage. This conversion takes
place in the X and Y channel digital-to-analog
converters (DACs). Each DAC holds the X and Y
deflection voltage until the input/output buffer and
control gates them to the HUD.
The third operation transfers the X2 data from
memory to the parameter register. The X2 data is a
digital quantity representing the line slope angle
In the first half of the fourth operation, the X2
data shifts from the parameter register to the X
channel rate register.
In the second half of this
operation, the Y2 data transfers to the parameter
register from memory.
The Y2 data is the digital
quantity representing the line slope angle sine.
In the first half to the fifth operation, the Y2 data
shifts from the parameter register to the Y channel
rate register. In the second half of this operation, the
T data transfers from memory to the parameter
register. The T data is a digital quantity representing
the length of the line draw (bright-up pulsewidth).
During the sixth and seventh operations, the BITE
circuits check all data in the X and Y channel rate
registers for correctness. All the data required to
draw a specific line is now within the symbol
generator. The specific line to be drawn could be a
symbol in itself or just part of a symbol.
When all the data to draw a specific line is in the
symbol generator (after the seventh operation), the
busy signal from the HUD is sampled. If the busy
signal is high, the operations stop until the busy signal
is low. The low signal signifies that the HUD is ready
to accept the data.
At this time, the input/output
buffer and control sends a start pulse to the parameter
register. This causes it to start down counting. The
down counting controls the length of time the
bright-up pulse is applied to the HUD. At the same
instant, a gating pulse goes to each DAC. This allows
an analog voltage to go to the deflection circuits of the
HUD. At the same time, the contents of the rate
registers either add to or subtract from the respective
residue register. This causes generation of either a
positive or negative overflow. The overflow drives
the deflection register up or down. This causes the
start point of the line to move in the direction of the
line slope angle. After a preset time delay, a bright-up
pulse goes to the HUD. The time delay compensates
for the slower response time of the deflection circuits
in the HUD. The bright-up pulse continues until the
parameter register has down counted to zero. When
the parameter register stops counting, the bright-up
pulse turns off. At this time the end of the line mode
is indicated to the digital computer. The line mode
may be repeated as many times as necessary to
complete the required symbol.
Circle Mode. When drawing a circle on the
CRT, a circle mode signal goes to the input/output
buffer and control. The signal starts the operation
counter as in the line mode. The data required to draw
a circle in a specific location transfers to the symbol
generator in the same manner as in the line mode.
However, the X2 data is equal to negative one, and the
Y2 data is all zeros. The T data set into the parameter
register is the digital equivalent of one over the circle
radius. Circles are drawn in a counterclockwise
direction, starting from the top. The overflow from
the residue registers controls the direction and amount
of change in each deflection register. However, in the
circle mode, the overflow also controls the circle
logic to the opposite channel. The circle logic is such
that a positive X channel overflow causes the contents
of the parameter register to add to the contents of the
Y channel rate register. If the X channel overflow is
negative, then the contents of the parameter register
subtract from the Y channel rate register.
opposite occurs if the overflow from the Y channel
residue register is positive.
This will cause the
contents of the parameter register to subtract from the
X channel rate register. If the Y channel overflow is
negative, the contents of the parameter register add to
the X channel rate register.
This cross-coupling causes the two channels to be
interdependent on each other. As the rate of change in