computer’s 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 cosine. 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. The 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 5-8