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15-9. Characters are displayed on the CRT as eight-by-eight dot matrices. Thirty-two dot matrices, of which
the last two are always blank, make one character line. Sixteen lines, of which the last line is always blank,
complete the display area. Therefore, the total number of matrices available for character display is 30 x 15 or
450 matrices. The blank matrices and the blank line is used for horizontal and vertical retrace blanking,
respectively. The display is generated by dot rows. As the CRT sweeps the first dot row of a character line, the
character generator outputs a serial bit pattern of 1 'sand D's that turn the crt intensity on and off. The result is a
row of dots that when combined with the next seven rows form a character.
15-10.
A select switch, on the data and address buses to the RAM, toggles at the master clock rate of 1 MHz.
This results in the processor and the character generator having access to the RAM alternately every other 0.5
microseconds. The processor stores in the RAM an 8-bit word that represents the character to be displayed.
The character generator scans the RAM in sequence with the CRT display scan. As each location in the RAM is
addressed, the 8-bit word stored at that location is latched by the 8-bit latch. Seven of the bits in the latch are
applied to the character ROM, the eighth bit is not used. The timing generator provides 3 bits which indicates
which row of dots are being scanned. The 10 bits applied to the character ROM, define a specific dot row of a
particular character. An 8-bit pattern, defining that row of the character, is parallel-loaded into the 8-bit shift
register. These 8 bits are serially shifted out of the register, at a 2 MHz rate, as the CHAR GEN Z-AXIS output
signal.
15-2
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