The System Manual of SOL-20 - History of Computers

PROCESSOR TECHNOLOGY CORPORATION
Sol THEORY OF OPERATION
SECTION VIII
In the Sol, the display format is 64 characters maximum per
character row, with a maximum of 16 rows per frame (page). Thus,
up to 1024 characters can be displayed per page.
A 9x13 (columns by lines) dot area, or character position,
is alloted on the monitor screen for each displayed character (see
Figures 8-2 and 8-3 on Page VIII-24). Consequently, each character
row consisting of sixty-four 9 x 13 dot areas requires 13 horizontal
scan lines. To provide spacing between both characters and rows,
only 12 dot lines and seven dot columns within the 9 x 13 matrix are
used for character display. Only nine of the available 12 dot lines,
however, are used for any given character.
Let's take a closer look at how the 9 x 13 dot matrix is used.
The first seven dot columns are available for all character displays;
the last two are used to provide a space between characters. The
first dot line in a character row is always blank to provide a space
between character rows. As shown in Figure 8-2, the second through
tenth dot lines are available for all upper case (capital) and control
characters, all symbol and punctuation marks (except the comma
and semicolon), and all lower case characters (except the g, j, p,
q and y). As shown in Figure 8-3, dot lines five through 13 are
available to display characters that normally extend below the base
line--lower case g, j, p, q and y plus the comma and semicolon.
Now that we have a feeling for how characters are formed on
the video monitor screen, we will move on to the circuit description.
Refer to Display Section Schematic in Section X, Page X-18.
The 14.31818 MHz DOT_CLOCK, which defines the period of one
dot (69.8 nsec) in a character display matrix, controls all timing
in the Video Display Generator. DOT_CLOCK is applied to pin 2 of
U28, a four-bit binary counter that is preset to count from seven
through 15 to divide DOT_CLOCK by nine. Two 1.591 MHz outputs are
supplied by U28: LOAD_CLOCK on pin 11 and !CHARACTER_CLOCK on pin 12.
Pin 11 is a low-active pulse of one DOT_CLOCK duration. Pin 12 is
high for five and low for four DOT_CLOCK periods. Both the LOAD_ and
!CHARACTER_CLOCK low-to-high transitions occur synchronously on the
same DOT_CLOCK.
!CHARACTER_CLOCK, which defines the period of one character
position (628 nsec), is inverted in U49 to become CHARACTER_CLOCK.
It performs most of the clocking functions in the Video Display Generator
and is made available on pin 4 of J4 for use by external
graphic display devices.
CHARACTER_CLOCK is in turn divided in U31 and U33, both of
which are presettable four-bit binary counters. Both start at count
3 when pin 8 of NAND gate U47 is low, and together they count 102
CHARACTER_CLOCKS to define horizontal timing at 64 usec (102 x 628
nsec = 64 usec).
VIII-23