Heathkit H9 Manual page 54

OU TPUTS
0r 0a 0
t " n" ut
@
ROW
ADD RESS
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
t0 l
tI 0
II l
O W O IS ALW AYS B LA NK
PICTORIAL 4.5
Since the electron beam in the CRT scans only one
line at a time, the dots that make up the characters
must be shifted out serially, one row at a time. To do
this, the character generator puts out up to five dots in
a row for each character. Pictorial 4-5 shows the row
outputs for the character H. The S-bit dot data is
always followed by three spaces (the absence of dots)
before the dots for the next character are shifted out.
Since the CRT scanning process requires the dots data
to be applied to the video circuits in a serial format,
the s-bit dot data is loaded into a shift register and
shifted eight times. This shifts out the five bits of dot
data followed by three spaces. The three spaces are
formed when a blanking signal is applied to the video
circuits during the fifth through seventh shift
sequences. They are also necessary because the
character location on the screen has been defined as
being eight rows wide. The spaces provide space
between characters.
During the fifth through the seventh shift sequence,
the shift register is loaded with new dot data for the
next character. The RAM address counter is also
incremented to its next address. This process of
addressing succeeding RAM location and outputting
dot data continues until a horizontal sync pulse is
generated at the end of a scan line. The horizontal
sync pulse increments the row counter and causes the
electron beam to begin a new scan line. The character
generator now outputs dot data for the next row in the
dot matrix, This process continues until all seven
rows of dot data have been outputted. Horizontal scan
lines 8 through 15 are normally blanked. The cursor,
will appear on lines 12 and 13. A new character line
(16 scan lines) begins after scan line 15 is completed.
The cursor is displayed on scan lines 12 and 13 when
an unblanking signal is applied to the video circuits.
This happens when the cursor location, which is
stored in the cursor latch, is coincident with the
address coming from the RAM address counter.
TECHNICAL DESCRIPTION
The master clock of the entire Terminal is located on
the character generator circuit board and it is com-
posed of IC210B, crystal Y2O'1., and transistor Q201,
The clock frequency is 12.395 MHz. The true and the
complemented outputs are provided for the dot clock
and the other system timing. lC's 214,27OC,227D,
277C, and 207,4' generate timing signals A, E, and F
from the master clock signal. Refer to timing diagram
#7 for these waveforms. Keep in mind as you study
the waveforms, that the trailing edge (the negative
transition) of the clock pulse always preceeds any
logic level transitions of the other IC's by approxi-
mately 30 nanoseconds.
At the start of the character generation cycle, the RAM
counter (on the RAM and counter circuit board) is
told by the trailing edge of waveform F to count up.
After 565 nS, the RAM data is latched into IC2O2 and
IC203 and then transferred to the input of the charac-
ter generator by the next leading edge of waveform F.
Eighty nS later, the trailing edge of waveform F tells
the RAM to count up again. In the meantime, the
character generator is starting to access its internal
dot information and, after approximately 450 nS, the
five bits of dot data appear at the output of the charac-
ter generator. Waveform A is used as a load pulse for
shift registerlC2O4. The dots are latched into the shift
register by the trailing edge of the dot clock before the
load pulse goes back to zero. Actually, the shift regis-
ter is loaded three times (at t6, t7, and t0), but only the
load at t0 is used. The next five cycles of the dot clock
shift dots out serially as video information, which, in
turn, is applied to the CRT, At the end of the fifth
pulse, waveform A is used as a blanking signal to
provide inter-character blanking, Besides latching
RAM data for the character generator, waveform F is
used in increment the character countet, which is
composed of IC's 215 and 220. After 80 characters
have been accessed and shifted out, the D output of
ICz15 provides a retrace blanking signal. This signal
blanks the video during the retrace time and it also
inhibits further RAM count-ups.