Heathkit H9 Manual page 55

Actually, the D output of IC215 does not turn on the
blanking directly. It is delayed by two character times
in IC211 to allow time for the last character accessed
to be shifted out. The outputs of IC211A generate
blanking and sync pulses and count up IC2 13 , the row
counter, which tells the character generator to access
the next row of dot information. When the retrace
blanking period is over, the process is repeated until
all eight scan lines of information for the same 80
characters have been shifted out, and a complete row
of characters is displayed on the screen. During the
next eight scan lines, the entire process is repeated
and the same 80 characters are shifted out once again,
but the video is blanked to provide spacing between
character lines.lC222C decodes the row counter out-
put of IC273 so that the cursor information can only
appear in scan rows 12 and 13 of the character row
cycle.
At the end of the 16th scan line, the D output of IC2 13
(the row counter) is used to count up the +12
(divide-by-12) counter in the RAM to access the next
row of 80 characters. It is also used to increment the
sync or frame counter, 1C21.6, which keeps track of the
number of rows of characters that are displayed on the
screen. At the end of the 16th scan line of the tZth
character row, the C and D outputs of IC216 generate
the vertical blanking signal. IC's 209C and 221,{,in
conjunction with the vertical blanking signal, gener-
ate the vertical sync pulse to start vertical retrace.
Refer to waveforms 1 through 22 on timing diagram
#2 for a description of the scan and frame cycles.
Cursor information from the RAM and TPU is latched
into IC202 along with the ASCII data by waveform F.
This cursor signal is delayed by IC208A for the
duration of the character generator access time and it
appears as video information at inputs 1 and 4 of
IC278 in the video chain.
If the ASCII character latched in IC's 2O2 and 203 is a
control character, bits o and 7 will be 0 and the Q
outputs of the latches will both be high causing the
output of IC2O1A to go to a logic 0. This logic 0 is
delayed by IC?O7B for the duration of the character
generator access time and it is used to blank the dot
information for that control character. The preset
input of IC207B is driven by a4Hz clock pulse, which
will unblank the control character dots 4 times a
second, causing the character to appear to blink on
the screen.
In the plot mode, the first 128 scan lines are used to
display information that corresponds to the binary
value of a 7-bit ASCII word. In this mode, the first row
of characters that would normally be seen on the
screen is repeated 4 times, starting with the 129th
scan line (in the 9th, 10th, 11th, and 12th character
lines). In the space directly above each character, the
beam is turned on to display a lighted dash in the scan
line that corresponds to the binary value of that
character. The 128 scan lines are arranged so that the
128th line (line 127) is at the top of the screen and the
first line (line 0) is at the bottom of the 8th character
line (128 lines from the top). For example, if the
character was a rubout, which is all 1's the dash
would appear at the very top of the screen. If the
character was a K (binary 1001011), its value would
be 75 and it would appear on plot line 75. Refer to
Pictorials 1.-2 (Page 6) and 1-3 (Illustration Booklet,
Page 2).
The four outputs of IC213 and the A, B, and C outputs
of IC216, count the scan lines from 0 at the top to'1.27
at the end of the 8th character row. The seven outputs
are inverted by 1C272 and IC219A so that the count
will be 727 at the top and 0 on the 127th row. The
outputs of the inverters are applied to the inputs of
exclusive NOR gates (IC206 and IC209A, B, and D).
The other inputs to the NOR gates are connected to
corresponding bits from the outputs of latches lC2O2
and IC203. When the seven data bits at the latches are
exactly coincident with the outputs of the inverters, a
logic 1 will appear at the outputs of IC206 and IC209.
If the plot function has been selected by the Plot
button on the keyboard, this coincidence signal is
gated through IC201B to the input of ICz08A. After a
delay of one character time, the plot coincidence sig-
nal will appear at the output of IC20BA as a plot video
signal that will then be displayed on the screen as a
short dash. During the plot time, the cursor informa-
tion is blanked by the output of IC201D and the ASCII
dot information is blanked by the output of IC221,8.
The video and blanking channels can be broken down
into three video channels and two basic blanking
channels. The dot information video channel consists
of IC210D, ICz19D, and IC218C. If there are no dot
blanking signals present, the dots that shift out of
IC2O4 will appear directly at pin 8 of IC218C. The dot
video will be blanked by the output of IC207B if the
dots represent a control character by the 9-to-15 line