Theory of Operation—2465B/2467B Service
STORING A CHARACTER. Displaying a character
starts with serially clocking 16 character data bits into a
16-bit shift register formed by registers U2960 and U2860.
The ROS1 strobe (readout strobe one) from the Address
Decode circuitry (diagram 1) is the clocking signal. The first
eight bits of the loaded data indicate the character to be
displayed,
while the last eight select the location on the crt
that the character is to be displayed.
by U2965A to produce a positive transition that shifts the
data bit present at U2960 pin 9 (Q
SH
) into U2860. After
15 ROS1 strobes have occurred, seven bits of character
data are latched into U2860, and the eighth character bit
and seven of the character address bits are latched into
character address register U2960 (though they have not
been shifted into their correct positions for addressing the
RAM).
On positive-going transitions of the ROS1 strobe, the
data bit present on the BDO data line is shifted into the
first latch of character address register U2960. The follow-
ing negative-going edges of the ROS1 strobe are inverted
At this point, the last character bit remains to be shifted
into the registers, but the operating mode must be set up
first to ensure correct operation upon shifting in the final
bit. The eight bits of mode data are shifted into the mode
8 HORIZONTAL DOT POSITIONS
WITHIN EACH HORIZONTAL
DISPLAY AREA-DETERMINED BY
BITS 0-2 OF CHARACTOR ROM
SMALL CHARACTERS
ARE DISPLAYED IN
TOP HALF OF A
GIVEN DISPLAY AREA
^^
16 VERTICAL
DOT POSITIONS
ABOVE LINE
REFERENCE- <
DETERMINED BY
BITS 3-6 OF
CHARACTER ROM
*CA5 LINE
REFERENCE'
/-
-■
"f
1
i_
rf
I"
1
■ ^H
^s
FIRST
\ READOUT
LINE
X
\
\
\
\ \
\ \
\
THIRD \
READOUT
LINE-
\ \
FOURTH \
READOUT —K
LINE -^^
ONLY SMALL CHARACTERS
MAY BE DISPLAYED ON
BOTTOM LINE
LARGE CHARACTERS
REQUIRE 2 ADJACENT
DISPLAY AREAS
25
Rlp|S|-|P|K| | | |11.5:8| |V
v
Om
*CA5 S
CA7 HI
(LINE
REFERENCE)
1
*CA5 HI S
CA7 LO
(LINE
REFERENCE)
XCA5 LO &
CA7 HI
(LINE
REFERENCE)
32 HORIZONTAL DISPLAY
AREAS-DETERMINED BY
CHARACTER COUNTER
*N0TE REFERENCE LEVEL IS
WITH DD3. DD4. DD5. AND
DD6 ALL LO
*CA5 S
CA7 LO
(LINE
REFERENCE)
6863-20
Figure 3-6. Developing the readout display.
3a-28
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