Omron 8025G Maintenance Manual page 108

THEORY OF OPERATION
SECfIOt
The blink function is generated by decoding 2Q, 3Q, and the BLINK sig-
nal at XC3-S. With 2Q and 3Q (XC3-l0 and -9, respectively) high, XC3 is enabled
when XC3-l1 is high. BLINK occurs 2.5 times per second. The resulting high out-
put at XE3-10 inhibits XE3-13 25% of the time to produce a blinking video display.
Reverse blink occurs in the same manner if the reverse video function enables XE2-
Reverse video is produced with lQ low. With XE2-l2 low, the exclusive
OR gate does not invert the output (pin 13) of XE3. Thus, the output at XDI-6 is
inverted with respect to the output of XB2 and the video display shows
up
as black
on a white field.
The dim function results from decoding 2Q (XD3-l2) and 3Q (XD3-13).
When both of these are high, the low level at XD3-ll is NOR-gated to XDl-9
(an
open-collector gate) to turn it
on.
Turning XDI on places R19 in the video mixer
network and R19 thus reduces the video voltage to produce a dim character. Dim
reverse is accomplished in the same way when the reverse video function is present
Underline is generated by decoding lQ (XD4-2), 2Q (XD4-l), and 3Q (XD4-
in XD4. With all three signals high, XD4-6 is partially enabled and then fully en
abled when CURS LINE is high, during the ninth scan line (LS) of the character.
T
resulting low-level at XD4-6 is NOR-gated to XD3-3, whose output is inverted by XD
to apply a low level on XDl-4. This low level holds XDl-6 high (video on) for the
duration of CURS LINE.
Video off is controlled by lQ (XCS-ll), 2Q (XCS-lO), and 3Q (XCS-9).
With all of these low, the resulting high at XCS-8 is NAND-gated to a low level at
XEl-S. This level clamps the video signal at the mixer network to ground and caus
the CRT screen to go blank.
Blanking is also performed by the output at XEl-3, which is controlled
by the COMP BLNK and BLANK inputs. The video is blanked when either or both of
these inputs are
lo~.
The cursor function involves the logic whose inputs are
CST,
CSO, MATCH
and RATE. XD4-8 is low when MATCH (from the cursor control card) is high, video
status bit
CST
is high (underline cursor on), and HIO is high during L8 (CURS
LINE~
of the character. This low level is inverted by XD2-l2, partially ena5ling XDS at
pin 1 and XDI at pin 12. XDS and XDI are thus turned on and off at a S pps rate
by the RATE signal on XDS-2 and XDl-13, respectively. With XD2-l2 high, XDS-3 is
high (off), XDl-ll is on, and XDl-8 is off during the low half of RATE. The result
ing low level at XDl-ll puts R19 in the video mixer network to dim the display. Dl
ing the opposite phase of RATE, XDS-3 is low while XDl-8 and XDl-ll are high. The
low level at XD5-3 also causes XEl-6 to go high. With XDl-S and XEl-6 high, the
video voltage can rise to a fullS volts to produce an intensified video. Even wit
reverse video the cursor will not be lost, since it blinks from dim to bright.
A blinking block cursor can also be generated by gating a low
CSO
(block
cursor on) at XD3-9 to XD4-9. HIO is high for each scan line in the character ma-
trix. Hence, the cursor gates function as described in the preceding paragraph
during the entire character matrix period to produce a blinking (dim to bright)
block.
The remaining logic on the video control card blanks control characters
and nulls.
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