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DIDS-402-2AM13
The unblanking input to A2B. 3 consists of short duration pulses which occur
12 times per character.
During the interval 3(CTS+l) to 3(CTS+l), approximately
12 of the se pulse s, are gated through A28. 3 to form the horizontal portion of the
cur s or symbol (
..J ).
The 012 MEM CLOCK input to A22. 8 is high once per bit during phases
1 and 2.
During the 3(CTS+l) to 3(CTS+l) interval, A22. 8 is enabled approxi-
mately six times to form the vertical portion of the cursor symbol
(.J).
These
two outputs are combined by gate A28.6 which produces a cursor video output.
This output is coupled to an alternate input of the video amplifier and occurs in
coincidence with the character symbol associated with the cursor.
In
this
manner, both the character symbol and the cursor symbol are displayed simul-
taneously.
4-24. 10.2 Blanking
The output of the blanking circuit is used to cut off the video amplifier
during the inactive scan periods of the monoscope ,and CRT (see figure 4-69).
The most frequent blanking level occurs once per character during 3(CTS+l).
This pulse is used to cut off the video amplifier while the monos cope beam is
being directed to the next character symbol 'to be displayed. The next most
frequent blanking level (as suming a full screen display) is the H DRIVE bla:Oking
level.
This level is eight characters wide and occurs during horizontal retrace.
This in turn cuts off the video amplifier during retrace to protect the CRT screen
phosphor.
The V DRIVE blanking level operates in a similar manner but is present
for 80 character times.
The V DRIVE pulse (which is immediately followed by
H DRIVE) cuts off the video amplifier once per frame to protect the CRT phosphor
during vertical retrace.
The fourth blanking output occur s when an Idle code is decoded in the CE
register.
The ALL lIs condition enables NAND-gate A39.6 which blanks the
video amplifier during CTS+4 and CTS+5 of the character time associated with
the display of the short underline symbol.
3(CTS+I)
H DRIVE
V DRIVE
~...JVVV-_
BLANKING
ALL I'S .....
---~
HI=>CTS+4 ·CTS+5
UNBLANKING
..... --<"---"
Figure 4-69.
Blanking Circuit,
Logic Diagram
4-152
OIDS 68-568
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