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THEORY OF OPERATION
SECI'ION 4
Video at the mixer (R13, R17, R18) can be blanked and changed in inten-
sity.
Output signals from the mixer are direct-coupled to emitter follower Q3
which drives the CRT video amplifier section via Jl-6.
In some terminals, composite video signals are provided by Ql, Q2, and
Q4.
The composite sync signal from the timing control card is amplified by Q4,
coupled through CRS, and added to the video in a second mixer consisting of R8, R9,
and R12.
The resulting composite video signal is amplified by Ql, Q2, and is made
available at J2-l and a BNC connector on the rear of the terminal.
If the most significant three bits of the character code are 100 (i.e.,
HLLXXXXX), the character is a nondisplayable video modification character.
This
kind of character specifies some modification to be made to the video presentation
until either the next video modification character or the end of the line occurs.
The modification character format is HLLXXYYY, with YYY defining the
modification (e.g., reverse video, blink, etc.).
The
XX
portion is available for
defining software functions (e.g., define protected fields).
Modification codes
and their functions are given in Table 4-7 •
•
Video modification codes on CGO through CG2 are stored in XC4, a type
74175 flip-flop.
XC4 is loaded by
LOAD F,
which is generated on the refresh buffer
card when a 100 (HLL) is decoded in the most significant three bits out of the re-
fresh shift register.
CLR F, also generated on the refresh buffer card, clears XC4.
Table 4-7 shows the XC4 input and output levels for the various video modification
functions.
Table .4-7.
Video modification register (XC4) input-output parameters
XC4 INPUTS
XC4 OUTPUTS
VIDEO MODIFICATION
-
-
-
CG2
CGl
r.GO
IQ
2Q
2Q
3Q
3Q
L
L
L
Normal Display
H
L
H
L
H
L
L
H
Video Off
L
L
H
L
H
L
H
L
Underline
H
H
L
L
H
L
H
H
Reverse Video
L
H
L
L
H
H
L
L
Dim Normal
H
L
H
H
L
H
L
H
Dim Reverse
L
L
H
H
L
H
H
L
Blink
Kormal
H
H
L
H
L
H
H
H
Blink
Reverse
L
H
L
H
L
tt-6S
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