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A latch is incorporated into a display adapter to provide means for measuring independently clocked video signals, and determining whether a display is operative.
English (United States)
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Video Output Checking
A latch is incorporated into a display adapter to provide means for measuring
independently clocked video signals, and determining whether a display is
If displays are to be customer-replaceable units in keyboard/display systems,
the ability to isolate display failures at a display/adapter connection is required.
There are four interface lines connecting the adapter and display. These lines
are (1) VRT (Vertical Retrace), (2) HRT (Horizontal Retrace), (3) BRT (Bright),
and (4) Video. For test purposes, signals on each of these lines are sensed, and
a timing circuit is used to measure the time between high and low transitions for
all but video signals. Since video signals are clocked by an oscillator which is
faster than the timing circuit, measuring presents a problem.
To overcome this problem, a latch is incorporated into the illustrated circuit in
the video line to a group of diagnostic data bus drivers. Any high-to-low transition
on the video line will clock the latch on. The latch will then remain on until the
next VRT signal which resets the latch. If blank screen video information is
considered, the video line will be high, and the latch will not be clocked on.
However, if one or more characters make up the video, there will be at least one
pulse which will clock the latch on. The latch will remain on for the remainder of
a frame and until a VRT signal. The latch output can then be measured and its
on time controlled by the...