Retrace Blanking; Brightness Control And Video Output I - Facit 4431 Service Instruction

The video signal coming out of 044-6, is exclu-
sive ORed with the reverse video bit in 035-9,10
to allow any particular bit of information to be
reversed out on a character by character basis.
The underline or reversed screen blanking output
of the attribute latch 058 determines whether
the video signal at 035-8 is to be unaltered or
made completely white.
The use of reverse screen blanking is as fol-
lows:
In SETUP modes only a few rows of information on
the screen (top and bottom rows) are saved by
the firmware. This requires that the center rows
of the screen be undisturbed and also blanked
out so that they are not visible. If bright back
ground is selected by the user, it is necessary
to white out the center portion of the screen.
The reverse screen blanking signal is used to a-
chive this. Also if an underline is required,
scan line 8 needs to be whitened out anef the att
ribute PROM will set the underline attribute at
the appropriate time.
The video signal that emerges from the OR gate
044-8 goes to three open collector NANO gates
060, to provide three levels of video signal in-
tensity. All of these gates are combined with
the blanking signal coming from 058-5. This
blanking signal is the OR of retrace blank sig-
nal which is required to blank out the screen
during screen retrace periods, and the Video
blank signal which comes out of the attribute
PROM 057-13 whenever a row of data or the whole
screen needs to be blanked.
The blinking bit out of the attribute latch is
combined with one of the three NANO gates and
when this gate is turned on its output goes low
along with the video signal and provides a more
intense picture. The bottom NANO gate, (060-1,
2, 13), is gated with the intensity bit, but in
a non-blinking fashion such that when turned on,
causes the video to be constantly brighter.
When the cursor bit is active (057-23), the cur-
sor blink bit is active (057-3), and the display
is in scan line 9 (057-22) or some other row
with block type cursor selected (057-4), the re-
verse video output 057-9 becomes complemented to
effect a blinking reverse image cursor.
7.4.3.5 Retrace blanking
During retrace the screen must be blanked to
black while data continues to emanate from the
character generator memory 040, and shift regi-
ster 039, and 038. The blanking output at 049-17
representing both vertical and horizontal blan-
king is buffered through 046 and then through
035.
The blank output at 02-11 is ANDed (037-1,2)
with the LSB data out of the character genera-
tor. This data bit DO is the extension bit of
the line drawing set which runs into the next
character so that continuous horizontal lines
may be drawn. This bit passes through to the
shift register during non blank periods but is
masked during blanking so that this random bit
23
will be loaded into the shift register 039 and
thus appear in the first dot position of the
first character after blanking.
This line drawing input extends into the next
character position as its output occurs at the
first clock position before jam of the new cha-
racter at 039-9. This output is looped back to
the first parrallel input 039-6, so the output
is jammed in and displayed once again at the be-
ginning of the new character position.
As explained above, the blanking signal also co-
mes out of the attribute latch and forms an in-
put to all three NANO gates 060 to completely
shutdown video during retrace. To keep the edges
of the display absolutely uniform, it is neces-
sary to reset the attribute latch 058 during ho-
rizontal and vertical blanking periods. This is
done by the OR gate 044-3 when the load pulse to
the shift registers clocks the blank attribute
into 058 while the other input 044-1 remains
low. The reset is released when blanking is over
synchronously with the MEMSHARE signal. At the
same time the jam pulse loads the last random
attributes into 058 but the blank output at 058-
5 is still low insuring that the random data
will not be displayed.
7.4.3.6 Brightness control and video output
To allow for keyboard control of brightness and
to set the brightness value stored in the EAROM,
the octal latch 056 is used which drives the
buffers 059, terminated with approximately bina-
ry weighted resistors. These are paralleled in
various combinations with R18 to trim the video
out voltage to the monitor.
Q2 is wired as a voltage control current source
where the voltage across R15 sets the current
dumped into R18 and binary weighted resistors.
As the voltage across R15 is greater, the video
level is brighter and as the resistors R8, R10
and R9 come on, the video level is made brighter
by pulling the non emitter end of R14 further
toward ground.
V103 is wired as an emitter follower which com-
bines the video signal from the collector of
V102 with the composite sync from 059-2 to gene-
rate the composite video to drive an external
monitor.