Tektronix 7834 - Page 82

Theory of
Operation--7834
time, the output level at pin
4
goes LO
where it remains for
about 0 .5 microsecond which is
equal
to
the period of the
Clock pulse (two-megahertz repetition
rate)
.
The Horizontal Chopped Blanking time must be longer than
the Vertical Chopped Blanking time, since it takes more
time for the display to switch between horizontal units
than
between vertical units . During the time that the level
at
pin 4 is LO, the crt is blanked and the Vertical Chopping
Signal and the
Plug-In Chop Drive cannot change levels . The
Clock pulse at
T3 changes only the Horizontal Chopped
Blanking
output
at pin 4 . The level on this pin goes HI after
the delay time to
unblank the crt .
For the next three Clock pulses, the Vertical Chopping
Signal output and Plug-In Chop Drive operate in the
normal
manner . However, just prior to the fourth Clock pulse
(time T4), both
outputs are again at their HI level . The
fourth clock pulse at
T4 switches the output at pin 1, pin 8,
and pin
4
(after delay)
to the LO level to start the next
cycle . Notice that
a Horizontal Chopped Blanking pulse
is
3-
2
6
To
CLOCK,
PIN 10
VERTICAL
CHOPPING
SIGNAL,
PIN 1
PLUG-IN CHOP
DRIVE
PIN 8
CHOP MODE
(HORIZ), PIN 6
HORIZONTAL I
CHOPPED
1
BLANKING, PIN 4

1
I
VERTICAL CHOPPED

'
BLANKING, PIN 4

I
OF U4320B

I
produced at pin 4 with every fifth Clock
pulse . Also notice
that with the HORIZONTAL MODE
switch set to CHOP,
two complete cycles of the
Vertical Chopping Signal are
produced with each
five Clock pulses (repetition rate two-
fifths clock
rate) and one complete cycle of the
Plug-In
Chop
Drive for every five clock pulses (one-fifth clock
rate)
.
Notice that
the large shaded area produced by the Horizon-
tal Chopped
Blanking pulse (see Fig . 3-19B) is not part of
the display
time (crt display blanked) . However, about
the
same
time segment is displayed from the vertical
signal
source with or without Horizontal Chopped
Blanking, due
to
the change in repetition rate with the HORIZONTAL
MODE
switch set to CHOP .
The Vertical Chopping Signal at pin 1 of
U4340 is connected
to the Vertical Mode Logic stage (see
following description)
through LR4342 . This signal is Hl when the
RIGHT VERT
unit is to be displayed and it is LO when
the LEFT VERT
unit is to be displayed . The Plug-in Chop
Drive at pin 8 is
connected to the plug-in units in the vertical
compartments
through LR4344, via the Main
Interface board . When this
signal is HI, Channel 2 of the plug-in
units can be displayed ;
Ti
T2
mill
I
DELAY
(1195-19) 1988-44
Figure 3-19 . (A) Input and output pins for Chop Counter IC, U4340 ; (B) Idealized waveforms
for Chop Counter stage .