Tektronix 7834

The Z-Axis Readout output at pin 12 is produced next . This
current is connected
to
the crt circuit
to
unblank the crt
to
the
intensity level
determined by
the voltage on the Gated
Readout Intensity line . The Character Scan ramp at pin 16
started to go negative as this timing sequence began . How-
ever, character generation does not start until the readout
intensity level has been established . The triangular Character
Scan ramp runs from about
---2
volts
to
about
-8
.5 volts,
then returns back
to
the original level . This waveform pro-
vides the scanning signal for the Character Generator stages .
Character Scan adjustment R3437 sets the do level of the
Character Scan ramp for complete characters on the display .
The Timer
stage operates
in
one of two modes, as controlled
by the Display-Skip level
at
pin 4 . The basic mode
just
described is a condition that does not occur unless all ten
characters of each word (80 characters total) are displayed
on the crt . Under typical conditions, only a few characters
are displayed in each word . The Display-Skip level
at
pin 4
determines the period of the Timer output signal . When a
character is to be generated, pin 4 is LO and the circuit
operates as just described . However, when a character is not
to be displayed, a HI level is applied to pin 4 of U3433
through CR3432 from the Display-Skip Generator stage .
This signal causes the Timer to shorten its period of opera-
tion to about 210 microseconds . The waveforms in Figure
3-32 show the operation of the Timer stage when the Display-
Skip condition occurs for all positions in a word . Notice
that there is no output at pins 12, 13, 14, and 16 under this
condition . This means that the crt display is not interrupted
to display characters . Also notice that the triangle waveform
at pin 6 does not go as far negative, and that the negative
portion of the trapezoidal waveform at pin 10 is shorter .
Complete details on operation of the Display-Skip Genera-
tor are given later .
The Timer operation is also controlled by the Single-Shot
Lockout level
at
pin
2 . If this level is LO, the Timer operates
as just described . However, if the
Single-Shot Lockout stage
sets a H I level
at
this pin, the Timer stage
is locked
out and
can not produce any output signals (see Single-Shot Lockout
description for further information) .
A negative voltage on the Gated Readout Intensity line sets
the intensity of the readout display independently of the
A
or
B INTENSITY
controls . The Gated Readout Intensity
line
also
provides
a
means
of turning the Readout System
off when a readout display is
not
desired
.
When the Gated
Readout Intensity line
is left open, the
current from pin 11
of U3433 is interrupted, and
at
the same time, a positive
voltage is applied
to
pin 4 through CR3431 . The positive
voltage switches the
stage to
the same conditions as were
present under the Display-Skip condition . Therefore, the
crt display is not interrupted to present characters . How-
ever, time-slot pulses continue to be generated .
PIN 6
PIN 10
VERTICAL AND
HORIZONTAL
OFF
COMMAND
PIN 13
Theory of
Operation-7834
Figure 3-32 . Timer stage operation when display-skip condition
occu
rs .
NOTE
:
TEST OSCILLOSCOPE EXTERNALLY TRIGGERED
FROM TP3450

(1195-31) 1988-57
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