Tektronix 5441 Instruction Manual page 49

Theory of Operation-54411~5441
form provides the scanning signal for the character genera-
tor stages. Full character scan adjustment R1006 sets the
dc level of the character scan ramp to provide complete
characters on the display.
The timer stage operates in one of two modes, as con-
trolled 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 UlOOO
through CR1003 from the display skip generator stage. This
signal causes the timer to shorten its period of operation to
approximately 210 ps. The waveforms in Fig. 3-6 show the
operation of the timer stage when the display skip condi-
tions occur 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 char-
acters. 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 generator are given later.
READOUT INTENS control R1000 sets the intensity of
the readout display independently of the INTENSITY con-
trol. The READOUT INTENS control also provides a means
of turning the readout system off when a readout display is
not desired. When RlOOO is turned fully counterclockwise,
switch S1000 opens. The current to pin 11 of U1000 is inter-
rupted and, at the same time, a positive voltage is applied to
pin 4 through R1003 and CR1002. This positive voltage
switches the stage to the same conditions that were present
under the display-skip conditions. Therefore, the crt display
is not interrupted to present characters. However, time-slot
pulses continue to be generated.
Time-Slot Counter
Time-Slot counter U1025 is a sequential switch that di-
rects the trapezoidal waveform input at pin 8 to one of its 10
output lines. These time-slot pulses are used to interrogate
the plug-in units to obtain data for the readout system. The
trigger pulse at pin 15 switches the time-slot counter to the
next output line; the output signal is sequenced consecu-
tively from time-slot 1 through time-slot 10. Figure 3-7
shows the time-relationship of the time-slot pulses. Notice
that only one of the lines carries a time-slot pulse at any
given time. When time-slot 10 is completed, a negative-
going end-of-word pulse is produced at pin 2. The end-of-
word pulse provides a drive pulse for the channel counter
and also provides an enabling level to the display-skip gen-
erator during time-slot 1 only. The end-of-word pulse also
resets the decimal point logic and zeros logic.
Fig. 3-6. Timer stage operation when display-skip condition
occurs.
~
7--
-~ r
7
Character Scan
REV MAY 1982