Tektronix 7834

states, the resulting
negative-going edge is connected to
pin
8 of U3447C . This results in
a negative-going Word Trigger
output at pin 13 to advance
the Channel Counter to the
next word . When the
next Trigger pulse is received at
pin
15, the Time-Slot
Counter returns to the
normal sequence
of operation
and produces an output on the
time-slot 1 line .
Channel Counter
Channel
Counter U3450 is a binary counter that
produces
the Channel
Address Code for the Column and
Row Decod-
er stages and the Format
Generator stage . This code
in-
structs these stages to
sequentially select and display
the 8
channels of data
from the plug-ins . Table 3-5 gives
the 8
combinations of
the Channel Address Code
and the result-
ant channel selected
with each combination .
TABLE 3-5
Channel Address Code
Single-Shot Lockout
Channel l
Left
Vertical
Channel~2
Right Vertical
Channel l
A Horizontal
Channel W 2
B Horizontal
Channel l
__
The
Single-Shot Lockout stage allows a
single readout frame
(8 complete words) to be displayed on
the crt, after which
the Readout System is locked out,
so further readout dis-
plays are not presented until
the circuit is reset . U3449C
and U3449D are connected to form
a bistable flip-flop . For
free-run operation, pin 8 of U3449C is
held HI . This acti-
vates U3449C and results in a
LO output level at pin 10,
enabling the Timer stage
to operate in a
free-running
manner .
The output of
the Single-Shot Lockout
stage remains LO to
allow U3433 to operate in the
free-running made until a
LO is received at pin 8 of
U3449C . When this occurs, the
output level at
pin 10 of U3449C does
not change im-
mediately .
However, the Readout
System is now enabled as
far as the
single-shot lockout
function is concerned . If
the
Channel Counter has not
completed word 8, the Readout
System continues to operate in
the normal manner . How-
ever, when word 8 is completed,
the negative-going
End-of-
Frame
pulse is produced at pin 11
of U3450 as the
Channel
Counter
shifts to the code necessary
to display word
one .
This
pulse is coupled to pin 5 of
U3449B . The momentary
H I
at
pin 5 activates U3449D
and its output stage goes LO
to
disable U3449C (pin 8 is already
LO)
.
The output of
U3449C
goes H I to disable the Timer
stage, so it operates
in
the Display-Skip
mode . The HI at
pin 10 of U3449C also
holds U3449D enabled, so it
maintains control of
the
flip-flop .
The Single-Shot Lockout stage
remains in this
condition
until a positive-going trigger
pulse is applied to
pin
8
of
U3449C . This trigger pulse
produces a LO at pin 10
of
U3449C
to enable U3433 and
disable U3449D . Now,
the
Timer stage can operate in the
normal manner for another
complete frame . When word 8 is
completed, the Channel
Counter produces another
End-of-Frame pulse to again
lock
out the
Timer stage .
Encoding the
Data
Theory
of Operation-7834
Data is conveyed
from the plug-in units to
the Readout Sys-
tem in
the form of an analog
(current level) code . The
characters that can be selected
by the encoded data are
shown on the Character Selection
Matrix (see Fig . 3-27) .
Each character requires two currents
to define it ; these
currents are identified as the
column current and the row
current, corresponding to
the column and row of the
matrix .
The column and row data
is encoded by the
programming
of the plug-in units .
Figure 3-34 shows a typical
encoding
scheme using resistors for
a voltage-sensing amplifier
plug-in
unit . Notice that the 10 time-slot
(TS) pulses produced
by
the Time-Slot Counter stage
are connected to the
plug-in
unit . However, time-slots
5, 6, and 10 are not
used by the
plug-in unit to encode
data when using the Standard
Read-
out Format . (See
Table 3-4 for Standard Readout
Format
.)
The amplitude of the
time-slot pulse is exactly -15
volts as
determined by the 'Timer stage
. Therefore, the
resultant
output current from
the plug-in units can be
accurately
controlled by the
programming resistors in the
plug-in units .
For example, in
Figure 3-34 resistors R10, R20,
R30, R80,
and R90 control
the row analog data, which is
connected
back to the Readout
System . Figure 3-35 shows an
idealized
output
current waveform of row analog data,
which results
from the
time-slot pulses . Each of the row levels
of current
shown in
these waveforms correspond to 100
microamperes
of
current . The row numbers on the
left-hand side of the
waveform correspond to the rows
in the Character Selection
Matrix (see Fig . 3-27) .
The row analog data is connected
back to the Readout
System via terminal B37 of the
plug-in
interface .
3- 4 3