Tektronix 7834

Theory of Operation---7834
In addition to the
encoding data inputs from the plug-in
units, inputs
are provided to the Column Data Switch IC
from the VERTICAL MODE and
HORIZONTAL MODE
switches to inhibit
the readout for any plug-in unit(s) not
selected for
display
.
When a unit is not selected, the line
corresponding
to the opposite channel is HI to forward bias
the
associated diodes ; CR3217 and CR3216, CR3219 and
CR3218,
CR3213 and CR3212, or CR3215 and CR3414
.
The forward-biased diodes cause the channel switches to
bypass the encoded data from the inhibited
channel . How-
ever, since it may be desired to display
information from
special-purpose plug-in units (even
though they do not
produce a normal waveform display on
the
crt),
a feature is
provided to over-ride the channel
inhibit
.
This is done by
applying a LO to the associated
Force Readout input . The
LO level diverts the HI
channel-inhibit current and allows
the data from
this plug-in unit to reach the Column Data
Switch,
even though it has not been selected for display by
the mode
switch .
Row Match adjustment
R3422 sets the gain of the Row
Data Switch to
match the gain of the Row Decoder for cor-
rect output .
Column Match adjustment R3407 performs
the same function for the
Column Data Switch stage .
Display-Skip
Generator
The Display-Skip
Generator is made up of 03406, 03411,
03414, and 03416 .
This stage monitors the time-multi-
plexed column
data (at the output of the Column Data
Switch)
during each time-slot to determine if the informa-
tion at
this point is valid data that should result in a crt
display .
Quiescently, there is about 100 microamperes of
current flowing through R3409 from 03419 and the Zeros
Logic and Memory stage . (The purpose of this
quiescent
current will be discussed in connection with
the Zeros Logic
and Memory stage .) This current
biases Q3406A so that its
base is about 0 .2 volt more positive
than the base of
Q3406B in the
absence of column data . Therefore, since
Q3406A and Q3406B
are connected as a comparator,
Q3406A
will remain on unless its base is pulled more
negative than the base of Q3406B .
- I - he analog data output from the Column Data
Switch pro-
duces a 0 .5 volt (approximately) change
for
each
unit of
column current that has been encoded by
the plug-in unit .
Whenever information appears at
the output of the Column
Data Switch, the base of Q3406A
is pulled more negative
than the base of Q3406B, resulting in
a negative (LO)
Display-Skip output to the Timer stage through
03416 .
Recall that a LO was necessary at
the skip input of the
Timer so it could perform
the complete sequence necessary
to display a character
.
Transistors 03411 and
03414
also
provide Display-Skip
action . The End-of-Word
level connected to their emitters
is LO only during
time-slot 1 . This means they are enabled
3- 4 6
only during this time-slot . These transistors
allow the Zeros
Logic and Memory stage to generate a Display-Skip
signal
during time-slot 1 when information that
is not to be dis-
played on the crt has been stored
in memory . (Further
information is given
under Zeros Logic and Memory .)
Column and Row Decoders
The
Column Decoder U3418 and Row
Decoder U3429
sense the magnitude of the analog voltages at
their inputs
(pin 10)
and produce a binary output on one of
ten lines
corresponding to
the column or row data encoded
by the
plug-in unit . These outputs provide the
Column Digital Data
and Row Digital Data, which
is used by the Character
Generator stages to select
the desired character for display
on the crt . The column and row data is
also used through-
out the Readout System to perform
other functions .
The input current at pin 9 of the
Column Decoder stage is
steered to only one of the ten
Column Digital Data outputs .
When a Display-Skip signal is present
(collector of 03416
HI), pin 9 is pulled HI
through CR3416 . This ensures that
no current is connected to the
Character Generator stage
under this condition . Notice the
corresponding input on the
Row Decoder . This input is connected to
ground and causes
only one of the ten row outputs to saturate to
ground .
The network at the input of the
Row Decoder, made up of
03427 and its associated components,
is
a
Row-14 detector
that produces the Jump Command .
This row current is
encoded by special-purpose
plug-ins
to
cause all or part of a
word to be jumped . Whenever
row 14 (13 units of row
current, or 1 .3 milliamperes) is
encoded, the base of 03427
is pulled negative enough so that this
transistor is reverse
biased to produce a HI Jump
Command output at its col-
lector .
The
Jump
Command is connected to the Word
Trigger stage to
advance the Channel Counter to the next
word and to reset the Time-Slot Counter to time-slot 1 .
Zeros Logic and Memory
The Zeros Logic and
Memory
stage
U3401 stores data
encoded by the plug-in units to
provide zeros-adding and
prefix-shifting logic for the
Readout System . The Strobe
pulse at pin 15 goes positive
when the data has stabilized
and can be inspected . This activates
the Zeros Logic and
Memory stage so that it can
store the encoded data . A
block representation of
the memory sequence is shown in
Figure 3-36 .
Typical output
waveforms of the five possible input condi-
tions
that
can occur are shown in Figure 3-37 . When
time-slot 1 occurs, a store command is given to all of the
memories . If the plug-in units encoded data for column 1, 2,
3,
4, or
10
during time-slot 1, the appropriate memory (or
memories) is set .
Notice that row 3 information from the
Row Decoder must also be present at pin 16 for data to be
stored in the memory of U3401 .