Tektronix 7834 - Page 95

The analog outputs of the Column Data Switch and Row
Data Switch are connected to the Column Decoder and
Row Decoder stages respectively . These stages sense the
magnitude of the analog voltage input and produce an out
put current
on one of ten lines . The outputs of the Column
Decoder stage are identified as C-1 through C-10 (column 1
through 10) corresponding to the encoded column informa-
tion . Likewise, the outputs of the Row Decoder stage are
identified as R-1 through R-10 (row 1 through 10) corres-
ponding to the encoded row information . The primary
function of the row and column outputs is to select a
character from the Character Selection Matrix (Fig . 3-27)
to be produced by the Character Generator stage . These
outputs are also used at other points within the system to
indicate when certain information has been encoded . One
such stage is the Zeros Logic and Memory . During time-slot
1
(TS-1 ), this stage checks if zero-adding or prefix-shifting
information
has been encoded by the plug-in unit, and
stores
it in the
memory
until time-slots 5, 6, or 8 . After
storing
this information, it triggers the Display-Skip Genera-
tor
stage so that there
is
no display during time-slot 1 (as
defined by Standard Readout
Format ;
see Table 3-4) . When
time-slots 5, 6, and 8 occur, the memory is addressed and
any information stored there during time-slot 1 is transferred
to the input of the Column Decoder stage to modify the
analog data during the applicable time-slot .
Also,
the Zeros Logic and Memory stage produces the
IDENTIFY function
.
When time-slot 1 is encoded for
IDENTIFY (column 10, row 3), this stage produces an out-
put level, which connects the Column Data Switch and Row
Data Switch to a coding network within the Readout
System
.
Then, during time-slots 2 through 9, an analog
current
output is produced from the Column Data Switch
and Row Data Switch, which addresses the correct points in
the Character Selection Matrix (Fig . 3-27) to display the
word "IDENTIFY" on the crt .The Zeros Logicand Memory
stage is reset after each word by the Word Trigger pulse .
The Character Generator stages produce the characters
which are displayed on the crt . Any of the 50 characters
shown on the Character Selection Matrix of Figure 3-27 can
be addressed by proper selection of the column and row
currents . Only one
character is addressable in any one time-
slot ; a space can be added into the displayed word by the
Decimal Point Logic and Character Position Counter stage
when
encoded by the plug-in . The latter stage counts the
number of characters generated
and produces an output
current to step the display
one character position to the
right for each character . In addition, the character position
is advanced once during each of time-slots 1, 2, and 3,
whether a character is generated during these time-slots or
not . This action fixes the starting point of the standard-
format display such that the first digit of the scaling factor
always
starts at
the
same point within each word regardless
of the information encoded in time-slot 1, 2,
or
3 preceding
this digit . Also, by encoding row
10
and column
0
during
any time-slot, a blank space can be
added to the display .
Theory of Operation--7834
Decimal points can be added to the display at any time by
addressing the appropriate row and column . (See Character
Selection Matrix, Fig . 3-27, for location of decimal points .)
The Decimal Point Logic and Character Position Counter
stage is reset after each word by the Word Trigger
pulse .
The Format Generator stage provides the output signals to
the vertical and horizontal deflection systems of the instru-
ment to produce the character display . The binary Channel
Address Code from the Channel Counter stage is connected
to this stage, so that the display from each channel is posi-
tioned to the area of the crt associated with the plug-in and
channel originating the word (see Fig . 3-26) . The position-
ing current or decimal point location current generated by
the Decimal Point Logic and Character Position Counter
stage is added to the Horizontal (X) signal at
the input
to
the
Format
Generator
stage to
provide horizontal position-
ing of the characters
within
each word . The X- and Y-
Readout
signals
are connected to
the Horizontal Amplifier
and Vertical Amplifier through the X- and Y-Buffer stages .
The Word Trigger stage produces a trigger from the End-of-
Word pulse generated by the - Time-Slot Counter stage after
the tenth time-slot . This Word Trigger pulse advances the
Channel
Counter to display the
information from the next
channel or plug-in . It also provides a reset pulse to the Zeros
Logic and Memory stage, and the Decimal Point Logic and
Character Position Counter stage
.
This Word Trigger stage
can
also be
advanced
to
jump
a
complete word or
a
portion
of a word when a Jump Command is received from the Row
Data Switch stage .
Timer
The timer
stage
establishes the timing sequence for all cir-
cuits within
the Readout System
. This
stage
produces 7
time-related output waveforms (see
Fig .
3-30) .
The triangle
waveform produced
at
pin
6
forms
the basis for
the remain-
ing signals . The
basic period of this
triangle waveform is
about 250 microseconds
as controlled by RC
network
R3434 and C3434 . The
triangle
waveform
is
clipped and
amplified by U3433 to form the trapezoidal output signal
at pin 10 . The amplitude of this output signal is exactly 15
volts as determined by U3433 (exact amplitude necessary
to accurately encode
data
in plug-in units
; see
Encoding the
Data) . The trigger output
at
pin 5 provides the switching
signal for the Time-Slot Counter and Word Trigger stages .
The signals at pins 12, 13, 14, and 16 are produced only
when the triangle waveform is on its negative slope and the
trapezoidal waveform has reached the lower level . The tim-
ing
sequence of these waveforms is important to the opera-
tion
of the Readout System (see expanded waveforms in
Fig . 3-31) . The Z-Axis
Inhibit command at pin 14 is pro-
duced first . This negative-going signal provides
a
blanking
pulse to the Z-Axis Logic stage (see diagram 4) to blank the
crt before the display
is
switched
to
the Readout System .
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