Tektronix 7623, Instruction Manual

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Circuit Description-7623/R7623 Service
Another operation of the Zeros Logic and Memory stage
is to produce the IDENTIFY function. When time-slot 1 is
encoded for IDENTIFY (column 10, row 3), this stage
produces an output 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 to display
the word "IDENTIFY" on the CRT. The Zeros Logic and
Memory stage is reset after each word by the Word Trigger
pulse.
The Character Generator stage produces the characters
which are displayed on the CRT. Any of the 50 characters
shown on the Character Selection Matrix of Fig. 3-24 can
be addressed by proper selection of the column and row
current. 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
how many characters have been 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 2
(normal/invert) or time-slot 3 (cal/uncal) which precedes
this digit. Also, by encoding row 10 and column 0 during
any time-slot, a blank space can be added to the display.
Decimal points can be added to the display at any time by
addressing row 7 and columns 3 through 7 (see Character
Selection Matrix for location of these 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
instrument to produce the character display. The binary
Channel Address No. 2 code from the Channel Counter
stage is connected to this stage so that the display from
each channel is positioned to the area of the CRT which is
associated with the plug-in and channel originating the
word (see Fig. 3-24). The positioning 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 positioning of the characters
within each word. The X- and Y-output signals are
connected to the Horizontal Amplifier and Vertical Ampli­
fier through the Horizontal Output and Vertical Output
stages respectively.
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. The
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 Decoder stage.
The Single-Shot Lockout stage allows the display se­
quence of the Readout System to be changed. Normally,
the Readout System operates in a free-running mode so the
waveform display is interrupted randomly to display
characters. However, under certain conditions (such as
single-shot photography), it is desirable that the Readout
System operate in a triggered mode where the readout
portion of the display is normally blanked out but can be
presented on command. The Readout Mode switch deter­
mines the operating mode of the readout system.
Circuit Analysis of Readout System
The following analysis of the Readout System describes
the operation of each stage in detail. Complete schematics
of the Readout System are shown on diagram 10 at the
read of this manual.
Timer
Timer U2126 establishes the timing sequence for all
circuits within the Readout System This stage produces
seven time-related output waveforms (see Fig. 3-28). The
triangle waveform produced at pin 6 forms the basis for the
remaining signals. The basic period of this triangle wave­
form is about 250 microseconds as controlled by RC
network C1214-R1214. The triangle waveform is clipped
and amplified by U1210 to form the trapezoidal output
signal
at pin 10. The amplitude of this output signal is
exactly 15 volts as determined by V2126 (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
timing sequence of these waveforms is very important to
the correct operation of the Readout System (see expanded
waveforms in Fig. 3-29). The Z-Axis Logic OFF Command
at pin 14 is produced first. This negative-going signal
provides a blanking pulse to the Z-Axis Logic stage (see
diagram 2) to blank the CRT before the display is switched
to the Readout System. It also produces the Strobe pulse
through R2137, Q2138, and CR2142 to signal other stages
within the Readout System to begin the sequence necessary
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