Tektronix 7623, Instruction Manual

Page: 86 / 266

Circuit Description —7623/R7623 Service
stage through C2155. Pin 13 of U2155D goes LO to enable
the Time-Slot Counter and Timer stages for the next
time-slot pulse. At the same time, the negative-going edge
produced at U2155D switches output states which is
connected to pin 3 of U2155D. This results in a negative­
going Word Trigger output at pin 4 of U2155B 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
The Channel Counter, made up of integrated circuit
U2250 is a binary counter which produces the Channel
Address code for the Column and Row Decoder stages and
the Format Generator stage. This code instructs these stages
to sequentially select and display the six channels of data
from the plug-in units. The input channel which is
displayed with each combination of the Channel Address
code is given in the discussion of the applicable stages.
Single-Shot Lockout
Q2108, Q2117, and U2120 makes up the Single-Shot
Lockout stage. This stage allows a single readout frame (Six
complete words) to be displayed on the CRT, after which
the Readout System is locked out so further readout
displays are not presented until the circuit is reset. U2120C
and U2120B are connected to form a bistable flip flop. For
normal operation, pin 3 of U2120 is pulled HI through
R2108. This activates U2120C to result in a LO output
level at pin 10. This level enables the Timer stage so it can
operate in the free-running manner as described previously.
The LO at pin 10 of U2120C is also applied to pin 5 of
U2120B. Since pin 6 of U2120A is LO, U2120B is disabled
and its output goes HI.
The output of this stage remains LO to allow U2126 to
operate in the free-running mode until a LO is received at
pin 8 of U2120C. When this occurs, the output level at pin
10 of U2120C does not change immediately. 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 six (Channel 2 of the Horizontal unit),
the Readout System continues to operate in the normal
manner. However, when word six is completed, a positive­
going End-of-Frame pulse is produced at pin 9 of U2120B
as the Channel Counter shifts to the code necessary to
display word one. This pulse, is coupled to pin 3 of U2120A
and
pin 12 of U2120D. The momentary HI at pin 3
activates U2120B and its output goes LO to disable
U2120C (pin 3 already LO). The output of U2120C goes
HI to disable the Timer so it operates in the display-skip
mode. The HI at pin 10 of U2120C also holds U2120B
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
U2120C. This trigger pulse produces a LO at pin 10 of
U2120C which enables U2120B and disables U2120C.
Now, the Timer can operate in the normal manner for
another complete frame. When word six is completed, the
Channel Counter produces another End-of-Frame pulse to
again lock out the Timer stage.
Encoding the Data
Data is conveyed from the plug-in units to the Readout
System in the form of an analog code having up to 11
current levels (from zero to one milliampere in 100
microampere steps). The characters which can be selected
by the encoded data are shown on the Character Selection
Matrix (see Fig. 3-25). Each character requires two currents
to define it; these currents are identified as the column
current and the row current which correspond to the
column and row of the matrix. The column and row data is
encoded by resistive programming in the plug-in units. Fig.
3-32 shows a typical encoding scheme 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, 7,
and 10 are not used by the plug-in unit to encode data
when using the Standard Readout Format (see Table 3-2
for Standard Readout Formal). The amplitude of the
time-slot pulses 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 Fig. 3-32, resistors R10 through R90
control the row analog data which is connected back to the
Readout System. These resistors are of fixed value and
define the format in which the information will be
presented by the Readout System. Fig. 3-33A shows an
idealized output current waveform of row analog data
which results from the 10 time-slot pulses. Each of the
steps of current shown in these waveforms corresponds 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 shown in Fig. 3-35. The row
analog data is connected back to the Readout System via
terminal B37 of the plug-in interface.
The Column analog data is defined by resistors R110
through R190. The program resistors are connected to the
time-slot lines by switch closures to encode the desired
data. The data as encoded by the circuit shown in Fig. 3-32
indicates a 100 microvolt sensitivity with the display
inverted and calibrated vertical deflection factors. This
results in the idealized output current waveforms shown in
Fig. 3-33B at the column analog data output, terminal A37
of the plug-in interface. Resistor R111, connected between
time-slot 1 and the column analog data output, encodes
two units of current during time-slot 1. Referring to the
3-40