Tektronix 7623, Instruction Manual

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Circuit Description —7623/R7623 Service
gives the eight combinations of the Channel Address No. 1
code and the resultant channel which is selected with each
combination. These stages have nine inputs and provide a
single time-multiplexed output at pin 7 which includes the
information from all of the input channels. Eight of the
nine inputs to each stage originate in the plug-in units; the
ninth input comes from a special data-encoding network
composed of resistors R2201 through R2209 and R2191
through R2199 (see Zeros Logic and Memory description
for further information on ninth channel).
Q2223-O2229 also provide display-skip action. The
End-of-Word level connected to their emitters through
R2229 is LO only during time-slot 1. This means that
Q2223-Q2229 are enabled only during time-slot. These
transistors allow the Zeros Logic and Memory stage to
generate a display-skip signal during time-slot 1 when
information has been stored in memory which is not to be
displayed on the CRT (further information given under
Zeros Logic and Memory discussion).
In addition to the data inputs from the plug-in units,
channel-inhibit inputs are provided from each of the plug-in
units. The channel inhibit lines are LO only when the
associated plug-in unit has been selected for display. When a
plug-in
unit is not selected, the respective line is HI which
forward biases the associated diode CR2162, CR2163,
CR2167, CR2166, CR2171, CR2170, CR2175, or CR1174
to by-pass the encoded data from this plug in. However,
since it may be desired to display information from
special-purpose plug-ins 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 forcing over-ride 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 switches.
Display-Skip Generator
The Display-Skip Generator, Q2215, Q2223, Q2229,
and Q2225 monitors the time-multiplexed column data at
the output of the Column Data Switch during each
time-slot to determine if the information at this point is
valid data which should result in a CRT display. The voltage
at the base of Q2215B is set by divider R2219, R2220, and
R2221. Quiescently, there is about 100 microamperes of
current flowing through R2213 and R2214 from Q2240
and the Zeros Logic and Memory stage (purpose of this
quiescent current will be discussed in connection with the
Zeros Logic and Memory stage). This current biases
Q2215A so its base is about 0.2 volt more positive than the
base of Q2215B in the absence of column data. Therefore,
since Q2215A and Q2215B are connected as a comparator,
Q2215A will remain on unless its base is pulled more
negative than the base of Q2215B. The analog data output
from the Column Data Switch produces a 0.5 volt change at
the base of Q2215A for each unit of column current that
has been encoded by the plug-in unit. Therefore, whenever
any information appears at the output of the Column Data
Switch, the base of 02215A is pulled more negative than
the base of Q2215B resulting in a negative (LO) Display-
Skip output to the Timer stage through Q2225. 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.
Column and Row Decoder
The Column Decoder U2244 and Row Decoder U2185
sense the magnitude of the analog voltages at their inputs
and produce a binary output on one of ten lines corres­
ponding to the column or row data which was encoded by
the plug-in. 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
throughout the Readout System to perform other func­
tions. The input current at pin 9 of the Column Decoder
stage is steered to only one of the ten Column Digital Data
outputs. The size of the character which will be displayed
on the CRT is determined by the value of R2227. When a
display-skip signal is present (collector of Q2225 is HI), pin
9 is pulled HI through CR2226. 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 Q2153 and its associated components, is a Row 13
detector which 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 13 (thirteen
units of row current; 1.3 milliamperes) is encoded, the base
of Q2153 is pulled negative enough so that this transistor is
reverse biased to produce a HI Jump output at its collector.
This Jump command is connected to the Word Trigger stage
(diagram 10) to advance the Channel Counter stage 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 U2232 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 it can store the encoded data. A block
representation of the memory sequence is shown in Fig.
3-34. Typical output waveforms for the five possible input
conditions that can occur are shown in Fig. 3-35. When
time slot 1 occurs, a store command is given to all of the
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