Zeros Logic And Memory; Character Generator - Tektronix 7140 Instruction Manual

ZEROS LOGIC AND MEMORY
current, column 1) to no prefix (0 column current, column 0)
so only the unit of measurement encoded during time-slot 9
The Zeros Logic and Memory stage 02232 stores data en-
is displayed. Notice that reducing the prefix program from
coded by the plug-in units to provide zeros-adding and pre-
column 1 to column 0 programs the Readout System to not
fix-shifting logic for the Readout System . The Strobe pulse
display a character at this readout location .
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 further feature of the Zeros Logic and Memory is the Iden-
tify function . If 10 units of column current are encoded by
the plug-in unit along with row 3 during time-slot 1, the Ze
Typical output waveforms of the five possible input condi- ros Logic and Memory produces a negative-going output
tions that can occur are shown in Figure 3-17 . When time- pulse at pin 1 to switch the Column Data Switch and Row
slot 1 occurs, a store command is given to all of the
Data Switch to the ninth channel . Then, time-slot pulses 2
memories . If the plug-in units encoded data for column 1, 2, through 9 encode an output current through resistors
3, 4, or 10 during time-slot 1, the appropriate memory (or R2191 through R2199 for column data and enable pin 10 of
memories) is set. Notice that row 3 information from the 02186. This provides the addresses necessary to display
Row Decoder must also be present at pin 16 for data to be
the word IDENTIFY in the word position allotted to the
stored in the memory of U2232.
channel that originated the Identify command. After comple-
tion of this word, the Column Data Switch and Row Data
Switch continue with the next word in the sequence .
If data was encoded during time-slot 1, a negative-going
output is produced at pin 7 while the memories are being
set. This negative-going pulse is connected to the base of The end-of-word signal from the Time-Slot Counter is con-
02229 in the Display-Skip Generator to produce a Display- nected to pin 9 of U2232 through C2239. At the end of each
Skip output . Since the information encoded during time-slot
word of readout information, this pulse goes LO . This
1 was only provided to set the memories and not intended erases the four memories in the Zeros Logic and Memory in
to be displayed on the CRT at this time, the Display-Skip
preparation for the data to be received from the next
output prevents a readout display during this time-slot.
channel.
During time-slot 5, a memory within 02232 is interrogated . If
information was stored in this memory, a positive-going out-
CHARACTER GENERATOR
put is produced at pin 7. This pulse is connected to pin 10 of
the Column Decoder through 02243 to add one unit of cur- Each character to be displayed on the instrument CRT con-
rent at the input of the Column Decoder . This produces a sists of a series of connecting points developed on a possi-
zero after the character displayed during time-slot 4. During ble 8-point by 8-point grid (see Fig. 3-22). The 8-bit binary
time-slot 6, another memory within 02232 is interrogated to output from the Character Generator is used to determine
see if another zero should be added . If another zero is nec- the location of points within the grid, whether or not to pro-
essary, a second positive output is produced at pin 7, which vide a trace connecting two points, and the point at which a
again results in a column 1 output from the Column Decoder character has been completed. The Character Generator
and a second 0 in the CRT display. stage consists of an oscillator, the Lower Order Address
Generator, and an EPROM connected to a latch.
Finally, another memory within 02232 is interrogated during
time-slot 8 to determine whether the prefix should be
changed, or left at the value that was encoded. If data has
been encoded that calls for a shift in prefix, a negative-going
output level is produced at pin 7. This negative level sub-
tracts one unit of column current from the data at the input
to the Column Decoder. Notice, on the Character Selection
Matrix of Figure 3-20, that when row 4 is programmed, a
reduction of one column results in a one-column shift of the
prefix . For example, with the 100 ~V program shown in Fig-
ure 3-15 . If the data received from the plug-in called for a
shift in prefix, the CRT readout would be changed to 1 mV
(zeros deleted by program ; see Encoding the Data).
The 100 microamperes of quiescent current through R2242
provided by 02243 (see Display-Skip Generator) allows the
prefix to be shifted from m (100 microamperes of column
REV NOV 1985
Theory of Operation-7104
02151 and 02152 form a square-wave oscillator whose fre-
quency is adjustable with C2155 to provide 16 cycles within
the time allotted for developing a character. The base of
02152 goes LO when the Timer produces a negative going
Ready pulse at pin 13 . This starts the oscillator by turning
02152 on . The emitter of 02151 becomes more negative as
C2154 and C2155 discharge through R2154. The capac-
itors continue to discharge until the emitter-base junction of
02151 becomes forward biased . 02151 then begins to con-
duct and causes the oscillator to begin changing states . As
02151 conducts, the discharge through C2154 and C2155
stops and causes a collector current reduction in 02152.
The current reduction causes the emitter and base of 02152
to rise positive which pulls the emitter of 02151 along with
them through C2154 and C2155. This positive shift on the
emitter of 02151 turns it off. Now with C2151 conducting
and 02152 turned off, the voltage on the emitter of 02152
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