Tektronix 7623, Instruction Manual

Page: 90 / 266

Circuit Description— 7623/R7623 Service
Fig. 3-34. Block representation of memory sequence in U2232.
memories. If the plug-in unit encoded data for column 1, 2,
3, 4, or 10 during time-slot 1, the appropriate memory (or
memories) is set. Notice that row 3 information from the
Row Decoder must also be present at pin 16 for data to be
stored in the memory of U2232. If data was encoded
during time-slot 1, a negative-going output is produced at
pin 7 as the memories are being set. This negative-going
pulse is connected to the base of Q2229 in the Display-Skip
Generator to produce a Display-Skip output. Since the
information that was encoded during time-slot 1 was only
provided to set the memories and was not intended to be
displayed on the CRT at this time, the display-skip output
prevents a readout display during this time-slot.
During time-slot 5, memory A is interrogated. If
information was stored in this memory, a positive-going
output is produced at pin 7. This pulse is connected to pin
10 of the Column Decoder through Q2240 to add one unit
of current at the input of the Column Decoder. This
produces a zero after the character displayed on the CRT
during time-slot 4. During time-slot 6, memory B is
interrogated to see if another zero should be added. If
another zero is necessary, a second positive output is
produced at pin 7 which again results in a column 1 output
from the Column Decoder and a second zero in the CRT
display.
Finally, memory C is interrogated during time-slot 8 to
obtain information on whether the prefix should be
reduced or left at the value which was encoded. If data has
been encoded which calls for a reduction in prefix, a
negative-going output level is produced at pin 7. This
negative level subtracts one unit of column current from
the data at the input to the Column Decoder. Notice on the
Character Selection Matrix of Fig. 3-24 that a reduction of
one column when row 4 is programmed results in a one unit
reduction of the prefix. For example, with the 100µV
program shown in Fig. 3-31, if the data received from the
plug-in called for a reduction 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
R2213 and R2214 that was provided by Q2240 (see
Display-Skip Generator) allows the prefix to be reduced
344