Tektronix 5441 Instruction Manual page 53

Theory of Operation-5441/~~441
Q1040B are connected as a comparator, Q1040A will re-
main on unless its base is pulled more negative than the
base of (210408. The analog data output from the column
data switch produces a 0.5 V change at the base of Q1040
for each unit of column current that has been encoded by
the plug-in unit. Therefore, whenever any information ap-
pears at the output of the column data switch, the base of
Q1040A is pulled more negative than the base of Q1040B,
resulting in a negative (LO) display-skip output to the timer
stage through (21052. 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.
(21048-Q1050 also provide display-skip action. The end-
of-word level connected to their emitters through R1050 is
LO only during time-slot 1. This means that Q1048-Q1050
are enabled only during time-slot 1. These transistors allow
the zero logic and memory stage to generate a display-skip
signal during time-slot 1 when information that is not to be
displayed on the crt has been stored in memory (further
information given under zeros logic and memory
discussion).
Column and Row Decoder
The column decoder, U1070, and row decoder, U1035,
sense the magnitude of the analog voltages at their inputs
and produce a binary output on one of ten lines correspond-
ing to the column or row data that was encoded by the plug-
in unit. These outputs provide the column digital data and
row digital data that 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 read-
out system to perform other functions. The input current at
pin 9 of the column decoder stage is steered to only one of
the ten column digital data outputs. When a display-skip
signal is present (collector of (21052 HI), pin 9 is pulled HI
through CR1052. 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 one of the ten row outputs
to saturate to ground.
Zeros Logic and Memory
The zeros logic and memory stage, U1060, 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 representa-
tion of the memory sequence is shown in Fig. 3-10. If the
plug-in unit encoded data from column 1, 2, 3, 4, or 10
agrees with row 3, the appropriate memory (or memories) is
set.
If data is encoded, 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 Q1050 in the display-skip
generator to produce a display-skip output. Since the infor-
mation that is encoded is only provided to set the memories
and not intended to be displayed on the crt at this time, the
display-skip output prevents a readout display if this encod-
ing occurs in time-slot 1.
During time-slot 5, memory A is interrogated. If informa-
tion is stored in this memory, a positive-going output is pro-
duced at pin 7. This pulse is connected to pin 10 of the
column decoder through (21056 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 that was encoded. If data has been en-
coded that calls for a reduction 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 Fig. 3-8 that a reduction of one column when row 4
is programmed results in a one unit reduction of the prefix.
For example, with the 100 FV program, if data was received
from the plug-in calling for a reduction in prefix, the crt read-
out would be changed to 1 mV (zeros deleted by program;
see Encoding the Data).
The 100 pA of quiescent current through R1041, provid-
ed by Q1056 (see Display-Skip Generator), allows the prefix
to be reduced from
p
(200 pA column current; column 2) to
m (00 FA column current; column 1). (Notice that if the prefix
program is reduced from column 1 to column zero, the read-
out system does not display a character at this readout
location.)
A further function of the zeros logic is the blank function.
If ten units of column current are encoded along with two
units of row current (row 3, column lo), the zero logic pro-
duces a negative-going output pulse at pin 1 of U1060. This
pulse lasts until the end of time-slot 10. Pin 1 of U1060 is
connected to the base of (21018 through R1020. When
turned on, Q1018 prevents the readout intensity current
from reaching the Z-Axis amplifier.
The end-of-word signal from the time-slot counter is con-
nected to pin 9 of U1060 through C1065. At the end of each
word of readout information, this pulse goes LO. This
erases the four memories in the zeros logic and memory in
preparation for the data to be received from the next
channel.
REV MAY 1982