Tektronix 7623, Instruction Manual

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Circuit Description —7623/R7623 Service
Fig. 3-32. Typical encoding scheme for voltage-sensing amplifier plug-in unit. Coding shown for deflection factor of 100 microvolts.
Character Selection Matrix, two units of column current
along with the two units of row current encoded by R10
(row 3) indicates that two zeros should be added to the
display. Resistor R120 adds one unit of column current
during time-slot 2 and along with the one unit of current
from the row output, the Readout System is instructed to
add an invert arrow to the display. R130 is not connected
to the time-slot 3 line since the vertical deflection factors
are calibrated. Therefore, there is no column current output
during this time slot and there is no display on the CRT
(see Display-Skip Generator for further information).
During time slot 4, two units of column current are
encoded by R140. There is no row current encoded during
this time-slot and this results in the numeral 1 being
displayed on the CRT. Neither row nor column analog data
is encoded during time-slots 5, 6, and 7 as defined by the
Standard Readout Format. During time-slot 8, two units of
column current and three units of row current are encoded
by resistors R181 and R80 respectively. This addresses the
µ prefix in the Character Selection Matrix. The final data
output is provided from time-slot 9 by R190 connected to
the column output and R90 to the row output. These
resistors encode three units of column current and four
units of row current to cause a V (volts) to be displayed.
Time-slot 10 is not encoded in accordance with the
Standard Readout Format. The resultant CRT readout will
be I 100 µV.
In the above example, the row analog data was pro­
grammed to define which row of the Character Selection
Matrix was addressed to obtain information in each
time-slot. The column data changes to encode the appli­
cable readout data as the operating conditions change. For
example, if the variable control of the plug-in unit was
activated, R130 would be connected between time-slot 3
and the column analog data output lines. This encodes 10
units of column current (see shaded area in time-slot 3 of
the waveform shown in Fig. 3-33B). Since one unit of row
current is also encoded during this time-slot by R30, a >
symbol is added to the display. The CRT readout will now
say I >100 µV. In a similar manner, the other switches can
change the encoded data for the column output and
thereby change the readout display. See the descriptions
which follow for decoding this information.
The column analog data encoded by the plug-in can be
modified by attenuator probes connected to the input
3-41