Valhalla 2300, Operation Manual

Page: 78 / 115

the input for a precise time (10,000 counts of
the clock) by charging C2 linearly toward the
input voltage. The waveform on C2 is a linear
ramp with a slope proportional to the input
voltage. Thus, the charge on C2 at the end of
this phase is also proportional to the input
voltage.
During the third, or reference-integrate stage,
a precise reference voltage of polarity
opposite to the input voltage is applied to the
integrator. IC2 begins discharging C2 toward
the reference voltage. Because the reference
voltage is fixed, the discharge slope is always
the same. Therefore, the time required to
discharge C2 back to zero is proportional to
the original input voltage.
The time is measured in IC3 by counting the
oscillator pulses. The IC2-IC3 set is designed
to give a count of 10,000 for a 1.0000VDC
input. The count, which is numerically equal
to the value of the input to IC3, is displayed as
the measured voltage. The reference
integration counts, which are accumulated in
an internal counter of IC3, are latched into a
multiplexer also inside IC3, at the end of the
count period.
During the next series of auto-zero and
integration periods, each digit of the counts
previously latched into the multiplexer are
sequentially placed on the BCD outputs of
IC3 and on the inputs of IC1, a binary-to-
seven segment decoder/driver. While the
most significant digit data is applied to IC1,
transistor TR1 is turned on by the digit output
of IC3 to provide the anode voltage for DS1.
When the next digit data is applied to IC1,
TR1 is turned off and TR2 is turned on to
provide the anode voltage for DS2. The
sequence is continued for TR3/DS3, TR4/DS4
and TR5/DS5 and then repeated. The
repetition rate is sufficiently high that all
digits appear to be continuously illuminated.
When 3
φ
3-wire or 3
φ
4-wire displays are
selected, the voltage and current displays are
blanked by the control logic. The logic places
a low on pin 4 of IC1, disabling its outputs.
The "9" bars of the displays are illuminated by
CR22.
10-3-10. Control Logic
The schematics for this section may be found
on 2300-072 sheets 1, 2 and 3. The outputs
from either the keyboard or the IEEE interface
are selected by IC's 7, 15, and 23. The
remote/local line from the IEEE interface
determines which signals drive the control
logic.
The outputs of IC7 are decoded by IC8 to
drive the voltage range LED's, voltage display
decimal points, and signal processing board
voltage ranges. The outputs of IC7 also drive
IC20 and IC8 on the main board.
The outputs of IC15 are decoded by IC16 to
drive the current range LED's. The outputs
also drive IC13 and IC20. The outputs of
IC13 drive the current display decimal points
and the signal processing board current
ranges.
The outputs of IC23 are decoded by IC24 to
drive the display selection LED's. Diode logic
is used to control the phase selection relays.
The outputs are also used to blank the voltage
and current displays for 3-phase displays. The
output of IC23 also drives IC20 which
controls the power display decimal points and
KW-W LED's.
10-3-11. Keyboard
The schematics for this section may be found
on 2300-072 sheets 1, 2 and 3. The front
panel push-buttons are scanned by IC's 6, 14,
and 22. These IC's scan the voltage, current
and display selection push-buttons,
respectively. Their outputs are BCD coded
signals corresponding to the last push-button
pressed. A power-up reset circuit composed
of TR20, C34, R53, and R54
momentarily energizes relays K1-K3.
The contacts of these relays are