Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
instrument bus. If the encoder was requesting service, the
low on DBO, when written back, is clocked into U2018B
when POLL is removed by the microcomputer. The output
of U2018B then cancels the encoder strobe with its low-to-
high transition.
Switch Matrix. The switch matrix includes both momen
tary contact and rotary switches. One side of each switch is
connected to — 12 V through a resistor in parallel with a Y
input. The other side of the switch is connected in parallel
with the other switches in the column to an X output.
When an X output is asserted, the Y inputs remain at a
negative voltage unless a contact is closed. If the contact is
closed, the X output raises the Y input for that switch to a
positive voltage. Rotary switches occupy as many positions
in the matrix as they have contacts. The switches are wired
to yield the codes shown in Fig. 5-39.
Rotary switches for RESOLUTION BANDWIDTH and
SPAN/DIV are a special case. Although each occupies four
positions in the matrix, the two are used only as up/down
prompts to the microcomputer to change the corresponding
parameter. The microcomputer notes the initial setting and
changes the parameter accordingly when the switch is
moved, keeping track of the direction the switch was
changed by comparing its new position to its old.
Power-up Circuit. When the microcomputer performs its
power-up routine, it writes a 1 in bit 3 address 74. Because
this bit is not latched, the microcomputer continues to write
this
1
in the bit while the keyboard encoder is initialized.
The keyboard encoder is initialized by setting to zero the
bits in the shift register that represent the rotary switches.
This happens because:
1) writing a 1 to bit 3 sets both inputs of U3012A high,
turning off Q6028;
2) Q6028 off allows — 12 V to be applied through R6028 to
the rotary switch X inputs. This overrides the keyboard
encoder X scan signals, so the Y inputs remain low without
regard to the position of the switch;
3) because the microcomputer continues to write a 1 in bit
3, the keyboard encoder is given enough time to update its
shift register with all zeros representing the rotary switch
contacts.
After the keyboard encoder is initialized,
the
microcomputer resets bit 3. This restores the switch matrix
to normal operation, and the keyboard encoder reads the
position of the rotary switches as changes in the switch
matrix. It outputs these apparent changes to the
microcomputer, which interprets them as the power-up val
ues for TIME/DIV and MINIMUM RF ATTENUATION and
the initial switch position for REFERENCE LEVEL, FRE
QUENCY SPAN, and RESOLUTION BANDWIDTH.
FREQUENCY Encoder
The center frequency control is a rotary switch that gen
erates a gray code. It is decoded as shown in Fig. 5-40.
Up/Down Encoding. The gray code changes one bit at a
time, causing U6025A to change state for each position
change of the switch. This pulses a low on the input of either
U5025B or U5025C (the other input remains high), making
the inputs to U6025B momentarily unequal. As a result,
U6025B pulses the set input of U4015B to assert SER
REQ.
The same pulse is inverted to clock the up/down flip-flop,
U4015A. This flip-flop records the direction of change in the
switch, determined by the exclusive-OR of the previous
state of B and the current state of A. The trailing edge of the
pulse from U6025B updates U2018A to remember the cur
rent state of B for the next cycle.
Exclusive-OR U6025D detects the direction of change in
the FREQUENCY control because of the property of the
gray code. Down (counterclockwise) yields unequal inputs
when the previous state of B is compared to the current
state of A, while up (clockwise) yields the opposite. The
up/down condition is clocked into U4015A and is read by
the microcomputer as the MSB of the output port.
FREQUENCY interrupt Interface
In its quiescent stage, U4015B is held cleared by the
feedback from its Q output to its clear input. A low on its set
input temporarily forces U4015B to set both its outputs
high, allowing the low on the set input to set the flip-flop.
When set, U4015B asserts SER REQ and drives U3014F
to assert DB3 when the microcomputer performs a poll.
U3014F is enabled during a poll as noted above for inter
rupts under Switch Encoding. U4015B is cleared (if it was
asserting DB3) when the microcomputer writes back the
parallel poll byte to clear all interrupts that were read.
Potentiometers
Some controls generate analog signals used by other
functions in the instrument. These controls are non
programmable.
REV AUG 1981
5-91
