Theory of Operation
2715 Spectrum Analyzer Service Manual 3-83
All of the control information is shiftedin through four shift registers U211,
U312, U610, and U520.
Front Panel
The Front Panel acts as an interface betweenthe user and the instrument. These
circuits translate operatoractions on front panel controls into data for the micropro-
cessor to read and execute.The Microprocessor outputs data showing current
operating modes to the user throughLEDs (light emitting diodes) and CRT readout.
The Front Panel consists of five sections. These sections are the Input Shift
Registers, the Switch Matrix, theDebounce circuitry, the Gray Code Knob, and the
Service Request/Acknowledge circuitry.See Figure 3--26.
Switch
Debounce
InputShift
Registers
6/
FPREQEN--
SwitchMatrix
11x 6
(59Total)
Switch
Debounce
And
Wire
Or Service
Requestand
Acknowledge
Circuit
Gray Code
Knob
OutputShift
Registers
Vcc
17LEDs
Total
Vcc
FPLATCH
DATAO
CLKO
FPREQ--
TLVL
FPACK--
2/
DATAI
CLKI
CRTINTS--
VLVL
2/ FrontPanel
Potentiometers
6/
11/
Figure 3- 26: Front Panel Block Diagram
The Input Shift Registers consist of threeserial input, parallel output shift registers
cascaded together.The outputs of the shift registers drive the front panel LEDs and
provide two control signals. The LEDs areconnected from the +5 V supply to the
shift register output through a 274 Ωresistor.The nominal LED current is 10 mA.
This current is high enough to provide adequatelight for the LED to be seen in direct
sunlight. A low (zero) is required on the output of the shift register to light the LED.
Two controlsignals are provided on the shift register outputs. They are the front
panel request enable(FP REQEN--) and the clock in enable (CLKIEN--).Both of
these signals are active low.

Input Shift Registers