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Theory of Operation (2620A/2625A)
Introduction
2
2-3
2-1. Introduction
The theory of operation begins with a general overview of the instrument and progresses
to a detailed description of the circuits of each pca.
The instrument is first described in general terms with a Functional Block Description.
Then, each block is detailed further (often to the component level) with Detailed Circuit
Descriptions. Refer to Section 8 of this manual for full schematic diagrams. The
Interconnect Diagram in this section (Figure 2-1) illustrates physical connections among
pca’s.
Signal names followed by a ’*’ are active (asserted) low. All other signals are active high.
2-2. Functional Block Description
Refer to Figure 2-2, Overall Functional Block Diagram, during the following functional
block descriptions.
2-3. Main PCA Circuitry
The following paragraphs describe the major circuit blocks on the Main PCA.
2-4. Power Supply
The Power Supply functional block provides voltages required by the vacuum-
fluorescent display (-30V dc, -5.0V dc, and filament voltage of 5.4V ac), the inguard
circuitry (-5.4V dc VSS, +5.3V dc VDD, and +5.6V dc VDDR), and outguard digital
circuitry of +5.1V dc (VCC).
Within the Power Supply, the Raw DC Supply converts ac line voltage to dc levels. The
5V Switching Supply converts this raw dc to 5.1V ±0.25V dc, which is used by the
Inverter in generating the above-mentioned outputs. The Power Fail Detector monitors
the Raw DC Supply and provides a power supply status signal to the Microprocessor in
the Digital Kernel.
2-5. Digital Kernel
The Digital Kernel functional block is responsible for the coordination of all activities
within the instrument. This block requires power supply voltages from the Power Supply
and reset signals from the Display Assembly.
Specifically, the Digital Kernel Microprocessor performs the following functions:
• Executes the instructions in ROM.
• Stores temporary data in RAM.
• Stores instrument configuration and calibration data in nonvolatileRAM and
EEPROM.
• Communicates with the microcontroller on the A/D Converter PCA viathe Serial
Communication (Guard Crossing) block.
• Communicates with the Display Controller to display readings and userinterface
information.
• Scans the user interface keyboard found on the Display Assembly.
• Communicates via the RS-232 interface and optional IEEE-488interfaces.
• Reads digital inputs and changes digital and alarm outputs.