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Service Manual
2A-6
2A-6. Serial Communication (Guard Crossing)
This functional block provides a high isolation voltage communication path between the
Digital Kernel of the Main PCA and the microcontroller on the A/D Converter PCA.
This bidirectional communication circuit requires power supply voltages from the Power
Supply block.
2A-7. Digital Inputs and Outputs
This functional block contains the Totalizer and External Trigger Input Buffers, eight
bidirectional Digital I/O channels, four Alarm Outputs, and the Input Threshold control
circuits. These circuits require power supply voltages from the Power Supply and signals
from the Digital Kernel.
2A-8. A/D Converter PCA
The following paragraphs describe the major blocks of circuitry on the A/D Converter
PCA.
2A-9. Analog Measurement Processor
The Analog Measurement Processor (A3U8) provides input signal conditioning, ranging,
a/d conversion, and frequency measurement. This custom chip is controlled by the A/D
Microcontroller (A3U9). The A/D Microcontroller communicates with the Main PCA
Microprocessor (A1U1) over a custom serial interface.
2A-10. Input Protection Circuitry
This circuitry protects the instrument measurement circuits during overvoltage
conditions.
2A-11. Input Signal Conditioning
Here, each input is conditioned and/or scaled to a dc voltage for measurement by the a/d
converter. DC voltage levels greater than 3V are attenuated. To measure resistance, a dc
voltage is applied across a series connection of the input resistance and a reference
resistance to develop dc voltages that can be ratioed. DC volts and ohms measurements
are filtered by a passive filter. AC voltages are first scaled by an ac buffer, converted to a
representative dc voltage by an rms converter, and then filtered by an active filter.
2A-12. Analog-to-Digital (A/D) Converter
The dc voltage output from the signal conditioning circuits is applied to a
buffer/integrator which charges a capacitor for an exact amount of time. The time
required to discharge this capacitor, which is proportional to the level of the unknown
input signal, is then measured by the digital counter circuits in the Analog Measurement
Processor.
2A-13. Inguard Microcontroller Circuitry
This microcontroller (and associated circuitry) controls all functions on the A/D
Converter PCA and communicates with the digital kernel on the Main PCA. Upon
request by the Main PCA, the inguard microcontroller selects the input channel to be
measured through the channel selection circuitry, sets up the input signal conditioning,
commands the Analog Measurement Processor to begin a conversion, stops the
measurement, and then fetches the measurement result. The inguard microcontroller
manipulates the result mathematically and transmits the reading to the digital kernel.