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Theory of Operation (2635A)
Detailed Circuit Description
2A
2A-7
2A-14. Channel Selection Circuitry
This circuitry consists of a set of relays and relay-control drivers. The relays form a tree
that routes the input channels to the measurement circuitry. Two of the relays are also
used to switch between 2-wire and 4-wire operation.
2A-15. Open Thermocouple Check Circuitry
Under control of the Inguard Microcontroller, the open thermocouple check circuit
applies a small ac signal to a thermocouple input before each measurement. If an
excessive resistance is encountered, an open thermocouple input condition is reported.
2A-16. Input Connector Assembly
The following paragraphs briefly describe the major sections of the Input Connector
PCA, which is used for connecting most of the analog inputs to the instrument.
2A-17. 20 Channel Terminals
Twenty HI and LO terminal blocks are provided in two rows, one for channels 1 through
10 and one for channels 11 through 20. The terminals can accommodate a wide range of
wire sizes. The two rows of terminal blocks are maintained very close to the same
temperature for accurate thermocouple measurements.
2A-18. Reference Junction Temperature
A semiconductor junction is used to sense the temperature of the thermocouple input
terminals. The resulting dc output voltage is proportional to the block temperature and is
sent to the A/D Converter PCA for measurement.
2A-19. Display PCA
The Display Assembly controller communicates with the main Microprocessor over a
three-wire communication channel. Commands from the Microprocessor inform the
Display Controller how to modify its internal display memory. The Display Controller
then drives the grid and anode signals to illuminate the required segments on the
Display. The A2 Display Assembly requires power supply voltages from the Power
Supply, a reset signal from the Reset Circuit, and a clock signal from the Digital Kernel.
2A-20. Memory Card Interface PCA
The Memory Card Interface PCA is used to access the memory on an industry standard
memory card installed through the slot in the front panel of the instrument. This
assembly allows management of the memory card power, adapts timing of accesses by
the Digital Kernel to the memory card, and provides visible indicators for low battery
voltage and memory card busy status.
2A-21. Detailed Circuit Description
2A-22. Main PCA
>The following paragraphs describe the operation of the circuits on the Main PCA. The
schematic for this pca is located in Section 8.