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Theory of Operation (2620A/2625A)
Detailed Circuit Description
2
2-7
2-14. 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.
2-15. 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.
2-16. 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.
2-17. 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.
2-18. 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.
2-19. 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 and a clock signal from the A1U4 Microprocessor.
2-20. Memory PCA (2625A Only)
The Memory PCA is used by the Digital Kernel to store nonvolatile measurement data.
This block requires power supply voltages from the Power Supply, a reset signal from
the Display PCA, and signals from the Digital Kernel.
2-21. IEEE-488 Option (-05)
Theory of operation for the IEEE-488 Option (-05) is presented in Section 7 of this
manual. The related schematic diagram is found in Section 8.
2-22. Detailed Circuit Description
2-23. 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.