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LCD Display/Keypad Panel
The front panel user interface is comprised of a molded silicon rubber keypad that
actuates a membrane switch assembly. The keypad is environmentally sealed, designed
for long life and all keys with tactile feedback to the operator. The 2 1/2" x 4 3/4" LCD
is a high resolution graphic display with on/off backlite for visual clarity. Both keypad
and display are connected to the instrument I/O board via ribbon cables.
4.2.2 1865 Instrument Modules
Analog Instrument Board
The analog instrument board is the heart of the 1865, it includes the measurement
detector, high voltage source, and bus interface to the processor. This board also
contains a non-volatile memory in EEPROM (electrically erasable programmable read
only memory) for retaining vital calibration information. This is the main board in the
instrument compartment to the left of the power supply modules, all other boards are
mounted to it. A brief description on the operation of the measurement detector is as
follows.
Figure 4-1 shows the basic block diagram for the measurement module. A programmable
high voltage (0 - 1000V) is applied to the unknown component (Rx). The current
flowing through the unknown is measured by the current to voltage converter whose
output (E1) is applied to the A/D for processing by the instruments CPU. The measured
results can be expressed as E1 = -(Rs/Rx)Ex. This technique is dependent upon the
accuracy of the applied voltage source, so to improve measurement accuracy and
eliminate applied voltage dependency an extra current measurement based on another set
of reference standards (Rb/Ra) is made. This measurement results can be expressed as
E2 = -(Rb/Ra)Ex. The ratio of the E2/E1 results in the expression for the unknown Rx =
(Ra/Rb)(Rs)(E2/E1).
R
R
S
B
R
X
R
A
E
E
X
X
E1
E2
A/D
Figure 4-1: Detector Block Diagram
Page 80 of 87
Theory
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