SINGLE SLOPE
ENABLE/DISABLE
-
n
SINGLE SLOPE
BUFFER
R117C
V
V
A
fl
R117B
R128
CHARGE BALANCE
COMPARATOR
+
5v
CLOCK
1
U118B U117A
A
INPUT
ENABLE/
DISABLE
TO
1rP
4.4.2 Address Decoding
Figure
4-6.
A/D
Converter
U123 is used to latch in the address that is on the bus when the
address strobe of U124 goes high and presents
it
to the PROM
(U122) during data strobe.
4.4.3 PIA
u114 provides for most of the control of the instrument.
It
controls all ranging hardware, A/D converter, and data out-
put and input for the
IEEE
option.
The display board also houses the special function keys: dB,
REL, STO/CLR and RCL.
4.5 DIGITAL CALIBRATION
The Model 197 uses digital calibration
to
eliminate all poten-
tiometers in the instrument to facilitate calibration. The con-
stants that the Model 197 uses are stored in a nonvolatile elec-
trically alterable read only memory (U113), and are read on
power-up of the instrument. There is one constant for each
range on DCV, ACV and
0,
except for the 750VAC range
which uses the constant from the 2V range.
On
the DCA and
ACA functions the 200mV DC and 200mV AC constants are
used respectively.
4.4.4 Display Board
4.6 POWER SUPPLY
The LCD display is driven by a flat pack LCD controller chip
U201 and
it
communicates to the microprocessor through
4
control lines. During power-up the microprocessor configures
U201 to drive the triplexed display.
Fuse F102 is the LINE FUSE which is internally accessible.
SlOl is the power on/off switch and S102 selects
115V
or
230V operations by placing the transformer primary win-
dings in parallel
or
series.
In order to drive the display correctly four voltages are ob-
tained from R134. The clock required by U201 is obtained
from
U119.
4-6
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