Technical data

The main processor has an on chip 10-bit successive approximation ADC
with two selectable inputs: FLASH and FREQRNG. The FLASH input is
used to sample the residual charge on the main integrating
ADC output
of
U402. The FREQRNG input is used to make voltage ranging decisions
concurrent with frequency or period measurements.
The main
CPU’s pulse width modulated DAC outputs a 0 V to 5 V dc
level after filtering the 23 kHz output with
R507 and C512. This level is
used to adjust the precharge amplifier offset voltage in
U101. Port bits
are also configured to detect the front/rear input switch position (
FXR0)
and to measure the input power line frequency (
LSENSE). Frequencies
from 55 Hz to 66 Hz are measured as 60 Hz. All other input frequencies
are assumed to be 50 Hz.
The main
CPU communicates with the earth referenced processor U700
through an optically isolated (U506 and U704) asynchronous serial link.
Data is sent in an 11-bit frame at a rate of 187.5 k bits/second. When the
RS-232 interface is selected, data is sent across the isolated link at
93.75 k bits/second. The 11-bit data frame is configured for one start bit,
eight data bits, one control bit, and one stop bit.
Earth-Referenced Logic
The earth referenced logic circuits (schematic shown on page 9-14)
provide all rear panel input/output capability. Microprocessor
U700
handles GPIB (IEEE-488) control through bus interface chip U701 and
bus receiver/driver chips
U702 and U703. The RS-232 interface is also
controlled through
U700. RS-232 transceiver chip U706 provides the
required level shifting to approximate
±9 volt logic levels through
on-chip charge-pump power supplies using capacitors
C708 and C709.
Communication between the earth referenced logic interface circuits
and the floating measurement logic is accomplished through an
optically-isolated bi-directional serial interface. Isolator
U506 couples
data from
U700 to microprocessor U501. Isolator U704 couples data from
U501 to microprocessor U700.
5
Chapter 5 Theory of Operation
Earth-Referenced Logic
105