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DC
Current
On
changing
functions
to
DC
Current, the
Working
Reference
voltage
is
switched
to
drive
a
voltage-to-current converter,
and
the
OUTPUT
display
legend
is
changed
to
|liA,
m
A
or A.
Over-
voltage protection
is
provided,
and
the
Output
lines
are fused.
AC
Current
An
AC
Current output
is
produced
by
the voltage-to-current
converter.
The
100|iA
and
1
A
ranges
are
driven
directly
from
the basic
IV
range,
and
the others
from
the
10
V
range.
Range
selection
is
achieved
by
switching
internal shunts.
Output
protection against
over-voltage
is
provided,
and
the
output
lines
are fused.
The
OUTPUT
display
legend
is
altered to
|
0 .A,
m
A
or
A.
Resistance
Remote
Sense.
One
of a
set
of
eight precision
resistors
is
internally
4-wire
connected
to
the I+,
I-,
Hi
and
Lo
terminals
by
operation
of each
RANGE
key
.
Simultaneously
the 4-
wire
calibrated
value of
the
resistor
is
displayed
(OUTPUT
display).
Pressing
the
OUT-
PUT
Zero
key
connects a
true
4-wire
short
to
the terminals,
and
the
OUTPUT
display indicates
zero.
This zero
display value
cannot
be
recalibrated.
Local
Sense (Remote Sense
LED
Unlit).
The
connections
to
the
resistor
remain
the
same, but
the display
value includes
the resistance
of
the
connections
from
the
Hi
and
Lo
terminals
to
the
resistor.
The
arrangement
provides a
calibrated 2-
wire
facility
with
external
connection
to
the
Hi
and
Lo
terminals.
The Zero
key
shorts the
Hi and
Lo
terminals,
in
this
case
the resistance
between
the terminals
is
displayed
and
may
be
recalibrated.
When
Q
is
selected
from any
other
function, the
4708
is
forced
into
Remote
Sense, but
this
may
be
deselected for
2-wire
operation.
Autocalibration
By
setting the
CAL ENABLE
security
keyswitch
on
the rear
panel
to
ENABLE,
the
4708
can
be
calibrated.
(Refer
to
Section
8).
The
output value
is
measured and
the
microproces-
sor
is
activated, to
add any
new
corrections
to factors
already
retained
in
non-volatile
memory. The
updated
correction
fac-
tors
are
applied
in
the
normal
RUN
mode.
Processor
A
6802-series
microprocessor
controls the
internal
performance
of
the instrument,
employing 26k
bytes
of
program memory.
2k
bytes
of
memory
are
used
for
stack
and
work
space,
and 2k
bytes are
made
non-volatile
by
a battery-powered
back-up
supply, storing
calibration correction
factors.
With
the
exception of
the
Power
ON/OFF
switch,
each
front
and
rear
panel
control
provides an
input
to
the
microprocessor
system,
which
translates
the
information
to
command
the
4708
analog
and
calibration functions.
The
processor
also controls the display, the
IEEE
488
Interface
Bus and
the
operation
of
the
restart
and
error
circuitry.
1-6
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