Fluke 8505A Instruction Manual page 42

850G Series
The
response
data
from
the
DMM
will
be
in
3-byte format, as
shown
below, for
each voltage or current
reading.
The
first
byte
of this
response contains sign and
error
bits,
an
implied binary point,
and an
implied scale
factor
of ten.
Bytes
2
and 3
further define the reading.
If
the reading
is
negative, the sign
bits will
equal
"1",
and
all
three bytes
must
be
two's
complemented
before conversion.
If
the error
bit
is
equal
to
the
complement
of the
sign
bit,
an error
is
defined.
BYTE
2
10 11
1111
1
Implied Binary Point
Error
Bit —
Sign
Bits
In
this
example, the sign
bits
are "1"
and
the reading
is
negative. Since the
complement
of the sign
bit
does
not
equal the error
bit
("1"),
no
error
is
defined.
To
convert the response
in this
example, the two's
complement
must
first
be formed.
BYTE
3
1111 1111
BYTE
1
1111
0
111
0000 1000
+
0100 0000
+
0000
0001
-(0
5 + .015625 0 + 0 .000001)
=
0.515626
X
10 (the
implied scale
factor)
Further
conversion
to calculate the true reading Rt necessitates multiplication of the
A/D
Converter reading
(RAD)
by the scale
factor for
the instrument's
range and
function.
[R
t
=
Raq
X
Scale
Factor]
Ohms
Readings
In "I"
The
procedure
for
measuring
ohms
in
High Speed Reading
mode
is
more
complex. High
Speed
Ohms
readings
differ
from
Fast
Ohms
(Z1
)
readings;
when
using the
"!"
character, the
DMM
will
not
compute
the true
reading. This conversion
must
be performed by the
user.
Up
to
500 readings
a
second
are possible
when
using
High
Speed Ohms.
Refer
to
OPERATING
NOTES,
provided
with
Option -05
and -07
for
High Speed
Ohms
Reading procedures.
j
8/16 Bit
Toggle The
"/"
character
is
used
to
toggle
between
the
8-bit
v
J
and
the 16-bit
mode.
When
this
character
is
used
to
toggle from
one
mode
to
another, the
immmediate
and/or termination character
must
be placed
in
the
least significant
byte (LSB)
of
the
programming
word.
2A-6