Fluke 8600A Instruction Manual page 28

8600A
3-42. Control and
Display
343. The
control portion of
the
instrument
consists of
contacts of
the range
and
function switches, a three-pole-
double-throw
switch
U16,
a bipolar
ROM
U9,
and custom
integrated
circuit
U8.
The
range and
function switch
posi-
tions in
conjunction with
strobe
signal
inputs to
U16,
con-
trol
the illumination of the
polarity indicator
on
the display
and
the
selection
of
five
or
six
ranges for the autorange
feature.
Refer to Figure 8-1 for the schematic diagram of
the control
circuits.
3-44
All
timing and
control
information
is
developed
in the
DVM
IC,
U8, from an
external clock consisting
of
Y1
and U7. Range information can be manually programed
by
the front panel range
and
function switches, or auto-
matically
programed by U8.
Eight strobe
signals
(STO-
ST7)
are
developed
in
U8
and
are used for range
program-
ming and
display timing. Digit information
from
the
counter
is
strobed out
in
bed format on
lines
W,X,Y, and
Z
(weighted
8,
4,
2,
and
1
respectively).
The
strobe
signals
are
strobed sequentially but the
digit
information
pre-
sented
on W, X, Y, and
Z
is
interleaved.
The main
signi-
ficant digit
(MSD)
without
polarity
appears
at
STO. The
MSD
with
polarity
is
presented
at
ST7. For
the display,
either
STO
or
ST7
is
selected
by U16, which
is
programed
by
the function switches.
The
second
significant
digit
(2SD) appears on
W,
X, Y, and
Z
at
ST2,
the
3SD
at
ST4,
the
4SD,
at
ST6, and
the
5SD
at
ST1
.
W,
X, Y, and
Z
pre-
sent the
digit
information
to a seven
segment
decoder,
U10.
The
strobe
signals light
the appropriate display
LED, U1
1-
U15, through
the strobe
drivers,
Q29-Q39. The
decimal
point
logic
(DPL) from
U8
outputs
a signal at the
appro-
priate strobe
signal.
For
the lowest range and
the
20
range,
lower
or
upper
annunciators
will light.
ST
signal
(ST3
for the
20V
range
etc.).
In
autoranging
(3
is
always connected
to
ST1
as
the lower
limit
and a
is
connected to
ST5
(5
ranges)
for
DCV
and
ACV
functions
and
to
ST6
for the
Ohms
function.
3-46 For
A/D
Converter timing
all
outputs (INT,
AZ,
DE(+R), and
DE(-R)
)
from
U8
are logic
NOT
outputs.
For example
the
INT
output
(pin 40) varies
between
OV
and
-15V. During
the integrate
signal
pin
40
stops con-
ducting
and
goes to
—15V,
biasing the
driver,
Q10,
off.
The
gate
of Q14 goes
to
—
2V
allowing
it
to conduct.
At
the
end of
the integrate
signal
pin
40 conducts and
goes
to
OV,
biasing
Q10
on which
drives
the gate of
Q14
to
—15V,
cutting
Q14
off.
The
other control
signals
work
the
same way
except that the gates of
the
FETs
go to
OV
during
their signal
on
times.
3-45 Range
information outputs
from
U8
appear
at
a,
b,
and
c (pins 29, 28,
and
30)
which
are
weighted
4, 2,
and
1
respectively.
The range information
is
also
presented
on W,
X, Y, and
Z
at
ST5
for the
DOU
output.
The
out-
puts
at
a,
b,
and
c are
steady
state
outputs
applied to
U9.
U9
interprets the
information and
selects the appropriate
relays.
Range
truth tables are included
in table
4-12. In
all
cases the
output
of
U9
goes
low
to
select a relay
because
Relay
Common
is
connected
to
+5V
through
S4A
(DCmA).
Relay,
Kl,
is
energized only
in
the
20V,
200V
and
1200V
ranges
of
the
DCV
function.
Kl
inserts
the Input Divider
between VS2 HI
terminal
and
the
A/D
Converter.
The
lower two
ranges
are
applied
directly to
the
A/D
Converter.
In
the lowest range
for
all
functions
U17
selects the
.IV
reference
and
switches
R66
into the charge path of C28.
In
range programming
U8
the
a
input
sets
the
upper
range
limit
and
the
|3
input
sets
the lower
limit.
Strobe
signals
are applied
through U16 and
the range switches
to
the
a
and
(3
inputs.
For manual
range
programming a
and
j3
are tied together
by U16.
S6-S1
1
apply
the appropriate
3-8
1/77