HP 3457A - 8-69. 44492 A 10 Channel Reed Multiplexer; 8-70. Closing a Channel; 8-71. TROUBLESHOOTING

8-69. 44492A 10 Channel Reed Multiplexer
Refer to the 44492A schematic for the following discussion,
The
10
Channel
Reed
Multiplexer
is
used
to
connect
one
of
ten
channels
to
the
input
of
the
3457A’s
high
and
low
terminals.
The
44492A
can
be
the
instrument’s
input
source
for
DC
voltage,
AC
voltage.
AC+DC
voltage,
resistance,
frequency
and
period
measurements.
Current
and
4-wire
resistance
measurements
cannot
be
made
through
the
44492A.
To
identify
the
card
as
option
44492A
pin
6
of
connector
P851
is
tied
to
the
+5
volt
power
supply,
This
signal
is
used
by
the
instrument's
inguard
processor
as
an
ID
line.
8-70. Closing a Channel
To
close
a
channel
the
instrument's
inguard
controller
(A2U501)
sends
16
bits
of
configuration
data
to
shift
register
U851
at
pin
2.
Shift
register
U851
stores
the
last
8
bits
of
the
configuration
string
while
the
first
8
bits
are
shifted
into
register
U852.
After
all
16
bits
are
clocked,
the
output
of
the
shift
register
is
enabled
by
a
strobe
pulse
initiated
by
the
inguard
controller.
Outputs
of
shift
registers
U851
and
U852
are
normally
high
and
active
when
low.
Outputs
of
the
registers
drive
the
base
of
transistors
U853-U855.
When
the
base
of
the
transistors
are
driven
low,
the
transistors
are
turned
on
and
are
used
to
source
current
across
relay
coils.
An
example
of
how
to
close
channel
0
is
given
below.
©
A
serial
string
of
15
low
states
followed
by
one
high
state
is
clocked
into
U851
and
U852.
©
The
strobe
input
of
the
shift
registers
(pin
|
of
U851
and
U852)
is
driven
high.
©
Output
QI
of
shift
register
U851
is
low
while
all
other
outputs
remain
high.
©
The
low
QI
output
applied
to
transistor
U854
pin
13
causes
the
transistor
to
turn
on
and
act
as a
current
source.
©
Current
flows
from
the
emitter
of
U854
through
the
coil
of
K851
to
ground.
This
current
causes
an
electromagnetic
field
that
force
the
contacts
of
K851
closed.
All
ten
relays
used
on
this
card
are
reed
relays.
These
relays
must
have
current
applied
across
their
coils
for
the
contacts
to
be
closed.
As
soon
as
the
current
is
removed
the
relays
begin
to
open.
Resistors
R861
and
R862
combine
with
inductors
L851
and
L852
to
limit
surge
currents.
Surge
currents
result
from
inputs
with
varying
potentials
being
switched
to
the
instrument.
By
lowering
these
surge
currents
relay
life
is
extended.
8-71. TROUBLESHOOTING
This
information
will
assist
in
isolating
the
failure
to
the
component
level
on
the
HP
3457A
multimeter
module.
The
information
provided
assumes
a
problem
was
indicated
by
either
the
power-on
or
opera-
tion
verification
tests.
HP 3457A Multimeter
8-35