HP 3457A - Page 188

Microprocessor Inoperative Reset. Under certain conditions it is possible for the microprocessor to
"lock-up" or become non-operational. During normal operation, the microprocessor continually toggles
the
INOP
signal
at
USI0,
pin
19.
If
the
TNOP
fails
to
toggle,
the
processor
is
reset
by
the
Watch
Dog
circuit as follows:
A
continually
high
or
low
TNOP
signal
applied
to
capacitor
C505
allows
the
input
of
inverter
US9Ib
to
go
low.
The
high
output
of
U591b
and
the
normally
high
RESET
signal
applied
to
the
inputs
of
gate
U598c
cause
its
output
to
go
low
and
reset
counters
US14
and
USI5.
After
being
reset
the
counters
begin
counting
the
2
MHz
clock
signal
applied
to
U514,
pin
10.
After
counting
for
slightly
more
than
0.5
seconds
the
output
of
counter
U5I5
pin
9
is
set
high.
The
high
output
of
U515
is
applied
to
the
minus
input
of
comparator
U519a
causing
its
output
to
go
low.
The
low
output
from
U519a
is
applied
to
the
plus
input
of
comparator
U519d
causing
its
output
to
go
low.
The
low
output
of
US19d
is
coupled
through
buffer
U594d
to
the
RESET
input
of
the
microprocessor
to
reset
it.
The
RESET
signal
from
buffer
U594d
is
also
applied
to
gate
U598c
causing
its
output
to
go
low
and
reset
counters
USI4
and
USIS.
Calibration Reset. The microprocessor is reset prior to each calibration routine to insure that valid
measurements
are
obtained.
When
calibration
is
requested,
the
microprocessor
stops
toggling
the
TNOP
signal at U510 pin 19, and waits for the circuit to time out as follows:
A continually high or low INOP signal applied to capacitor C505 allows the input of inverter US91b to
go
low.
The
high
output
of
U591b
and
the
normally
high
RESET
signal
applied
to
the
inputs
of
gate
U598c
cause
its
output
to
go
low
and
reset
counters
U514
and
USI5.
After
being
reset
the
counters
begin
counting
the
2
MHz
clock
signal
applied
to
U514,
pin
10.
.After
counting
for
slightly
more
than
0.5 seconds the output of counter U515 pin 9 is set high. The high output of USI5 is applied to the
minus
input
of
comparator
U519a
causing
its
output
to
go
low.
The
low
output
from
U519a
is
applied
to
the
plus
input
of
comparator
U519d
causing
its
output
to
go
low.
The
low
output
of
US19d
is
coupled
through
buffer
U594d
to
the
RESET
input
of
the
microprocessor
to
reset
it,
The
RESET
signal
from
buffer
U594d
is
also
applied
to
gate
US98c
causing
its
output
to
go
low
and
reset
counters
U5I4
and USIS.
8-49. Main Controller (03457-66511)
Beginning with serial number 2538A02954, 3457A’s were shipped with a new main controller board, the
03457-66511.
This
board
works
similar
to
the
original
main
controller,
the
03457-66501,
and
is
com-
pletely
backwards
compatible.
The
following
sections
describe
operation
of
the
new
main
controller
in
detail.
Refer to schematic Al 1/1 for the following discussion.
The Main Control Assembly manages the overall operation of the instrument. This includes managing
the measurement process, performing mathematical calculations, storing data, and communicating with
the front panel and with other instruments connected to the interface bus.
The
Main
Controller
consists
of
microprocessor
U601,
one
ROM
(U602)
and
one
RAM
(U603).
The
ROM
contains
the
operating
routines
for
the
microprocessor
while
the
RAM
provides
both
volatile
and
nonvolatile
memory.
Items
stored
in
the
nonvolatile
memory
of
RAM
are
the
calibration
and
auto
calibration
constants,
the
HP-IB
address
of
the
instrument,
and instrument
configuration
states selected
by
the
user.
HP 3457A Multimeter
8-23