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Section
VIII
THEORY
OF OPERATION
Model
3455A
Figure
8-35. Simplified
Program
ROM
Circuit.
8-113.
Program
ROM.
Figure
8-35
shows
a
simplified sche-
malic diagram
of
the
program
ROM
circuitry
used
in
the
inguard
controller.
The
program
RO.M
contains the opera-
tion instructions for
the inguard controller routine.
There
are
512
eight
bit
program
storage locations
contained
in
the
ROM
which
are
addressed
by
nine input
lines
(10
through
18).
The
ROM
output
is
connected
to the
processor
data
lines
D0
through D7.
The
output
ofjlie
ROM
is
enabled
only
when
the
signal level
applied
to
18
is
thccomplement
of
the
level
applied to
18.
This function
is
accomplished
by
gates
Lf29C
and 1/29D and
occurs
wlieri
(he processor
sets
the
"program
gate"
signal high.
8-114.
Output
Circuit.
The
output
circuit
of
the inguard
controller controls
all
inguard switching
of
the voltmeter.
Switch
control
signals are
transferred
through
six
latches
(sec
Figure 8-36).
Hach
latch
is
set
individually to
output
the
proper switch
signals.
The
inguard processor
sets
the
switching information
for
the latches
on
the data
bus
(pro-
cessor
output
D0
through
DS)
and
the
select
code
for
the
particular latch to
accept the information
on
device
select
lines
DS0
through
DS3.
Transfer
of
information
from
the
processor to the latches
is
synchronized
by
(he
clock input
to the
processor
and
(he device
select
decoder U14.
8-115.
MAIN CONTROLLER.
8116.
General.
8-1 17.
The
purpose of
the
main
controller
is
to
control
communication between
the front panel.
HP-IB
interface,
display
and
inguard section of the multimeter
and
to per-
form mathematical
calculations to correct
measurement
data
and
provide
measurement
scaling
or
percent
error
readings.
The
following
is
an explanation
of
the operations
performed
by
the
main
controller as
illustrated
in
Figure
8-37.
a.
The
main
controller reads
and
stores
the status
code
of
(he front panel switches
flocal
operation) or
HP-IB
buf-
fers
(remote
operation)
to determiflO
inOaSUremeiU
para-
meters, such
as range, function,
mode,
and sample
time.
b.
Using
the status information, the
main
controller
generates switching information
for
the analog
section
and
transfers
this
information
to the
inguard
controller.
c.
During
(he
time
the inguard controller
is
setting
the
switches
and
making
the required
measurements,
the
main
controller
computes
(he constants used to correct the
mea-
surement
data.
This
step
is
shown
in
Figure
8-37
as
"per-
form
preliminary
math
calculations"
and
involves
combin-
ing
(he
offset error
and
full
scale
error readings, relevant to
the
measurement
being
made,
into
two
constants.
d.
The main
controller receives the
measurement
data
from
the inguard
controller, applies
the correction factors
found
in
the previous step
and
computes
the corrected
measurement
answer.
8-22
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