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22
1.3.
5.3.
Counter
The
counter
receives logic
input
pulses
from
the
ADC
ouput, or
from
the
comparator output
when
measuring
in
the
frequency
mode.
Selection
between
the
two
inputs
is
made
in
the counter
input control stage
by
a
control
signal
from
the
microprocessor.
The
counter
itself
consists
of
two
high-speed
4-bit
binary counters
in
cascade
which
give
an
8-bit
output
via
buffers
to the databus.
When
measuring
frequency,
additional
counting
capacity
is
provided
by
the
internal
counter
of the micro-
processor.
a)
Counter
input control
The
data input
from
the
ADC
is
the varying duty-cycle
pulses
that represent the charge-discharge cycle of
the
integrator capacitor.
In
the counter the
result
of the
first
measuring period
is
obtained;
i.e.
the
number
of
clock-pulses passed during the charge
parts
of the
cycles.
At
the
end
of
this
period the
jJ-P
timer carry
signal
AZ
changes
the
state
of the control
logic
and
also reverses
the input
voltages to
the
ADC
to
compensate
for
any
zero
drift.
A
second measuring
period follows
and
the
resultant
measurement
is
the
mean
of the
two
periods.
The
data input
from
the
comparator
consists
of
logic
pulses
representing the
frequency measurement.
These
are
gated to the counter
when
a
logic
1
is
sent
from
the
(XP
on
the
CT/ADC
line. In
this
measuring
function, the data input path
from
the
ADC
is
inhibited.
b)
Counter output
control
Whatever
input
is
selected,
the counter
reads
in
the
pulses after
it is
reset
an enabled
by
the
IJ.P
control.
The
8-bit
output
is
connected
to
output
buffers
and
applied to the
databus
when
the counter
output
is
enabled
by
the microprocessor.
In
the
frequency measuring
mode
the
most-significant
output
bit
from
the counter
is
routed to the
T1
input of the microprocessor to
use
the
internal
counter
as
extra capacity
in
the event of overflow.
1.
3.5.4.
Analog
control
The
analog
control section can
be
divided
into
two
parts;
—
The
inputs to the microprocessor that
indicate
the
status
of the analog section
—
The
output
circuits
of the
pP
that control
the analog
section
according to the
settings
of the
function
selector
and
the inputs
from
the
mode
switches
and
signals
to
be measured.
a)
Control
inputs
The
various inputs to the control section provide the
means
for
the
juP
to sense the
control
requirements
for
a
particular
measurement
situation.
These
inputs include:
—
The
function
selector
switch
position
—
The
manually
selected
mode
switches
—
The
counter output
(display
and
status
condition,
e.g.
overload)
—
The
ROM
program,
or
software control inputs
The
mode
switches
and
the function
selector are
scanned under software
control,
from
the
/tP
and
the
ROM
program,
in a
multiplexer
stage
and
the output conditions detected
are
fed
serially
to the
pP
as logic
control
signals.
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