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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