Racal Instruments 1994 Service Manual page 148

Figure
5.5
- Basic Recipromatic Counting
Technique
5.3.3.1.4
At
the
end
of
each
measurement
period,
MCC1
generates an
interrupt
request
for the
microprocessor.
The
registers within
MCC1
are addressed
using the
address bus
and
the
MCC
SELECT
line.
The measured
value
is
transferred
to
the
microprocessor
via the
multiplexed
bus.
5. 3.3.
1.5
The
internal
and
external frequency standard inputs are
both
fed to
MCC2.
The system
will
operate
from the external standard provided that the input
is
of
sufficient
amplitude.
A
10
MHz
output, derived
from
the
frequency standard
in
use,
is
made
available at
a socket on the rear
paneL
5.3. 3.2
Circuit Description
5. 3. 3.
2.1
Refer the the
schematic shown
on page 7-11.
5.3.
3.2.2
Measured
Signal
Input
5. 3. 3.
2. 2.1
For
all
measurement functions other than
FREQ A
and
PERIOD
A, the
differential outputs
from Channel
A
and Channel B
are applied
to
the
measuring
circuit
at
IC39-15,
16,
17 and
18.
For the
FREQ A
and
PERIOD A
functions, the
A
signal
frequency
is
divided
by two
in
IC41a and
fed
to
IC39-19.
5.3.3.
2. 2. 2
For
the
FREQ
C
and
RATIO
C/B
functions, the
C
signal
is
fed
to
IC39-19. For these functions
IC41a-5
is
held at logic
1
by the
PST1
control
line
(CHANNEL
A
INHIBIT) from
the
Channel
C
block.
As
a
result,
IC41a
is
held in set
and
the
A
signal
is
inhibited
from reaching IC39-19.