Racal Instruments 9901 Maintenance Manual page 48

Qll
will
be non-conducting
due
to
the
+5V
applied
via the
Pulse/Contact switch
1S6, R45
and D
18
to
the base of
Q1
1 .
4.22
When
the external timing contact closes
to
earth,
it
discharges
C21
via
D18
and
R42,
thus pulling
down
the
base and
turning
on
Q1
1 .
This
sends a
signal
to
the
Start
and
Stop
inputs of
the
CDI
Chip.
4.23
Contact bounce
pulses are prevented from causing a
spurious
'stop'
because
they
are of
too short a duration
to
re-charge C21,
thus
Qll
is
unaffected
until the
timing contact re-opens, C21
is
then able
to
re-charge and
Ql
l
turns off,
thus the
bounce
protection period
is
determined by
the value.
of C21
.
'A' Channel D.C.
Amplifier
4.24
The
'A'
amplifier and
its
bounce
protection circuit function
in the
same manner
as
the
'B'
amplifier described above.
The input
signal
is
supplied
when
the
AC/DC
switch
I'SI is
set
to
DC.
There
is
also
an
additional gating stage
in the
output (IC18a)
which
inhibits the
'A'
d.c.
amplifier output
when
the
B-B/B-A
switch
1
S2
is
set
to
B-B
for
single
line
Time
Interval
mode.
CHECK
Mode
^•25
In
CHECK
the
A
channel a.c.
amplifier
is
inhibited
and
the
1
MHz
reference
is
fed
to
the counter
in
the CDI
Chip. The
1
MHz
reference from
IC2/6
is
fed
to
IC15c which
is
enabled
by
a
'T on IC15c/9
when
switch 1S5
is
set
to
CHECK.
(The pull-up
resistor
is
R95 on
the right
hand
side of
the circuit
diagram).
From IC15c
the
1
MHz
is
fed
via
|C15b and
C14
to
the output stage
Q8, thence
to
the counter
via IC2/22.
4.26
To inhibit the
'A' amplifier
in
CHECK
mode
a
'O'
is
provided by switch 1S5
which
is
inverted
in
IC14d and
turns
on Q6, which
shunts the amplifier.
Hie same
inhibit
is
applied
when
the
AC/DC
switch
is
at
DC.
SLOPE SELECTION
4.27
The
polarities of the Start/Stop triggering
edges are
selected by
switches 1S3
(STOP
slope)
and
1S4 (START
slope)
in
conjunction with
the
Exclusive
-OR
gates
IC19a, 19b
and
19d.
An
Exclusive-OR
gate gives a
'1'
output when
the input
pins
have
opposite
logic
signals. By
means
of the slope selection switches alternative
Icgic levels
can
be applied
to these gates, which can
thus
determine which
edge
of
the input
pulse
shall
perform the
start or stop function.
4.28
The
selected
triggering
edge
is
differentiated by R40/C19
in
the stop line
and
R50/C22
in the
start line
to
provide
the appropriate
triggering
pulse. The
slope
gates
have
the following functions:-
4-5
9901 Vol.2