Racal Instruments 9901 Maintenance Manual page 49

Gate
Function
'B'
channel
stop (single
line timing
and
totalizing)
'B'
channel
start
(single
and double
line timing,
and
totalizing).
'A'
channel
stop in
Time
Interval
double
line.
4.29
The
emitters of the
Start
and
Stop
transistors
Q12
and
QIO
are connected
to
OV
via
the
snap-action
transistor
switch
Q16/Q13
and R56.
This circuit provides a pulse
when
manual
start/stop
is
used,
which
eliminates contact bounce
effects.
Under normal
conditions capacitor
C23
is
charged
up,
and
Q16
and
Q13
are turned on.
When
the START/
STOP
button
S50
is
pressed,
it
rapidly discharges
C23 and
turns off
Q16
and
Q13.
Turning
off
Q16
and
Q13
effectively
turns off
Q12
and QIO,
producing
the appropriate pulses
for
start
and
stop at the inputs to the
CDI
Chip. Because
Q16
and
Q13
cannot
turn
on again
until
C23
has charged up
the contact
bounce
pulses
are suppressed.
I
Cl 9a
I
Cl 9b
IC19d
MANUAL
START/STOP
DATA OUTPUT
AND
DISPLAY
SYSTEM
NOTE:
Refer
to the
Supplementary Data
in
the
Technical Specification
at the
front
of the
book
for
a
summary and diagram
of the data output format.
Multiplex Readout and
Display
4.30
The
Display
Assembly
contains a 6-digit readout system using light-emitting
diode
(LED) numerical
indicators. The display data
is
fed from IC2 on
pins
15
to 18,
this
4-way
output being
fed from a parallel-to-serial (multiplex) system within the
CDI
Chip,
to
the
7-segment decoder
IC17.
4.31
In
the multiplex system the b.c.d.
outputs from the data
stores
are fed
in
sequence
on
to the
4-way
output. This
is
determined by a 10
kHz
synchronising signal
(derived from the frequency
standard)
which
interrogates
each decade
store
in
turn
for
a
period of 100
ps. The
same
10
kHz
signal
is
used
(via IC10)
to
'enable' the
appropriate
LED
in
the
Display.
4.32
The system
Is
set
by
the
(Ro) signal
which
occurs
in
the
10°
state
of the data output
at
IC2/14.
This
sets
ICIO
to the
'O'
state,
and
via
IC16/1,
turns
on
Q19.
This
enables
the
10°
digital
Indicator
D.1.2 which
displays the digit determined by
the
7-segment
decoder IC17. The
other display LED's are then
enabled
in
sequence
for
a period of
100 ps
each.
Decimal
Point
and Overflow Decode
4.33
The d.p. and overflow data
is
fed out from the
CDI
Chip
during the ninth state of
4-6
9901 Vol.2