Resistor Arrays; Resistor Array - Racal Instruments 9901 Maintenance Manual

3.22
In
the
LED
indicators
each
displayed numeral
is
formed by
illuminating
an
appropriate
number
of short straight
segments.
The numeral
'8',
for
example,
is
formed from 7
segments,
whereas
the
numeral
'6'
will require
only
5
segments. The segments
can be checked by
selecting
CHECK
and
pressing the
RESET
button. This calls
up an
'all
eights'
display.
Decimal
Point
3.23
Decimal point (d.p.) information
is
obtained
by encoding
the four
line
b.c.d.
output data from the
CDI Chip
with the display
enable
signal. This
encode
turns
on a d.p. drive
transistor
at the correct instant to
illuminate the appropriate
decimal
point.
Control Information
3.24
Function selection
in
the instrument
is
performed
by
a 3-line binary
coded system
identified as lines a,
b
and
c.
The time base
('n')
units
are also selected by a
3-line system
identified as lines x,
y and
z.
The
inverse
of
the logic
used
in
the
CDI
Chip
is
available at
the rear
panel data output connector.
The
logic
decode
is
given
in
Table 2
and
Table 3
in
the
Technical Specification
at
the
front
of the
book.
Resistor
Arrays
3.25
Many
of the integrated circuits
are 'open collector' types.
For these
IC's
discrete 'pull-up'
resistors
are
provided
in
the circuit.
These
resistors
may
be
mounted
in
sealed dual-in-line
(d.i.l.)
packages,
for
example
R83
(Fig. 4)
which
has
seven
56Q
resistors
with a
common
connection to
+5V. Such
arrays
cannot be
serviced
and
must
be changed
in
the
event of a faulty
resistor.
Figure
3.9
shows a
typical
array
schematically.
Another
type
of
resistor
array comprises a d.i.l.
package
containing
separate
resistors
of identical
values, for
example
R83
which
contains seven
56
Q
resistors.
Pin
14
+5V
D.I.L.
Resistor
Array
Fig.
3.9