Circuit Description - Racal Instruments 1994 Service Manual

Figure
5.6
- Display-Block
Diagram
S.3.4.2
Circuit Description
5. 3.
4.2.1
The schematic
is
shown
on page 7-15.
The GPIB LEDs
LP3, LP4, and
LP5
are driven via
SKI from
the
GPIB
system.
The
GATE LED
LP7
is
driven
from
the
measurement
block via a
driver stage,
shown
on page
7-12,
and
SK2
pin
11.
The
TRIG
LEDs LP39
and LP40
are driven
from
the Channel
A/B
block
via driver stages,
shown
on
page
7-12,
and
SK2
pins 7
and
3.
The
STANDBY LED
LP1
is
driven
via
SKI
pin 8
from
the standby and
interrupt block.
The
remaining
LED
indicators
and
the
numeric
indicators DI5 and DI6 are controlled by the
display driver IC3. Numeric
indicators Dll
to DI4 are controlled by
IC4.
5. 3. 4.2. 2
Display
data
is
stored
in
memory
within IC3 and
IC4.
To
change
the data,
the microprocessor puts
a
control
word on
the port
B
bus.
The
microprocessor writes
this
word
into the display driver by means
of a
negative
pulse applied
to the
DISPLAY
STROBE
line at
SKI
pin 4.
The
control
word determines
the operating
mode
of the
display drivers.
5. 3. 4. 2.
3
The microprocessor
then
selects the display driver required by setting a
logic
0
on
the appropriate
MODE
line at
SKI
pin 3 or
6.
Eight
words
containing display
data
are written into the
selected display driver via the port
B
bus, controlled by eight
negative-going
pulses
on the
DISPLAY
STROBE
line.
5-11