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Ohmeda 3000 - Display Board

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1/Functional
Description
C.
Display
Board
This
is
a
functional
description
for
the
Infarit
Warmer
System
Display
Board
Part
No.
0631-5031-
700.
Refer
to
Schematic
No.
0676-0325-000
for
a
de-
tailed
circuit
diagram.
The
display
board
provides
the
interface
between
the
operator
and
the
control
system.
It
displays
the
status
of
the
unit,
the
patient
status,
and
can
also
be
used
as
a
diagnostic
aid.
The operator
controls
the
system
by
depressing
the
various
switches
on
the
front
display.
Operation
of
the
display
board
is
simplified
with
the
use
of
two
ICs:
the
8243
I/O
ex-
pander
which
is
used
in
conjunction
with
the
switches;
and
the
MM5451
(or
MM5450)
driver
used
in
conjunction
with
the
LED
display.
Switch
Decoding
The
I/O
expander,
U1,
is
always
enabled
in
the
read
mode
because
its’
sole
purpose
is
to
detect
switch
depressions.
The
8243
is
activated
by
the
microcon-
troller
sending
a
“Low”
signal
on
the
Chip
Select
(CS)
line.
A control
word
(4
bits)
is
latched
from
the
input
port
2
on
the
“High”
to
“Low”
transition
of
the
PROG
pin.
The
word
is
decoded
as
follows.
Instruction
Address
P23
P22
Code
P21
P20
Code
0 0
Read
0 0
Port
4
0
1
Write
0
1
Port
5
1
0
Or
1
0
Port
6
1
1
and
1
1
Port
7
As
soon
as
the
read
instruction
and
the
port
ad-
dress
are
decoded
the
corresponding
port
lines
are
set
to
a
“High”
impedance
state
and
the
input
buf-
fers
within
the
8243
are
switched
“On”.
When
a
switch
is
depressed
on
the
display,
the
respective
line
switches
“Low”
and
is
loaded
into
the
input
buffer.
The
“Low”
to
“High”
transition
on
the
PROG
line
terminates
the
read
instruction
and
transfers
in-
formation
back
to
port
2.
When
the
microprocessor
sets
the
CS
line
“High”
the
8243
is
disabled.
1-8
LED
Display
Driver
The
LED
display,
driver,
U2,
controls
the
LED
dis-
plays.
The
displays
are
multiplexed
with
a
duty
cycle
of
20%
and
a
refresh
rate
of
60
hertz.
Data
is
input
to
pin
22
synchronously
with
the
clock
(pin
21).
The
first
“1"
bit
activates
the
driver
and
35
data
bits
will
follow.
After
the
35th
bit
is
loaded
the
data
is
latched
to
provide
direct
output.
Note
that
a
logic
“High”
at
the
input
switches
the
output
“Low"
and
switches
“On”
the
LED
connected
to
the
output
(output
is
inverted).
Brightness
Adjust
R9
is
used
to
adjust
the
output
current
from
U2
and
in
tum
change
the
brightness
of
the
LEDs.
R9
is
ad-
justed
to
produce
3.30
+
0.10
volts
across
R10
(3.3V/221
ohms
=
15ma).
C6
is
used
to
prevent
os-
cillations
at
pin
19.
Multiplexing
of
Displays
Since
there
are
not
enough
data
bits
to
drive the
en-
tire
display,
the
displays
are
divided
into
four
sec-
tions.
Bits
1-28
are
used
to
supply
the
necessary
in-
formation
to
each
section.
Bit
29
is
unused.
Bit
30
is
tied
to
a
221
ohm
+
1%
resistor
which
is
used
for
calibration.
Bits
31-34
select
which
channel
of
the
display
is
activated
by
switching
ON
a
Darlington
transistor.
The
Darlington
provides
a
large
gain
so
that
a
small
drive
current
will
sustain
the
large
cur-
rent
draw
from
the
LEDs.
A
string
of
35
zeroes
are
sent
on
the
data
line
every
fifth
update
cycle.
The
driver
has
a
serial
input
and
does
not
have
a
master
reset.
This
string
of
zeroes
resets
the
driver
in
case
an
extra
pulse
was
entered
by
a
noise
spike.
The
basic
circuit
for
one
LED
segment
consists
of
the
5
volt
LED
supply
(reduced
to
4.3
volts
by
a
series
1N4001
diode,)
a
Darlington
switch
to
enable
the
supply
to
the
LED
group,
and
the
MM5451
driver
to
select
a
low
voltage
return
for
the
segment
(if
selected).

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