ViewSonic VG181 - Page 14

Service
Manual
ViewSonic
VG181
July
2000
-
Version
1.0
THEORY OF CIRCUIT OPERATION
THE OPERATION OF THE ANALOG PORT
The
analog
port
is
a
15
pin
mini
D-Sub
connector
for
receiving
the
video
signal
from
the
host
device.
It
features
an
EEPROM
which
complies
with
the
DDC1/DDC2B
protocol,
H-syne
and
V-sync
detecting
circuitry
which
regenerates
a
synchronous
signal
for
PW364
detection,
video
signal
matching
circuitry,
and
a
AD9884A
for
capturing
the
RGB
graphics
signal
and
digitizing
each
pixel.
The pin assignments of the 15 pin connector are shown below:
GGAl
15
The
RGB
graphics
signal
of
the
host
device
is
transmitted
to
the
analog
port
through
pins
1
to
3.
The
transmission
lines
should
be
75ohm
resistance
matched
at
R18,
R19
and
R20.
Over
voltage
protection
is
provided
by
D5,
D6
and
D7.
The
video
signal
should
be
coupled
to
RIN,
GIN,
BIN
and
SOGIN
of
AD9884A
through
C42,
C47,
C48
and
C49.
The
EDID
data
is
stored
in
EEPROM
(24LC21)
which
complies
with
the
DDC1/DDC2B
protocol
that
performs
a
plug
and
play
function.
When
in
DDC1
protocol,
the
host
device
accesses
the
EDID
data
through
RVSI
(pin14)
and
RGSA
(pin12)
while
RGSL
(pin15)
is
held
high.
However,
in
DDC2B
protocol
the
host
device
accesses
EDID
data
through
RGSA
(pin12)
and
RGSL
(pin15).
The
GSCL,
GSDA
and
GVSI
pins
should
be
pulled
up
through
R5,
R6
and
R7.
Over
voltage
protection
is
provided
by
D1,
D2,
D3
and
ZD4
with
a
clamping
limit
of
5
volts.
Page 12
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