Service manual
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In order
to
change from high
to
low,
the
input
must
be
above VTHH. On the
other
hand, for the
output
to
rurn from low
to
high, the
input
must
be
below
VTHl.
For
the
PC
·1600, three
res
isto
rs
(Al,
R2,
R3)
are
chosen
to
for maintain {bl level
is
in
between VTHH
and VT
Hl
under the normal ,tate,
® When
the
input
(a)
changes
from
low
to
high,
the
signal
(bl
is
sent
to
the VOO
si
de
as
a pulse above
VTHH
by
means
of
the capacitor Cl between
VTHH
and
GN
D
as
a normal level.
As
a result,
the
signal
{el
changes
from high
to
low
.
@
On
the
other
hand, when
input
(a) changes
from
high
to
low,
the
signal (bl
is
sent
to
the
VeE
side
as
a pulse
PC
- l600
above
VTH
L
by
means
of
the
capacitor C1 between
VTHL
and
GND.
As
a result,
the
signal (e) changes
from l
ow
to
high.
A signal transition
is
latched
on
the
output
side
us
i
ng
the pulse by means
of
the
capacitor and characteristics
of the Schmitt
IC
for convenion of a logic signal into
the
RS
·232C interface signat.
The role
of
the
R4
output
is
to
prevent the possible
destruction
of
the
IC4584 which may occur by an
accidental
shon
in connection with the RS.232C inter·
face
or
the connection
of
outputs
together.
The capacitor
C2
is
for increasing speed for conveying
a change
in
the MN4584.
8. Service precautions
Before
servicing
of
the
PC
·1600,
it
is
mandatory that you
release
static power
in
your body by using the
eanh
band.
(When
removing the key
PWB
from the
top
cabinet, it
is
recommended
that
you secure the key tops and display
filter using cellopane tape.)
In
order to open the cabinet, remo
ve
the RS.232C interface
connector cover, system bus connector cover, expansion
slot covers
{1
,
2l,
battery cover, batteries, and the ex·
pansion module.
Remove the f
ive
screws
(see
figu re) and slowly lift the
bottom cabinet with care so
that
you
do
not
damage the
chips installed on the
FPC
PWB.
F
ig.
8·
'
-
15
-
Now, you
will
see the signal leve
ls.
To get power by using
the
AC
adaptor or battery cells, connect the oscilloscope
probe to the negative side
of
the
battery.
Fig.
8
·2
8-1. Repl
ac
ing the
FPC
PWB
1. With
the
connector
PWB
secured on the bottom cabinet.
pry
the
holder
(A)
at
(A)
using a flat tip screwdriver.
Next. remove the holder
(B)
.
Bottom c:.binel
HOlder (A I
HoldM
(
8)
Fig.
8·3