In order to change from high to low, the input must
be above VTHH. On the other hand, for the outpur to
turn from low to high, the input must be below VTHL.
For the PC·1600, three resistors (Rl, R2, R3) are
chosen to for maintain (b) level is in between VTHH
and VTH L under the normal state,
® When the input (a) changes from low to high, the signal
(b) is sent to the VDD side as a pulse above VTHH by
means of the capacitor Cl between VTHH and GND
as a normal level. As a result, the signal (c) changes
from high to low.
@ On the other hand,when input (a) changes from high to
low, the signal (b) is sent to the VEE side as a pulse
-
PC-l600
above VTH L by means of the capacitor Cl between
VTHL and GND. As a result, the signal (c) changes
from low to high.
A signal transition is latched on the output side using
the pulse by means of the capacitor and characteristics
of the Schmitt IC for conversion of a logic signal into
the RS-232C interface siqnal.
The role of the R4 output is to prevent the possible
destruction of the IC4584 which may occur by an
accidental short 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, lt is mandatory that you
release static power in your body by using the earth band.
(When removing the key PWB from the top cabinet, it is
recommended that you secure the keytops and display
filter using cellopane tape.)
In order to open the cabinet, remove the RS-232C interface
connector cover, system bus connector cover, expansion
slot covers (1, 2), battery cover, batteries, and the ex-
pansion module.
Remove the five screws (see figure) and slowly lift the
bottom cabinet with care so that you do not damage the
chips installed on the FPC PWB.
Fig.8·1
-15-
Now, you will see the signal levels. 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. Replacing 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 cabinet
Holder (A)
Holder (B)
Fig.8-3
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