Epson PX-8 Technical Manual page 131

REV.-A
2.11 .2.6 Reception
The input interface line signals RXD, CTS, DSR, and CD can always be read, regardless of whether
power is on or off.
That is, the RS-232C interface supply voltage regulator needs not be turned on only for reception
provided that the CD (DCD) signal is monitored with the DSR signal held high or the reception
check is disabled .
• The 10 kohm current limiting resistor insert-
ed on the receive signal line as shown in Fig.
2-110 protects the connected device from
excess current or voltage drop. The next vol-
tage limiter circuit, consisting of two diodes,
and converts the RS-232C line levels from
the mating transmitter to the 0/+5V TTL le-
vels. The 100 kohm resistor pulls the limiter
output down to ensure that the inverter IC 98
to functions properly.
2.11.2.7 Operation Mode Switching
The transmit/receive lines between the option
unit (if connected) are connected to the trans-
mit/receive data lines (TXD/RXD) between the
serial controller 82C51 (2A), as well as to the
those lines going to and from the RS-232C in-
terface, as shown in Fig. 2-111.
These two pairs of lines cannot be controlled
simultaneously. It is necessary to enable one
either pair or no other. This is accomplished by
the AUX signal, issued from gate array
GAH40M. This signal is controlled by the main
CPU using bit 5 of its I/O address 0002. When
the bit is 0, the AUX signal is held low, disa-
bling the transmit/receive data lines to/from
the RS-232C interface at gates 3E and 12C re-
spectively. When the bit is 1; i.e., the AUX sig-
nal is high, the data lines are disabled. Thus,
the option unit data lines must be disabled at
the option unit in order to prevent mixture of
the signals.
2-107
9
RXD
Current limiting
resistor
Fig. 2-110
82C51 (2A)
TXD
.-----;l..-T
x
0
R36
GAH40M
~------~~-----oAUX
TXD
~------~~---~
RS-232C:
interface
Fig.2-111
Option
unit