Epson PX-8 - Page 133

REV.-A
2.12.3 Interface
The signal voltage levels are the same meaning that used in the RS-232C interface, as shown in
Table
2-22.
Table 2-22 Data Signal Voltage Levels
Voltage Level Data Signal State
Corresponds to Start
or Stop Bit State.
+8V
0
Space
Start bit
-8V
1 Mark Stop bit
• The Transmit (PTX) signal line is controlled by the INHRS signal via transistor Q16 inserted
across
that line and the -8V supply. This is required to suppress the regulator output, prevent-
ing
its rise time irregular voltage waveform from being recognized as a start bit by the connect-
ed device .
• The PRX and PIN signal lines have a 1 O-kohm
current limiting resistor, a voltage limiter circuit,
consisting of two diodes, and a 100 kohm pull-down resistor, as shown in Fig. 2-113.
The current limiting resistor protects the connected device from overload and voltage drop.
The
voltage limiter converts the ± 8V RS-232C levels to the 0/+5V TTL levels.
The
pull-down resistor ensures that the IC inverter to functions properly.
Input
Fig.2-113
2-109