Epson PX-8 - RS-232 C Interface

REV.-A
2.11 RS-232C Interface
This interface is controlled by the programmable serial controller 82C51 (lC 2C). The interface re-
quires special
voltage supplies that meet the RS-232C standard. ± 8V sources are provided from
a DC-DC converter regulator which generates the voltages from the battery voltage (VB) under the
control of IC 4C.
2.11.1
RS-232C levels
The RS-232C standard defines the space state as being between +25V and +3V inclusive, and the
mark state be between -25V and -3V inclusive. This circuit uses the DC-DC converter to generate
the voltage sources of ± 8V from the battery voltage (+5V) for the RS-232C levels. Table 2-19
summarizes the relationship between the levels and data signals.
Table 2-19
Voltage Level Data Signal Timing Signal State
Start/Stop Bit in
Start-Stop System
+8V
0
On Space
Start bit
-8V
1 Off
Mark Stop bit
The interface circuit uses the receiver circuit shown in Fig. 2-103, which can receive up to maxi-
mum levels of ± 25V.
+5
(Input waveform)
10K
GI---t---t-------1I--_t__
Input
'-----_ Output
(RS-232C levels)
(TTL levels)
(Output
waveform) Inn
G~-~-L-
I
Fig. 2-103
The RS-232C input signal is converted to TTL level by the two limiter diodes after passing
through the 10 kohm resistor.
2-100