Epson PX-8 Technical Manual page 130

REV.-A
2.11.2.5 Signal Level Conversion
An IC 75188 (USART) is inserted in two RS-232C interface output signal lines to convert the TTL
levels to the RS-232C levels of ± 8V.
A limiter circuit consisting of two diodes is used for the level conversion from RS-232C to TTL.
Fig. 2-109 shows the individual circuits.
<
Output signal lines
>
There are three output signal lines; RTS, TXD
and DTR. RTS and TXO are converted by the IC
12C (75188). DTR is normally pulled up to the
-8V supply and it is driven to +8V only when
DTR goes low.
<
Input signal lines
>
There are the four input signal lines; RXD, CTS,
DSR, and CD. These signals are converted by
the limiter diode circuit shown in the figure.
Mark (negative level) causes a current through
the diode B, clamping the input to IC 4049 at
the ground level. Space (positive level) causes a
current through the diode A to the +5V supply,
clamping the input at +5V.
-sv
RTS~12~
,.=3 _ _ _ _ _ ___._--
0
RTS
C31
::L
14
~
AUX
TXD
+8V
-:tf
DTR
5V
GND
n - - - -__ - __
--~
TXD
(INHRS)
-8V
Lv
w.
o DTR
~
I
I
I
To RS-232C
interface
+5
j
A
10K
B
(Input)
(Output)
-
-- -
=_JlJl ___
Fig. 2-109
*
This interface circuit uses RS-232C levels of ± 8V. The standards define the levels as follows:
• Mark-logical "1" (stop-bit level): -3V
~
-25V
• Space -logical "0" (start-bit level): +3V
~
+25V
2-106