Epson LQ-2550 Technical Manual page 100

Circuit Description
Figure 2-48 shows the RS-232C serial interface circuit. Data transmitted from the host computer is
converted from EIA (+3 to +27 V, –3 to –27 V) to ITL O V, + 5 V voltage levels by the RS-232C
driver 75189 (3A). The converted data is sent to the main CPU via buffers in the M546 10P (1 1 B). On
the contrary, data transmitted from the main CPU is sent to the 75188 (4A), converted from TTL to EIA
voltage levels, and transmitted to the host computer.
The sequence for the serial interface operation is shown in Figure 2-49, and that for handshaking in
Figure 2-50.
+ 1 2
D1
CN3
- 6
"
1
& 2
3
1
1
1
1
1
I
1
1
1
7 5 1 8 9 ( 3 A )
1
10
RxD
1
FG ~~~
I
- 1 2
D2
7 5 1 8 8 ( 4 A )
4
5
L - - - - - –– - - - J
2
- 1 2
R22
9
,- - - - - – - – - - - -
— C7
7
4 13
777
Figure 2-48. RS-232C Serial Interface Circuit
1
36 6 ,0
B)
- 1
2-53
REV.-A
'
MAIN-CPU
( 1 3A)
line