Circuit Description; Operation At Power On; Rs-232C Interface - Oki ML421 Maintenance Manual

2.2

Circuit Description

A block diagram is shown in Figure C-1.
80C51
RST
TD
RD
IF WR
+5 VD
+12VDC
-8VDC
+5 V

2.2.1 Operation at power on

After power is turned on, an RST OUT signal is sent from the printer control board to reset the
printer. When the reset is canceled, the 80C51 CPU performs initialization. Initialization consists
of setting the 80C51 timer, and setting the serial mode.

2.2.2. RS-232C interface

The DTR, SSD, TD and RTS signals output by the 80C51 are converted to RS-232C signals
by line driver 75188 (Q1) and sent to the interface.
In addition, signals DSR, CTS, CD, and RD on the RS232C interface are converted to TTL level
by line receiver 75189 (Q2) and input to the 80C51.
42113901 Rev.2
DB0 to DB9
LS245
±9 V power
supply circuit
Figure C-1 Block Diagram
ADR latch
ROM
64Kbit
RAM
64Kbit
+9 V
–9 V
+5V
0V
Oki Data CONFIDENTIAL
CN1
75188
TD
75189
RD
Serial data
control line
Bus line
Control line
127 /