C.2. Serial
RXD - Receive Data - (Pin 3)
This input carries serial asynchronous data sent by the computer to the printer.
RTS - Request To Send - (Pin 4)
This output is always held high (above 3 volts).
CTS - Clear To Send - (Pin 5)
DSR - Data Set Ready - (Pin 6)
Unused.
SG - Signal Ground - (Pin 7)
All signals can transmit between the printer and the host computer to send each signals with a signal ground.
+5V DC - (Pin 11)
This line is connected to the printer’s +5V DC line (+5V ±0.5V, 250 mA [Parallel and Serial total] maximum, fused).
DTR - Data Terminal Ready - (Pin 20)
This output is used as a buffer
RS-232C Interface Voltage Levels
The voltage levels of the interface signals conform to EIA