SP238A Datasheet, PDF (8/12 Pages) Exar Corporation – +5V Powered Multi-Channel RS-232 Drivers/Receivers

Application Examples
Transmitted Data (TD or TX)
This is the pin on which the computer sends the
actual data signal to be transmitted, i.e. a positive
voltage (+3V to +15V) to represent a logic “0”,
and a negative voltage (-3V to -15V) to represent
a logic “1”. The PC would send the data on this
line to be transmitted by the modem.
Ring Indicator (RI)
This line is used by the peripheral device to tell
the computer that a remote device wants to start
communicating. The modem would activate the RI
line to tell the computer that the remote modem
was calling, i.e. the phone is ringing.
Data Carrier Detect (DCD)
This line is used by the modem to tell the com-
puter that it has completed a transmission path
with the remote modem, and to expect to start
receiving data at any time.
Received Data (RD or RX)
This is the pin on which the modem sends the
computer the incoming data signal, i.e. a positive
voltage (+3V to +15V) to represent a logic “0”,
and a negative voltage (‑3V to ‑15V) to represent
a logic “1”.
INTERFACE EXAMPLE - A PRINTER ON
THE IBM PC SERIAL PORT
The RS-232 standard defines 22 serial interface
signals. These signals consist of ground lines,
timing, data, control and test signals, plus a set
of signals rarely used for a second data channel.
Many of these signal lines are not used in typical
RS-232 applications; in fact, the IBM^® PC serial
port is implemented using only nine pins. For ex-
ample, consider the case of a PC using this nine
pin port to communicate with a peripheral device
such as a printer. We see the following activity on
each of the RS-232 lines as the computer and
printer are activated and communicate.
Data Terminal Ready (DTR)
This is the pin the computer uses to tell peripheral
devices that it is on-line and ready to communi-
cate. Once the computer is powered-up and ready,
it will send out a signal on the DTR to inform the
printer that it is powered-up and ready to go. The
printer really doesn’t care, since it will simply print
data as it is received. Accordingly, this pin is not
needed at the printer.
Data Set Ready (DSR)
Peripheral devices use this line to tell the computer
that they are on-line and ready to communicate.
When the printer is turned on and has completed
its self-test routine (assuming it does one), it will
send a signal to the PC by asserting this line.
Request To Send (RTS)
The computer activates this line to notify the
peripheral device that it is ready to send data. In
this example, the computer notifies the printer
that it is ready to send data to be printed by the
printer.
Clear To Send (CTS)
This is the line on which the peripheral device tells
the computer that it is ready to receive data from
the computer. If the printer was not ready, i.e. it
was out of paper, for example, it would not assert
this line. Once the printer was ready to receive
data from the PC, it would assert this line. When
it receives the CTS signal from the printer, the PC
knows that a data transmission path has been
established between itself and the printer.
Signal Ground (GND)
The Signal Ground pin acts as a reference for all
the other signals. This pin is simply maintained
at a 0V level to serve as a level to which all other
signals are referenced. Both the PC and the printer
will have this line connected to their respective
internal ground lines.
IBM Printer Port Interconnections
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SP238A_100_071410