SP238A Datasheet, PDF (7/12 Pages) Exar Corporation – +5V Powered Multi-Channel RS-232 Drivers/Receivers

Protection From Shorts Exceeding
±15V
The driver outputs are protected against shorts
to ground, other driver outputs, and V⁺ or V^‑. For
protection against voltages exceeding ±15V, two
back-to-back zener diodes connected to clamp
the outputs to an acceptable voltage level are
recommended. (Refer to Figure 2.)
Figure 2. High Voltage Short-Circuit Protection
INTERFACE EXAMPLE - A MODEM
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 example, consider the case of a PC using this
nine pin port to communicate with a peripheral
device such as a modem. We see the following
activity on each of the RS-232 lines as the com-
puter and modem are activated and communicate
with each other as well as the remote modem at
the other end of the phone line.
Application Examples
Data Terminal Ready (DTR)
This is the pin the computer uses to tell pe-
ripheral devices that it is on-line and ready to
communicate.
Data Set Ready (DSR)
Peripheral devices use this line to tell the computer
that they are on-line and ready to communicate.
When the modem 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 modem
that it is ready to send data to be transmitted by
the modem.
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 modem was not ready,
i.e. it was performing a loop-back self-test, for
example, it would not assert this line. Once the
modem was ready to receive data from the PC,
it would assert this line. When it receives the
CTS signal from the modem, the PC knows that
a data transmission path has been established
between itself and the modem.
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 modem will have this line connected to their
respective internal ground lines.
IBM Modem Port Interconnections
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SP238A_100_071410