Datasheet

Application Note
www.tektronix.com/oscilloscopes18
When connecting two RS-232 devices, a null modem is
commonly required. This device swaps several lines, including
the Tx and Rx lines. That way, each device can send data on
its Tx line and receive data on its Rx line.
Table 5 shows the pinout used for a 9-pin connector,
commonly used with RS-232 signals. Remember that if
your signal has passed through a null modem, many of the
signals will be swapped. Most importantly, Tx and Rx will be
swapped.
RS-232
Background
RS-232 is a widely-used standard for serial communication
between two devices over a short distance. It is best known
for its use in older PC serial ports, but it is also used in
embedded systems as a debug port or for linking two devices.
The RS-232-C standard was introduced in 1969. The
standard has been revised twice since then, but the changes
are minor and the signals are interoperable with RS-232-C.
There are also related standards, such as RS-422 and
RS-485, which are similar but use differential signaling to
communicate over longer distances.
How it Works
The two devices are referred to as the DTE (data terminal
equipment) and DCE (data circuit-terminating equipment). In
some applications, the DTE device controls the DCE device;
in other applications, the two devices are peers and the
distinction between DTE and DCE is arbitrary.
The RS-232 standard specifies numerous signals, many
of which are not commonly used. The two most important
signals are Transmitted Data (Tx) and Received Data (Rx). Tx
carries data from the DTE to the DCE. The DTE device’s Tx
line is the DCE device’s Rx line. Similarly, Rx carries data from
the DCE to the DTE.
The RS-232 standard does not specify which connectors
to use. Twenty-five-pin and nine-pin connectors are most
common. Other connectors have ten, eight, or six pins. It’s
also possible to connect two RS-232 devices on the same
board, without using standard connectors.
Signal Abbreviation Pin
Carrier Detect DCD 1
Received Data Rx
2
Transmitted Data Tx
3
Data Terminal Ready DTR
4
Common Ground G
5
Data Set Ready DSR
6
Request to Send RTS
7
Clear to Send CTS
8
Ring Indicator RI
9
Table 5. Common RS-232 connector pinout.