In telecommunications, RS-232, Recommended Standard 232 is a
standard introduced in 1960 for serial communication transmission
of data. It formally defines the signals connecting between a DTE
(data terminal equipment) such as a computer terminal, and a DCE (data
circuit-terminating equipment or data communication equipment), such
as a modem. The
RS-232 standard had been commonly used in computer
serial ports. The standard defines the electrical characteristics and
timing of signals, the meaning of signals, and the physical size and
pinout of connectors. The current version of the standard is TIA-232-F
Interface Between Data Terminal Equipment and Data Circuit-Terminating
Equipment Employing Serial Binary Data Interchange, issued in 1997.
RS-232 serial port was once a standard feature of a personal
computer, used for connections to modems, printers, mice, data
storage, uninterruptible power supplies, and other peripheral devices.
RS-232, when compared to later interfaces such as RS-422,
Ethernet, has lower transmission speed, short maximum cable length,
large voltage swing, large standard connectors, no multipoint
capability and limited multidrop capability. In modern personal
USB has displaced
RS-232 from most of its peripheral
interface roles. Many computers no longer come equipped with RS-232
ports (although some motherboards come equipped with a COM port header
that allows the user to install a bracket with a DE-9 port) and must
use either an external USB-to-
RS-232 converter or an internal
expansion card with one or more serial ports to connect to RS-232
peripherals. Nevertheless, thanks to their simplicity and past
RS-232 interfaces are still used—particularly in
industrial machines, networking equipment, and scientific instruments
where a short-range, point-to-point, low-speed wired data connection
1 Scope of the standard
3 Limitations of the standard
4 Role in modern personal computers
5 Physical interface
5.1 Voltage levels
5.3.1 3-wire and 5-wire RS-232
6 Data and control signals
6.1 Ring Indicator
6.2 RTS, CTS, and RTR
7 Seldom-used features
7.1 Signal rate selection
7.3 Timing signals
7.4 Secondary channel
8 Related standards
9 Development tools
10 See also
12 Further reading
13 External links
Scope of the standard
Electronic Industries Association
Electronic Industries Association (EIA) standard RS-232-C as of
Electrical signal characteristics such as voltage levels, signaling
rate, timing, and slew-rate of signals, voltage withstand level,
short-circuit behavior, and maximum load capacitance.
Interface mechanical characteristics, pluggable connectors and pin
Functions of each circuit in the interface connector.
Standard subsets of interface circuits for selected telecom
The standard does not define such elements as the character encoding
(i.e. ASCII, EBCDIC, or others), the framing of characters (start or
stop bits, etc.), transmission order of bits, or error detection
protocols. The character format and transmission bit rate are set by
the serial port hardware which may also contain circuits to convert
the internal logic levels to
RS-232 compatible signal levels. The
standard does not define bit rates for transmission, except that it
says it is intended for bit rates lower than 20,000 bits per second.
RS-232 was first introduced in 1960 by the Electronic Industries
Association (EIA) as a Recommended Standard. The original DTEs
were electromechanical teletypewriters, and the original DCEs were
(usually) modems. When electronic terminals (smart and dumb) began to
be used, they were often designed to be interchangeable with
teletypewriters, and so supported RS-232. The C revision of the
standard was issued in 1969 in part to accommodate the electrical
characteristics of these devices.
Because the standard did not foresee the requirements of devices such
as computers, printers, test instruments, POS terminals, and so on,
designers implementing an
RS-232 compatible interface on their
equipment often interpreted the standard idiosyncratically. The
resulting common problems were non-standard pin assignment of circuits
on connectors, and incorrect or missing control signals. The lack of
adherence to the standards produced a thriving industry of breakout
boxes, patch boxes, test equipment, books, and other aids for the
connection of disparate equipment. A common deviation from the
standard was to drive the signals at a reduced voltage. Some
manufacturers therefore built transmitters that supplied +5 V and
−5 V and labeled them as "
RS-232 compatible".
Later personal computers (and other devices) started to make use of
the standard so that they could connect to existing equipment. For
many years, an RS-232-compatible port was a standard feature for
serial communications, such as modem connections, on many computers
(with the computer acting as the DTE). It remained in widespread use
into the late 1990s. In personal computer peripherals, it has largely
been supplanted by other interface standards, such as USB.
still used to connect older designs of peripherals, industrial
equipment (such as PLCs), console ports, and special purpose
The standard has been renamed several times during its history as the
sponsoring organization changed its name, and has been variously known
as EIA RS-232, EIA 232, and, most recently as TIA 232. The standard
continued to be revised and updated by the Electronic Industries
Association and since 1988 by the
Association (TIA). Revision C was issued in a document dated August
1969. Revision D was issued in 1986. The current revision is TIA-232-F
Interface Between Data Terminal Equipment and Data Circuit-Terminating
Equipment Employing Serial Binary Data Interchange, issued in 1997.
Changes since Revision C have been in timing and details intended to
improve harmonization with the
CCITT standard V.24, but equipment
built to the current standard will interoperate with older
ITU-T standards include V.24 (circuit identification) and V.28
(signal voltage and timing characteristics).
In revision D of EIA-232, the
D-subminiature connector was formally
included as part of the standard (it was only referenced in the
appendix of RS-232-C). The voltage range was extended to ±25 volts,
and the circuit capacitance limit was expressly stated as
2500 pF. Revision E of EIA-232 introduced a new, smaller,
standard D-shell 26-pin "Alt A" connector, and made other changes to
improve compatibility with
CCITT standards V.24, V.28 and ISO 2110.
RS-232 (May 1960) "Interface Between Data Terminal Equipment &
EIA RS-232-A (October 1963)
EIA RS-232-B (October 1965)
EIA RS-232-C (August 1969) "Interface Between Data Terminal Equipment
and Data Communication Equipment Employing Serial Binary Data
EIA EIA-232-D (1986)
TIA TIA/EIA-232-E (1991) "Interface Between Data Terminal Equipment
and Data Communications Equipment Employing Serial Binary Data
TIA TIA/EIA-232-F (1997-10-01)
TIA TIA-232-F (R2012)
Limitations of the standard
RS-232 is used beyond the original purpose of interconnecting
a terminal with a modem, successor standards have been developed to
address the limitations. Issues with the
RS-232 standard include:
The large voltage swings and requirement for positive and negative
supplies increases power consumption of the interface and complicates
power supply design. The voltage swing requirement also limits the
upper speed of a compatible interface.
Single-ended signaling referred to a common signal ground limits the
noise immunity and transmission distance.
Multi-drop connection among more than two devices is not defined.
While multi-drop "work-arounds" have been devised, they have
limitations in speed and compatibility.
The standard does not address the possibility of connecting a DTE
directly to a DTE, or a DCE to a DCE.
Null modem cables can be used to
achieve these connections, but these are not defined by the standard,
and some such cables use different connections than others.
The definitions of the two ends of the link are asymmetric. This makes
the assignment of the role of a newly developed device problematic;
the designer must decide on either a DTE-like or DCE-like interface
and which connector pin assignments to use.
The handshaking and control lines of the interface are intended for
the setup and takedown of a dial-up communication circuit; in
particular, the use of handshake lines for flow control is not
reliably implemented in many devices.
No method is specified for sending power to a device. While a small
amount of current can be extracted from the DTR and RTS lines, this is
only suitable for low-power devices such as mice.
The 25-pin D-sub connector recommended in the standard is large
compared to current practice.
Role in modern personal computers
PCI Express x1 card with one
Main article: Serial port
In the book
PC 97 Hardware Design Guide,
support for the
RS-232 compatible serial port of the original IBM PC
RS-232 has mostly been replaced in personal computers
USB for local communications. Advantages compared to
USB is faster, uses lower voltages, and has connectors that are
simpler to connect and use. Disadvantages of
USB compared to RS-232
USB is far less immune to electromagnetic interference
(EMI)[dubious – discuss] and that maximum cable length is much
shorter (15 meters for
RS-232 v.s. 3 - 5 meters for
USB depending on
USB speed used).
In fields such as laboratory automation or surveying,
may continue to be used. Some types of programmable logic controllers,
variable-frequency drives, servo drives, and computerized numerical
control equipment are programmable via RS-232.
have responded to this demand by re-introducing the
DE-9M connector on
their computers or by making adapters available.
RS-232 ports are also commonly used to communicate to headless systems
such as servers, where no monitor or keyboard is installed, during
boot when operating system is not running yet and therefore no network
connection is possible. A computer with an
RS-232 serial port can
communicate with the serial port of an embedded system (such as a
router) as an alternative to monitoring over Ethernet.
In RS-232, user data is sent as a time-series of bits. Both
synchronous and asynchronous transmissions are supported by the
standard. In addition to the data circuits, the standard defines a
number of control circuits used to manage the connection between the
DTE and DCE. Each data or control circuit only operates in one
direction, that is, signaling from a DTE to the attached DCE or the
reverse. Because transmit data and receive data are separate circuits,
the interface can operate in a full duplex manner, supporting
concurrent data flow in both directions. The standard does not define
character framing within the data stream, or character encoding.
Diagrammatic oscilloscope trace of voltage levels for an
character (0x4B) with 1 start bit, 8 data bits, 1 stop bit. This is
typical for start-stop communications, but the standard does not
dictate a character format or bit order.
RS-232 data line on the terminals of the receiver side (RxD) probed by
an oscilloscope (for an
ASCII "K" character (0x4B) with 1 start bit, 8
data bits, 1 stop bit, and no parity bits).
RS-232 standard defines the voltage levels that correspond to
logical one and logical zero levels for the data transmission and the
control signal lines. Valid signals are either in the range of +3 to
+15 volts or the range −3 to −15 volts with respect to the "Common
Ground" (GND) pin; consequently, the range between −3 to +3 volts is
not a valid
RS-232 level. For data transmission lines (TxD, RxD, and
their secondary channel equivalents), logic one is defined as a
negative voltage, the signal condition is called "mark". Logic zero is
positive and the signal condition is termed "space". Control signals
have the opposite polarity: the asserted or active state is positive
voltage and the deasserted or inactive state is negative voltage.
Examples of control lines include request to send (RTS), clear to send
(CTS), data terminal ready (DTR), and data set ready (DSR).
RS-232 logic and voltage levels
+3 to +15 V
−15 to −3 V
The standard specifies a maximum open-circuit voltage of 25 volts:
signal levels of ±5 V, ±10 V, ±12 V, and ±15 V
are all commonly seen depending on the voltages available to the line
driver circuit. Some
RS-232 driver chips have inbuilt circuitry to
produce the required voltages from a 3 or 5 volt supply. RS-232
drivers and receivers must be able to withstand indefinite short
circuit to ground or to any voltage level up to ±25 volts. The slew
rate, or how fast the signal changes between levels, is also
Because the voltage levels are higher than logic levels typically used
by integrated circuits, special intervening driver circuits are
required to translate logic levels. These also protect the device's
internal circuitry from short circuits or transients that may appear
RS-232 interface, and provide sufficient current to comply with
the slew rate requirements for data transmission.
Because both ends of the
RS-232 circuit depend on the ground pin being
zero volts, problems will occur when connecting machinery and
computers where the voltage between the ground pin on one end, and the
ground pin on the other is not zero. This may also cause a hazardous
ground loop. Use of a common ground limits
RS-232 to applications with
relatively short cables. If the two devices are far enough apart or on
separate power systems, the local ground connections at either end of
the cable will have differing voltages; this difference will reduce
the noise margin of the signals. Balanced, differential serial
connections such as RS-422, RS-485, and
USB can tolerate larger ground
voltage differences because of the differential signaling.
Unused interface signals terminated to ground will have an undefined
logic state. Where it is necessary to permanently set a control signal
to a defined state, it must be connected to a voltage source that
asserts the logic 1 or logic 0 level, for example with a pullup
resistor. Some devices provide test voltages on their interface
connectors for this purpose.
RS-232 devices may be classified as Data Terminal Equipment (DTE) or
Data Circuit-terminating Equipment (DCE); this defines at each device
which wires will be sending and receiving each signal. According to
the standard, male connectors have DTE pin functions, and female
connectors have DCE pin functions. Other devices may have any
combination of connector gender and pin definitions. Many terminals
were manufactured with female connectors but were sold with a cable
with male connectors at each end; the terminal with its cable
satisfied the recommendations in the standard.
The standard recommends the
D-subminiature 25-pin connector up to
revision C, and makes it mandatory as of revision D. Most devices only
implement a few of the twenty signals specified in the standard, so
connectors and cables with fewer pins are sufficient for most
connections, more compact, and less expensive. Personal computer
manufacturers replaced the
DB-25M connector with the smaller DE-9M
connector. This connector, with a different pinout (see Serial port
pinouts), is prevalent for personal computers and associated devices.
Presence of a 25-pin D-sub connector does not necessarily indicate an
RS-232-C compliant interface. For example, on the original IBM PC, a
male D-sub was an RS-232-C DTE port (with a non-standard current loop
interface on reserved pins), but the female D-sub connector on the
same PC model was used for the parallel "Centronics" printer port.
Some personal computers put non-standard voltages or signals on some
pins of their serial ports.
Main article: Serial cable
The standard does not define a maximum cable length, but instead
defines the maximum capacitance that a compliant drive circuit must
tolerate. A widely used rule of thumb indicates that cables more than
15 m (50 ft) long will have too much capacitance, unless
special cables are used. By using low-capacitance cables,
communication can be maintained over larger distances up to about
300 m (1,000 ft). For longer distances, other signal
standards are better suited to maintain high speed.
Since the standard definitions are not always correctly applied, it is
often necessary to consult documentation, test connections with a
breakout box, or use trial and error to find a cable that works when
interconnecting two devices. Connecting a fully standard-compliant DCE
device and DTE device would use a cable that connects identical pin
numbers in each connector (a so-called "straight cable"). "Gender
changers" are available to solve gender mismatches between cables and
connectors. Connecting devices with different types of connectors
requires a cable that connects the corresponding pins according to the
table below. Cables with 9 pins on one end and 25 on the other are
common. Manufacturers of equipment with
8P8C connectors usually
provide a cable with either a
DB-25 or DE-9 connector (or sometimes
interchangeable connectors so they can work with multiple devices).
Poor-quality cables can cause false signals by crosstalk between data
and control lines (such as Ring Indicator).
If a given cable will not allow a data connection, especially if a
gender changer is in use, a null modem cable may be necessary. Gender
changers and null modem cables are not mentioned in the standard, so
there is no officially sanctioned design for them.
3-wire and 5-wire RS-232
A minimal "3-wire"
RS-232 connection consisting only of transmit data,
receive data, and ground, is commonly used when the full facilities of
RS-232 are not required. Even a two-wire connection (data and ground)
can be used if the data flow is one way (for example, a digital postal
scale that periodically sends a weight reading, or a GPS receiver that
periodically sends position, if no configuration via
necessary). When only hardware flow control is required in addition to
two-way data, the RTS and CTS lines are added in a 5-wire version.
Data and control signals
The following table lists commonly used
RS-232 signals (called
"circuits" in the specifications) and their pin assignments on the
DB-25 connectors. (See
Serial port pinouts) for other
commonly used connectors not defined by the standard.)
Data Terminal Ready
DTE is ready to receive, initiate, or continue a call.
Data Carrier Detect
DCE is receiving a carrier from a remote DCE.
Data Set Ready
DCE is ready to receive and send data.
DCE has detected an incoming ring signal on the telephone line.
Request To Send
DTE requests the DCE prepare to transmit data.
Ready To Receive
DTE is ready to receive data from DCE. If in use, RTS is assumed to be
Clear To Send
DCE is ready to accept data from the DTE.
Carries data from DTE to DCE.
Carries data from DCE to DTE.
Zero voltage reference for all of the above.
Connected to chassis ground.
The signals are named from the standpoint of the DTE. The ground pin
is a common return for the other connections, and establishes the
"zero" voltage to which voltages on the other pins are referenced. The
DB-25 connector includes a second "protective ground" on pin 1; this
is connected internally to equipment frame ground, and should not be
connected in the cable or connector to signal ground.
Ring Indicator (RI) is a signal sent from the DCE to the DTE device.
It indicates to the terminal device that the phone line is ringing. In
many computer serial ports, a hardware interrupt is generated when the
RI signal changes state. Having support for this hardware interrupt
means that a program or operating system can be informed of a change
in state of the RI pin, without requiring the software to constantly
"poll" the state of the pin. RI does not correspond to another signal
that carries similar information the opposite way.
On an external modem the status of the Ring Indicator pin is often
coupled to the "AA" (auto answer) light, which flashes if the RI
signal has detected a ring. The asserted RI signal follows the ringing
pattern closely, which can permit software to detect distinctive ring
The Ring Indicator signal is used by some older uninterruptible power
supplies (UPSs) to signal a power failure state to the computer.
Certain personal computers can be configured for wake-on-ring,
allowing a computer that is suspended to answer a phone call.
RTS, CTS, and RTR
Flow control (data) § Hardware flow control
The RTS and CTS signals were originally defined for use with
half-duplex (one direction at a time) modems such as the Bell 202.
These modems disable their transmitters when not required and must
transmit a synchronization preamble to the receiver when they are
re-enabled. The DTE asserts RTS to indicate a desire to transmit to
the DCE, and in response the DCE asserts CTS to grant permission, once
synchronization with the DCE at the far end is achieved. Such modems
are no longer in common use. There is no corresponding signal that the
DTE could use to temporarily halt incoming data from the DCE. Thus
RS-232's use of the RTS and CTS signals, per the older versions of the
standard, is asymmetric.
This scheme is also employed in present-day
RS-232 to RS-485
RS-485 is a multiple-access bus on which only one device
can transmit at a time, a concept that is not provided for in RS-232.
RS-232 device asserts RTS to tell the converter to take control of
RS-485 bus so that the converter, and thus the
RS-232 device, can
send data onto the bus.
Modern communications environments use full-duplex (both directions
simultaneously) modems. In that environment, DTEs have no reason to
deassert RTS. However, due to the possibility of changing line
quality, delays in processing of data, etc., there is a need for
symmetric, bidirectional flow control.
A symmetric alternative providing flow control in both directions was
developed and marketed in the late 1980s by various equipment
manufacturers. It redefined the RTS signal to mean that the DTE is
ready to receive data from the DCE. This scheme was eventually
codified in version RS-232-E (actually TIA-232-E by that time) by
defining a new signal, "RTR (Ready to Receive)", which is
circuit 133. TIA-232-E and the corresponding international standards
were updated to show that circuit 133, when implemented, shares the
same pin as RTS (Request to Send), and that when 133 is in use, RTS is
assumed by the DCE to be asserted at all times.
In this scheme, commonly called "RTS/CTS flow control" or "RTS/CTS
handshaking" (though the technically correct name would be "RTR/CTS"),
the DTE asserts RTR whenever it is ready to receive data from the DCE,
and the DCE asserts CTS whenever it is ready to receive data from the
DTE. Unlike the original use of RTS and CTS with half-duplex modems,
these two signals operate independently from one another. This is an
example of hardware flow control. However, "hardware flow control" in
the description of the options available on an RS-232-equipped device
does not always mean RTS/CTS handshaking.
Equipment using this protocol must be prepared to buffer some extra
data, since the remote system may have begun transmitting just before
the local system deasserts RTR.
The EIA-232 standard specifies connections for several features that
are not used in most implementations. Their use requires 25-pin
connectors and cables.
Signal rate selection
The DTE or DCE can specify use of a "high" or "low" signaling rate.
The rates, as well as which device will select the rate, must be
configured in both the DTE and DCE. The prearranged device selects the
high rate by setting pin 23 to ON.
Many DCE devices have a loopback capability used for testing. When
enabled, signals are echoed back to the sender rather than being sent
on to the receiver. If supported, the DTE can signal the local DCE
(the one it is connected to) to enter loopback mode by setting pin 18
to ON, or the remote DCE (the one the local DCE is connected to) to
enter loopback mode by setting pin 21 to ON. The latter tests the
communications link, as well as both DCEs. When the DCE is in test
mode, it signals the DTE by setting pin 25 to ON.
A commonly used version of loopback testing does not involve any
special capability of either end. A hardware loopback is simply a wire
connecting complementary pins together in the same connector (see
Loopback testing is often performed with a specialized DTE called a
bit error rate tester (or BERT).
Some synchronous devices provide a clock signal to synchronize data
transmission, especially at higher data rates. Two timing signals are
provided by the DCE on pins 15 and 17. Pin 15 is the transmitter
clock, or send timing (ST); the DTE puts the next bit on the data line
(pin 2) when this clock transitions from OFF to ON (so it is stable
during the ON to OFF transition when the DCE registers the bit). Pin
17 is the receiver clock, or receive timing (RT); the DTE reads the
next bit from the data line (pin 3) when this clock transitions from
ON to OFF.
Alternatively, the DTE can provide a clock signal, called transmitter
timing (TT), on pin 24 for transmitted data. Data is changed when the
clock transitions from OFF to ON, and read during the ON to OFF
transition. TT can be used to overcome the issue where ST must
traverse a cable of unknown length and delay, clock a bit out of the
DTE after another unknown delay, and return it to the DCE over the
same unknown cable delay. Since the relation between the transmitted
bit and TT can be fixed in the DTE design, and since both signals
traverse the same cable length, using TT eliminates the issue. TT may
be generated by looping ST back with an appropriate phase change to
align it with the transmitted data. ST loop back to TT lets the DTE
use the DCE as the frequency reference, and correct the clock to data
Synchronous clocking is required for such protocols as SDLC, HDLC, and
A secondary data channel, identical in capability to the primary
channel, can optionally be implemented by the DTE and DCE devices. Pin
assignments are as follows:
7 (same as primary)
Secondary Transmitted Data (STD)
Secondary Received Data (SRD)
Secondary Request To Send (SRTS)
Secondary Clear To Send (SCTS)
Secondary Carrier Detect (SDCD)
Other serial signaling standards may not interoperate with
RS-232 ports. For example, using the TTL levels of
near +5 and 0 V puts the mark level in the undefined area of the
standard. Such levels are sometimes used with NMEA 0183-compliant GPS
receivers and depth finders.
A 20 mA current loop uses the absence of 20 mA current for
high, and the presence of current in the loop for low; this signaling
method is often used for long-distance and optically isolated links.
Connection of a current-loop device to a compliant
requires a level translator. Current-loop devices can supply voltages
in excess of the withstand voltage limits of a compliant device. The
original IBM PC serial port card implemented a 20 mA current-loop
interface, which was never emulated by other suppliers of
Other serial interfaces similar to RS-232:
RS-422 (a high-speed system similar to
RS-232 but with differential
RS-423 (a high-speed system similar to
RS-422 but with unbalanced
RS-449 (a functional and mechanical interface that used
RS-423 signals - it never caught on like
RS-232 and was withdrawn by
RS-485 (a descendant of
RS-422 that can be used as a bus in multidrop
MIL-STD-188 (a system like
RS-232 but with better impedance and rise
EIA-530 (a high-speed system using
properties in an EIA-232 pinout configuration, thus combining the best
of both; supersedes RS-449)
EIA/TIA-561 8 Position Non-Synchronous Interface Between Data Terminal
Equipment and Data Circuit Terminating Equipment Employing Serial
Binary Data Interchange
EIA/TIA-562 Electrical Characteristics for an Unbalanced Digital
Interface (low-voltage version of EIA/TIA-232)
TIA-574 (standardizes the 9-pin
D-subminiature connector pinout for
use with EIA-232 electrical signalling, as originated on the IBM
When developing or troubleshooting systems using RS-232, close
examination of hardware signals can be important to find problems. A
simple indicator device uses LEDs to show the high/low state of data
or control pins. Y cables may be used to allow using another serial
port to monitor all traffic on one direction. A serial line analyzer
is a device similar to a logic analyzer but specialized for RS-232's
voltage levels, connectors, and, where used, clock signals. The serial
line analyzer can collect, store, and display the data and control
signals, allowing developers to view them in detail. Some simply
display the signals as waveforms; more elaborate versions include the
ability to decode characters in
ASCII or other common codes and to
interpret common protocols used over
RS-232 such as SDLC, HDLC, DDCMP,
and X.25. Serial line analyzers are available as standalone units, as
software and interface cables for general-purpose logic analyzers and
oscilloscopes, and as programs that run on common personal computers
CCITT V.24 (de)
CCITT V.28 (de)
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"RS232C Standard". Knowledgebase. National Instruments. Archived from
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ITU-T Recommendation V.24 - Data Communication over the telephone
network - List of definitions for interchange circuits between data
terminal equipment (DTE) and data circuit-terminating equipment (DCE).
International Telecommunication Union
International Telecommunication Union (ITU-T). March 1993. Archived
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Wikimedia Commons has media related to RS-232.
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