In telecommunications, the recommended standard 232 [1], RS-232, refers to a standard introduced in 1960 [2] for serial data transmission. It formally defines the signals which are connected between a DTE (data terminal), for example a computer terminal, and a DCE (data line termination device or data communication device), for example a modem. The standard defines the electrical characteristics and timing of the signals, their significance and the physical dimensions and pin assignments of the connectors. The current version of the standard is TIA-232-F, published in 1997. Interface between data terminals and data lines with serial binary data exchange. The RS-232 standard has been widely used in serial computer ports.

An RS-232 compliant serial port was standard on many computer types. They used personal computers not only for modem connections but also for printers, mice, data storage, UPS and other devices.

Compared to the latest interfaces such as RS-422, RS-485 and Ethernet, the RS-232 has a lower transmission rate, short maximum cable length, high voltage, large standard plugs and no multipoint capacity and limited multipoint capacity. In modern personal computers, USB has replaced the RS-232 interface with most peripheral interface features. Many computers are no longer equipped with RS-232 ports and must use an external USB / RS-232 converter or an internal expansion card with one or more serial ports for connection to RS-232 devices. However, due to their simplicity and their ubiquitous past, RS-232 interfaces continue to be used, especially in industrial machines, network devices, and scientific equipment where a short-range wired data cable points to low-speed points.

Purpose of the standard

  • The RS-232-C standard [3] of the Electronic Industries Association (EIA) since 1969 defines:
  • Characteristics of electrical signals such as voltage level, signaling speed, signal synchronization and sampling frequency, voltage resistance, short-circuit behavior and maximum load capacity.
  • Mechanical properties of the interface, connectors and pin identification.
  • Functions of individual circuits in the interface connection.
  • Standard subcircuits of interface circuits for certain telecommunication applications.

The standard does not define elements such as character encoding (ASCII, EBCDIC or others), character frames (start or end bits, etc.), bit transfer order or bit registers. error detection. The character format and the transfer rate are defined by the hardware of the serial port, usually a UART, which may also include a circuit to convert the internal logic levels to RS-232 compatible signal levels. The standard does not define transmission rates, except that transmission rates are less than 20,000 bits per second.


RS-232 was first used as a standard recommended by the Electronic Industry Association (EIA) in 1960 [4] [1]. Initial DTEs are electromechanical teletypes and original DCEs (mostly) modems. When used with electronic terminals (intelligent and scary), they are usually designed to be TTY interchangeable and thus support RS-232.

Because the standard does not have any requirements for external devices, such as computers, printers, testing tools, payment terminals, etc., designers have implemented a compatible RS-232 interface on their devices, usually explaining the idiosynthetic standard. The common problems that have arisen are the determination of unusual switches for unusual or missing connectors or control signals. Independence leads to a growing sector for connecting connectors, connection boxes, test equipment, books and other equipment to other sectors. A general deviation from the standard is signal control at a lower voltage. Some manufacturers have built-in transmitters that operate at +5 V and -5 V and “RS-232 compatible” calls.

Later, personal computers (and other devices) use standards to connect existing devices. A compatible RS-232 port is a standard serial feature for many years, such as: modem connections on multiple computers (the computer has worked as a DTE). It is widely used until the late 1990s, and has been widely replaced by other interface standards, such as USB for PC peripherals. The RS-232 is still used to connect older device types, industrial equipment (eg programmable logic controllers), console ports, and special devices.

This standard has been changed many times in history after the name of the sponsored organization has changed. It also has the names of EIA RS-232, EIA 232 and more recently TIA 232. The standard was updated and updated by the Electronic Industries Association and since 1988 the Telecommunications Industry Association (TIA). [5] Revision C was published in a document in August 1969. Revision D was published in 1986. The current revision is the TIA-232-F interface between data terminal systems and devices. the termination of the data circuit using serial binary data exchange, published in 1997. Changes since the change of C relating to synchronization and details to improve connecting to CCITT V.24, although the current standard is in harmony with the standards. previous versions.

The associated ITU-T standards include V.24 (circuit identification) and V.28 (voltage characteristics and signal synchronization).

In the revision of the EIA-232, the D-subminiature connector is formally included in the standard (this is only referred to in Appendix RS-232-C). The voltage range is extended to ± 25 volts and the limit of the capacitance of the circuit is specifically referred to as 2500 pF. EIA-232 Revision E introduces a new “Alt A” standard connector for 26-pin D-shell housing and other modifications made to improve compatibility with the standards of CCITT V.24, V .28 and ISO improve. 2110.


  • EIA RS-232 (May 1960) “Path between data and data terms”
  • RRP RS-232-A (October 1963)
  • RRP RS-232-B (October 1965)
  • EIA RS-232-C (August 1969) “Data Devices Interface and Data Transmission, Serial Data Transmission”
  • EIA EIA-232-D (1986)
  • TIA TIA / EIA-232-E (1991) “Data and Stylish Lighting Data Transfer Devices with a Work Act Launched by a Man” \ t
  • TIA TIA / EIA-232-E (1997-10-01)
  • ANSI / TIA-232-F-1997 (R2002)
  • TIA TIA-232-F (R2012)

Standard limitations

  • Because RS-232 is used outside of the original purpose of connecting to a terminal in a modem, standards are developed later to withstand these limits. The RS-232 standard problems are: [7]
  • High voltage fluctuations as well as positive and negative current requirements increase the energy consumption of the interface and intensify energy planning. The voltage change request also limits the maximum speed of a compatible interface.
  • Asymmetric signaling with respect to a common signal of mass limitations in immune to intrusion and distance transmission.
  • The multipoint connection between more than two devices is not defined. Although multipoint “alternatives” are developed, their speed and compatibility are limited.
  • The standard does not relate to the possibility of connecting DTE directly to a DTE or DCE in a DCE. Zero modem cables can be used to make connections, but they are not defined by the standard and some cables use different connections.
  • The definitions of the two ends of the connection are asymmetric. It does the role assignment of a new concept device with a problem. The designer must choose a DTE or DCE interface and pinned assignments.
  • The handshake interface and line controls are used to configure and remove the dial-up communication circuit. In particular, the use of connection lines for power control is unreliable in many devices.
  • There is no way to use a device. While it is possible to get a small amount of energy from the DTR and RTS lines, this is only suitable for low-power devices, such as mice.
  • The standard 25-pin D-sub connector is great compared to current practice.

Roll into modern PCs

PCI Express x1 card with RS-232 port with a 9-pin connector

In PC Hardware Guide [8], Microsoft does not recommend RS-232 compatible serial port support for IBM PC’s original design. Currently, the standard RS-232 is replaced by PCs for local communication via USB. The advantages of RS-232 are USB faster, utilizing lower voltages and offering simpler connections for connectivity and use. USB disadvantages via RS-232 have USB with less interference (electromagnetic) (EMI) and the maximum length of used USB cable speeds is shorter.

RS-232 devices can still be used in areas such as laboratory automation or checking. Some types of programmable logic controllers, frequency converters, servo drives, and computer control control units can be used by RS-232. Computer manufacturers respond to this request by reconnecting the DE-9M connector on their computer or offering adapters.

RS-232 ports are often used for communication with non-UI systems, such as servers without a monitor or control panel installed. This happens at startup when the operating system is not ready for use and a network connection is not possible. A computer with RS-232 serial interface can interact with a combined system’s serial interface (such as a router) instead of tracking via Ethernet.

Physical interface

In RS-232, the user data is sent as a series of time bits. The quality supports simultaneous and abnormal transitions. In addition to data circuits, the standard refers to a number of control circuits used to manage the connection between DTE and DCE. Not all data or control circuits operate only in one direction, or by indicating the DTE signal to the DCE linked or vice versa. Transmission and data reception are separate circuits, the interface can operate in full duplex mode and support the same data flow in both directions. The rules do not apply to character frames in the data stream or character encoding.

voltage levels

Oscilloscope track voltage diagram for ASCII character “K” (0x4B) with 1 bit boot, 8 bit data (first bit significant at least), 1 bit stop. This is common in startup communication, but the quality does not support a character format or bit.

RS-232 data on receiver terminals (RxD) is tested with oscilloscope (for ASCII character “K” (0x4B) with 1 boot bit, 8 bit data, 1 stop bit and no equality).

The RS-232 standard refers to the voltage levels that match the logic level and zero logic level for data transmission lines and sign control lines. The correct sign is from +3 to +15 volts or from -3 to -15 volts around the origin point (GND). The range between -3 and +3 volts is therefore not a valid level of RS-232. For data transmission lines (TxD, RxD and the second equivalent channel), a negative voltage logic is indicated and the position of the signal is called a “marker”. Zero logic has a signal with a positive voltage and the position of the sign is called “space”. The other polarity is the control polarity: the certified or active state is a positive voltage and the deactivated or inactive state is a negative voltage. Examples of control lines are Send Request (RTS), Send Clear (CTS), Data Ready Terminal (DTR) and Data Resolution Set (DSR).

Logic level and voltage RS-232

Data circuits Control circuits Voltage
0 (space) Asserted +3 to +15 V
1 (mark) Deasserted −15 to −3 V

The maximum standard voltage without voltage of 25 volts in particular: ± 5V, ± 10V, determined 12V and V 15V levels are generally determined based on the voltages available to the line control line. Some RS-232 routing chips are distributed to generate the required voltages for a 3 or 5 volt supply. RS-232 drivers and receivers must enter an unspecified level of ground fault or voltage up to ± 25 volts. The speed of change is controlled or the speed that the signal changes between levels.

Since the voltage levels are higher than the logic levels normally used with integrated circuits, special intermediate control circuits are required to convert logic levels. They also protect the device’s internal circuit from possible short circuits or transistors at the RS-232 interface and provide sufficient power to meet the speed requirements of the data transfer scan.

As both of the circuit RS-232 depends on the ground pin being at zero volts, problems with connecting machines and computers, the voltage between the land pin on one end and the ground pin is not another. It can also create a hard loop. General mass RS-232 limiting to relatively short cables. If the two devices are long enough apart or in separate power systems, the local land connections at each end of the cable have different voltages. This difference reduces the signal to noise ratio. Equal differential layers such as RS-422 or RS-485 tolerate more potential land differences due to differential signage. [9]

Unused interfaced signals have an unallocated logic state whose termination is established. If a control signal needs to be fixed permanently in a defined state, it must be connected to a voltage source that accepts the level 1 or 0 logic, for example with a resistor. Some devices provide test voltages at their interface terminals for this purpose.

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