RS- 232 is a standard for a commonly existing in computers serial interface that has been developed in the early 1960s by the U.S. standardization committee Electronic Industries Association ( EIA).


Mainframes and text terminals were merged into the early 1990s with the help of modems by point-to -point connections over the telephone line. The transmission of data in both systems was carried out sequentially. Due to the original purpose, the interface includes some asymmetries in the definition of the control lines, which may result in the usual later became applications in completely different areas to Verschaltungsproblemen.

Current Usage

There are devices worldwide barely produced that have an RS- 232 interface. Modern bus and data transmission protocols provide reliable and faster connectivity. Only a few more PCs are shipped with a COM port, notebook manufacturers offer almost not at all to this equipment option. To microcontroller programming or older, still in use devices, such as telephone systems and control devices to be able to operate with modern computers and programming, interface converter USB are on the market to serial available.

American standard

The current American version is officially called (ANSI EIA / ) TIA -232 -F and is from the year 1997. Usual in the U.S. and Europe designation is RS -232 ( RS stands for Radio Sector, bringing the initial competent authority of the department is meant, but is often read as Recommended Standard ). On the question of the correct designation, see the section labeling standards in EIA - Electronic Industries Alliance.


RS- 232 defines the connection between the terminal (data terminal equipment (DTE ), Eng. Data terminal equipment ( DTE) ) and the modem (data communication equipment (DCE ), Eng. Data communication equipment (DCE ) ) with regard to timing and voltage level. When the connector 25 -pin D -Sub recommended ( not required). The transmission protocol is not part of the standard. In general, the parameters are explained in serial data transmission.

Further transmission standards such as RS- 422, RS -485 can be found in the article Serial interface.

  • The transmission is in words. A word corresponds, depending on the configuration five to nine bits, where then a single character is encoded. Most of the coding is done according to ASCII. Often ( ASCII ) control codes for controlling a VT100 terminals are as used, they are in the RS232 standard but not defined. Usual, therefore, to transfer seven or eight data bits. However, the processing of the five -bit telex code is for example also possible ( by adjustment of the signal level).
  • An RS -232 link is operating ( bit ) in series, each with a data line for both transmission directions. That is, the bits are successively transmitted on a line, as opposed to the parallel data transmission. The necessary serial-to- parallel conversion happens mostly in the so-called UARTs ( either as an integrated module in a microcontroller or as a single block).
  • Although there are countless other serial interface types is the traditional RS-232 " serial port " because it specifically in the PC sector was the only common earlier.
  • The data transfer is asynchronous, so there is no common clock. Each participant can line is free, at any time, transfer complete data words. The synchronization in the transfer is made by the recipient as the so-called word synchronization, ie at the beginning of the signal edge of the start bit. The synchronization of the receiver is done with the start of transmission on the data line, because the stop bit and the idle state on the line has the inverse level of the start bit. The receiver synchronizes itself to the center of each data bit and samples the bits of the data word with its own bit rate from. For this to work, the bit rate of the transmitter and receiver may vary only a few percent of each other. Each transmitted word must therefore be initiated and at least one stop bit (logical 1) be signed by a start bit (logical 0). The stop bit is not a bit in the true sense, but means the minimum length of the break or hibernation. Therefore, can be present between two words Any number of stop bits, even non-integer values ​​such as 1.5 stop bits. By this is meant that the minimum duration of the pause time duration of 1.5 bit cells corresponds. The reason is that some UARTs need a longer break of more than one bit time between the reception of two words. Between start and stop bit ( s) is the actual payload (data ) on the cycle time unchanged ( NRZ encoded) to be transferred.
  • RS- 232 is a power interface (as opposed for example to a power interface). The binary states are realized by different electrical voltage levels. For the data lines ( TXD and RXD ), a negative logic is used, wherein a voltage of between -3 V and -15 V ( ANSI/EIA/TIA-232-F-1997 ) is a logical one, and a voltage between 3 V and 15 V representing a logical zero. Signal level between -3 V and 3 V are undefined. At the control lines (DCD, DTR, DSR, RTS, CTS, and RI) is represented by the active state of a voltage between 3 V and 15 V, the inactive state to observe by a voltage between -15 V and -3 V. however, is that given here ( and mostly used ) designations for the control lines in the original standard does not happen that way. There are only certain circuits described which, although they may be associated with these names, but they are named differently in the standard. The above-mentioned voltages are related to the receiver ( inputs). Wherein the transmitters ( output), the voltage must be at least 5 V and -5 V at a load of 3 ... 7K, in order to ensure sufficient signal to noise ratio. Common is the use of 12 V and -12 V.
  • As a plug-in connection according to the original standard 25 -pin D- sub connector for DTE and DCE for terminals were used. Since many of the 25 lines pure printer or terminal control lines are from the electromechanical era, which are not required for most compounds with more modern peripherals, today 9-pin D -Sub plugs and sockets have been established, which often DB 9 or more correctly called DE- ninth These were originally introduced as a pure stopgap saving space with the IBM PC / AT ( back then it was a matter of the plug together with a similarly reduced Centronics port on a plug-in card to accommodate ). The 9-pin connector is therefore not to be found in the RS- 232 standard, but the standard EIA/TIA-574. For the RS- 232 data transmission and other connectors are rarely used, such as Modular 8P8C ( often incorrectly referred to as RJ -45, specified in EIA / TIA 561 ) or completely firm-specific.
  • To avoid loss of data, the receiver must be able to stop data transmission, when no more data can be processed. This so-called handshake can be implemented in two ways, either by software on certain control codes or special cables (hardware handshake). When software handshake, the receiver sends the flow control special characters to the transmitter. Only three lines ( RxD, TxD, and GND) are in accordance with the data transfer required, but this way handshake is only possible if the two control codes are not present in the user data. When Xon / Xoff protocol, the receiver sends the flow control special characters to the transmitter ( Xon Xoff = 11h and 13h = ).
  • With a hardware handshake, the two devices to signal via additional control lines to their respective status. A minimal interface with hardware handshake example, consists of five lines ( TxD, RxD, GND, RTS and CTS).

The standard does not specify the bit rate, although it is mentioned that it is intended for transmission rates up to 20,000 bit / s. Usual UARTs are used in conjunction with the RS -232, support data transfer rates of 115.2 kbit / s and more. In order to achieve a defined transmission characteristics, writes the standard maximum slope at the transmitter and one ( on the bit rate -dependent ) minimum slope in the transition region -3 V ... 3 V at the receiver before.

Cable length and transmission rate

Since signal quality decreases with increasing line length, the line length is limited.

A limiting factor is the transit time of the signal. Since an RS- 232 interface at the cable end can not be terminated with its characteristic impedance ( to large power loss ), there are inevitably line reflections. With increasing transmission speed and cable length the reflections interfere more and more data transfer. The standard requires that the slope value of 30 V / us shall not exceed the transmitter, so that the effects of the reflections are limited. The receiver side, a square wave signal with a very high slope is produced by a Schmitt trigger again.

Another aspect is that the signal transmission is not differentially, but asymmetric (single-ended or unbalanced) takes place. The transmitted signal contains a DC voltage component, and is therefore relatively sensitive to common mode noise. Such disorders may be caused by inductive coupling into the loop RxD Gnd eg. Because relate all the signals to the same Gnd signal, a stream on the TxD line can produce a voltage drop on the Gnd line, which leads to a potential shift between the two communication parties and is seen, for example, on the RxD line, causing interference.

According to original standard is a cable capacity of max. 2500 pF allowed, which over standard cables with cable length of max. 15 m ( 50 feet) corresponds. With cables, which have a very low capacity ( for example UTP CAT -5 cable with 55 pF / m ), can be compliant reach 45 m from the definition. The accompanying table gives experience of Texas Instruments again.

The problems of mutual influence over Gnd, terminating resistor missing, etc. can be solved by a differential transmission as RS- 485, LVDS, etc..

Cabling and connectors

In order to connect two devices via the serial interface, which must be " listening" are connected to the " speaking " lines. On terminals or computers ( DTE - data terminal equipment ) are "speaking " lines TxD, RTS and DTR, " listening " lines are RxD, CTS, DSR, DCD, and RI. For modems (DCE - data circuit -terminating equipment ), it is exactly the reverse; there are further from the terminal " spoken " signals to the opposite side and therefore must to these "hear" the other way around, the " included " from the opposite side signals to the terminal " further said."

  • If it is a connection from the terminal or computer (DTE) (usually with a plug ) to a modem (DCE) (usually with socket ), a 1:1 cable is necessary.
  • If, in contrast to connect two devices of the same type (eg two PCs), so the lines are to cross. Such a cable is called a null modem cable as a modem (ie modems '0 ') will be used. Due to the unbalanced defined set of control signals and their sometimes quite liberal use but there is not THE crossed cables per se, that always works. One end of the cable, which is designed for connection to a DTE device, may possibly cause to a DCE device a short circuit (which may move normally according to V.28 specification no hardware damage by itself, but already in practice has occurred ).
  • Through a loopback plug or jack the serial transmission signal of the device is fed directly to the receiving part of the same apparatus. Application finds such a loopback device, inter alia, in the development of communication programs. If even then the control lines " looped ", is also to be observed here that, depending on the device type (DTE or DCE), the control signals DCD and RI are each both either input or output, and have no clear predefined " counterpart". They must therefore be connected in suitable, so that no short- circuit between outputs or undefined input levels can occur.

The practical identification of DTE and DCE devices is determined by measuring the quiescent level (voltage between GND and TxD or RxD, different occupancy 9 - and 25-pin cable note ) possible. Some modern devices detect unused input connections and turn order to save energy, the output driver. In this case, the putative outputs a connected opposite to be fooled by a suitable resistance between the signal terminal and GND.

The names and descriptions of the main signal lines are based on the original use of the interface. With " remote site" in this table is not meant the opposite side (the one at the other end of the phone line in the classic use), but the local partner of the DTE ( ie, in the classical case, a DCE (modem) ). Line numbers have been formulated in both the DTE (PC ) and DCE (modem), and the same from the perspective of the DTE, but the characteristic of the terminal ( input or output) is different in each case.

Additional standards

Color coding of the wires ( for RS -232 lines are no standards prescribed )

  • RJ45: connector commonly occur in network components such as switches, routers or firewalls, color- coding of the TP cable according to this standard:
  • V.24: The ITU standard (1964 ) defines more than 50 interface lines. The RS -232 interface uses it 22
  • V.28: the ITU standard (1972) describes the electrical properties of an interface that is often used with the V.24.
  • DIN 66020-1: The V.24 largely taken over by the German Institute - for - standardization.
  • ISO 2110: defining the mechanics of a connector.