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This standard was originally used for connecting a teletypewriter, which used ASCII code, asynchronous communication and electromechanical components, with a modem. Many early minicomputers from companies such as DEC used 33-ASR (Automatic Send Receive) teletypes as cheap system consoles (for a fancier machine one got a 35-ASR). When electronic terminals (smart and dumb) began to be used, they were often designed to be interchangeable with teletypes, 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. Then, because it was there, it was used for all sorts of remote communications, especially through modems, including computer to computer. Later personal computers (and other devices) started to make use of the standard so that they could connect to existing equipment. When IBM introduced the PC with an RS-232 port, this interface became truly ubiquitous. The IBM Selectric typewriter with RS-232 supplanted teletypes in many applications. For many years some form of RS-232 port was a standard feature for serial communications on almost all computers. It remained in widespread use into the late 1990s. For much of this time it was the standard way to connect modems.
One exception is mainframe computers, which generally do not communicate directly with terminal devices. These large systems generally have specialized I/O processors attached to them for this purpose. For instance, some IBM mainframes had a telecommunication control unit (TCU) attached to their multiplexer channel. The TCU would support multiple terminals, sometimes hundreds. Many of these TCUs could support RS-232 when it was required, although there were other serial interfaces as well.
There is some confusion about what the EIA actually standardized in RS-232. This standard only specifies electrical characteristics and circuits and pin numbers. The other characteristics of the teletype interface such as the D connector, use of the ASCII code and data format and asynchronous (or start-stop) communications are not part of RS-232, but were lumped into the name because these features usually all came together, so that in the end they became, effectively, mandatory, and the original meaning of the standard name was lost. There were about 100,000 33-ASR teletypes made in total, there are now almost that many PCs made every week, but they can all act as virtual teletypes. About the only feature that was required by teletypes that has been generally abandoned is that a real teletype required two stop bits to work reliably, so that a character took 11 bit times. This is why 100 word per minute teletypes transmitted at 110 baud. Today everyone uses one stop bit. In the rest of this article RS-232 means the 33-ASR teletype simulation, not the document.
IBM favored the use of the eight-bit EBCDIC code instead of the seven-bit ASCII code. IBM also favored a big endianWhen integers or any other data are represented with multiple bytes, there is no unique way of ordering of those bytes in memory or in a transmission over some medium, and so the order is subject to arbitrary convention. This is similar to the situation i transmission format instead of the little endian ASCII format. IBM did support all of these however. In order to send little endian characters, for instance, the mainframe would simply bit reverse each character in a string using a block translate instruction.
Earlier teletype machines had 3 rows of keys and only supported upper case letters. They used the 5 bit baudot codeThe Baudot code named after its inventor Emile Baudot, is a character set predating EBCDIC and ASCII and used originally and primarily on teleprinters. Baudot's original code, developed around 1874 is known as International Telegraph Alphabet No 1, and is and generally worked at 60 words per minute. Teletypes with 4 row keyboards and ASCII code with upper and lower case letters were an innovation that came into use in the same period as computers began to become widely available. The UART serial communications integrated circuits, introduced in the early 1970s, and still emulated by most chipsets, still support the earlier teletype formats, including 1.5 stop bits, but these features are rarely used.
The importance of serial ports began to wane when high speed networking became available for computer to computer communications. Although there were many competing standards originally, today this usually means 10 or 100 Base T EthernetEthernet is a packet-based computer networking technology for local area networks (LANs). It defines wiring and signaling for the physical layer, and packet formats and protocols for the media access control (MAC)/ data link layer of the OSI model. Ethern type connections. In the near future (post 2004) even higher speeds will be common.