RS-232

RS-232
Aeroid · CC BY-SA 4.0 · source
Name	RS-232
Caption	A common DB-25 connector used for RS-232.
Type	Serial communication standard
Designer	Electronic Industries Alliance
Design date	1960
Manufacturer	Various
Production date	1960–present
Superseded by	USB, Ethernet
Superseded	Teletypewriter current loop
Length	Typically up to 15 meters

RS-232. It is a standard for serial communication transmission of data, formally defined by the Electronic Industries Alliance. Originally introduced in 1960 for connecting data terminal equipment to data communication equipment such as modems, it became ubiquitous for connecting computers to peripherals like printers and mice. The standard defines the electrical characteristics, timing, and meaning of signals, as well as the physical size and pinout of connectors.

Technical specifications

The standard, formally known as TIA-232-F, governs asynchronous communication where data is sent without a shared clock signal. It employs a start bit and stop bit framing structure, with common character encodings like ASCII transmitted LSB first. Standard data rates, defined by the standard, range from 75 to 115,200 bits per second, though many implementations support non-standard higher speeds. The interface is designed for point-to-point communication, typically over distances up to 15 meters, though this is heavily dependent on cable quality and electrical noise.

Electrical characteristics

RS-232 uses single-ended signaling with voltage levels that are bipolar relative to a common ground. A logic '1' (marking state) is represented by a voltage between -3 and -15 volts, while a logic '0' (spacing state) is represented by a voltage between +3 and +15 volts; the region between -3 and +3 volts is undefined. These large voltage swings, compared to later logic families like TTL, provide better noise immunity for longer cables. The driver circuits, often implemented with specialized integrated circuits like the 1488 and 1489, must be capable of sourcing sufficient current into the typically high impedance of a receiver.

Connectors and pin assignments

While the standard originally specified a 25-pin D-subminiature connector (DB-25), the widespread adoption of RS-232 in personal computers popularized the 9-pin DE-9 connector. The IBM PC/AT was instrumental in establishing this de facto standard. Key pins on the DE-9 include TD on pin 3, RD on pin 2, and RTS/CTS on pins 7 and 8 for hardware flow control. Adapter cables between the 25-pin and 9-pin variants are common, and many devices implemented only a subset of the full signal set, leading to the proliferation of null modem cables for direct computer-to-computer connections.

Data and control signals

The interface defines a set of control signals for managing the communication session, following a model derived from teleprinter and modem interactions. Primary data signals are TXD and RXD. Essential handshaking signals include DTR, DSR, RTS, and CTS, which together can implement hardware flow control. Other signals like RI and CD are vestiges of telephony, indicating an incoming call and modem carrier presence, respectively. In many applications, especially with short cables, these control signals are often ignored or "strapped" active, a configuration sometimes called "three-wire serial".

Applications and history

Developed by the Electronic Industries Alliance, RS-232 was a foundational technology for the computer revolution. It was the standard interface for external modems, connecting computers to the public switched telephone network and early bulletin board systems. It was also the primary means to connect terminals like the VT100 to minicomputers from Digital Equipment Corporation and Data General. In the personal computer era, it was used for mice from companies like Microsoft, printers, and for direct file transfer between machines. The standard's longevity ensured its use in industrial control systems, scientific instruments, and point of sale terminals long after its consumer decline.

Limitations and successors

The primary limitations of RS-232 include its low data rate, limited cable length, large voltage swings requiring special driver chips, and lack of support for multiple devices on a single port. These shortcomings led to the development of newer standards. RS-422 and RS-485 offered differential signaling for longer distances and multi-drop networks. In the consumer space, USB, developed by a consortium including Intel, Microsoft, and IBM, comprehensively replaced RS-232 for peripheral connectivity due to its higher speed, power delivery, and plug-and-play capability. For networking, Ethernet and later Wi-Fi superseded serial connections for terminal access and data transfer.

Category:Computer buses Category:Telecommunications standards Category:American inventions