Black Box Explains...RS-232.
RS-232, also known as RS-232C and TIA/EIA-232-E, is a group of electrical, functional, and mechanical specifications for interfaces between computers, terminals, and modems. The RS-232 standard was developed by the Electrical Industries Association (EIA), and it defines requirements for connecting data communications equipment (DCE)—modems, converters, etc.—and data terminal equipment (DTE)—computers, controllers, etc.) devices.
The standard, which is functionally equivalent to ITU V.24/V.28, specifies the workings of the interface, circuitry, and connector pinning. Both sync and async binary data transmission fall under RS-232. Most PCs use the RS-232 interface to transmit data to modems, and some printers even use it.
Although RS-232 is a “standard,” you cannot necessarily expect seamless communication between two RS-232 devices. Why? Because different devices have different circuitry or pinning, and different wires may be designated to perform different functions.
The typical RS-232 connector is DB25, but some PCs and other data communication devices have DB9 connectors. To connect 9-pin PC ports to devices with 25-pin connectors, you need a simple adapter cable. The most commonly used interface, RS-232 is ideal for transmitting data at speeds up to 20 kbps and over distances up to 50 feet (15.2 m).