RS-232 - (Recommended Standard-232) - RS-232 is an Electronics Industries Association standard asynchronous serial port connection that's used commonly for modems, computer terminals, and serial printers. RS-232 uses a 25-pin or 9-pin connector. RS-232 data is a stream of bits at a constant speed. Each character is preceded by a start bit and followed by one or two stop bits; a positive voltage is transmitted for a 0 bit, a negative voltage for a 1.  It can transfer data over relatively long distances.