This is a standard for Asynchronous Transfer between computer equipment and accessories. Data is transmitted bit by bit in a serial fashion. The RS-232 standard defines the function and use of all 25 pins of a DB-25 type connector. In the minimal configuration, 3 pins are used, namely: Ground, Transmit Data, and Receive Data. Some systems use 6 additional lines for a total of 9, including "handshaking" signals and modem control signals. Virtually no systems require more than 9 connections, and therefore a modified RS-232 standard has been developed which uses the smaller DB-9 type connector instead of the DB-25 connector. On the Macintosh, a Mini-DIN-8 is used. This is a round connector about 1/4" in diameter and has 8 pins. Macintosh actually uses a most robust standard, RS-422, that is also compatible with RS-232 devices.