An external bus that is beginning to replace parallel and serial ports. With a maximum transfer speed of 12 Mbps (megabits per second), USB is designed for low- to mid-speed peripheral devices (such as keyboards, mice, printers, joysticks, and modems) whereas more bandwidth-intensive devices (such as digital video cameras and storage devices) use the IEEEFireWirestandard. The main advantage of USB over traditional ports is that it offers easy expandability; you can daisy chain up to 127 devices (far more than the number of devices supported by traditional ports). All USB devices support plug-and-play and hot plugging. The computer automatically recognizes any USB device as soon as it's plugged in or added to the chain. Desktop computers that support USB typically have two four-pin USB ports (one for a keyboard and mouse daisy chain, the other to daisy-chain all other USB devices). USB was introduced in computers in 1997 and has received a boost from Windows 98, which offered better support for the standard than Windows 95. Macintosh computers support the USB standard. The iMac, for example, has no serial or SCSI ports, only USB ports.