A controller, or interface, is a device that takes charge of the data through various parts of the system. Most motherboards have controllers for the keyboard, serial devices, parallel devices, floppy drives, and IDE/EIDE or SCSI devices. One can also add video sound, joystick, and other controllers. These all work with the master Input/Output controller. Every device must have a controller

Buses also have controllers. It also works with the bus clock to make sure that only the right amount of data goes through. The controller is kind of like the foreman on a construction site. On the hard drive data transfer controller it also bridges the I/O controller with the hard drive / floppy drive controller.

The most common type of controllers are SCSI and ATA (which is based on IDE /EIDE). ATA is the controller and internal data transfer protocol. IDE/EIDE is the interface. SCSI is faster and more expensive.

A SCSI allows more devices to be attatched to the system than an IDE does.

Some other common interfaces are RAID and Fiber Channel.