-> I/O units often consist of a mechanical component and an electronic component.
-> The electronic component is called the device controller or adapter.
-> The mechanical component is the device itself.
-> The controller card usually has a connector on it, into which a cable leading to the device itself can be plugged.
-> Many controllers can handle two, four, or even eight identical devices.
-> If the interface between the controller and device is a standard interface, either an official ANSI, IEEE, or ISO standard or a de facto one, then companies can make controllers or devices that fit that interface. Many companies, for example, make disk drives that match the SATA, SCSI, USB, Thunderbolt, or FireWire (IEEE 1394) interfaces.
-> The controller’s job is to convert the serial bit stream into a block of bytes and perform any error correction necessary.
-> The block of bytes is typically first assembled, bit by bit, in a buffer inside the controller. After its checksum has been verified and the block has been declared to be error free, it can then be copied to main memory.
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