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System Bus Interface

Every hard disk interface communicates with the PC over one of the system's I/O buses. On modern systems, the main system bus is PCI, with some support for the old ISA standard still provided. Older systems used the VESA local bus (VLB) in combination with ISA. Logically, the hard disk interface is one device on the system I/O bus, which is connected to the memory, processor and other parts of the system.

The choice of bus type has a great impact on the features and performance of the interface. Higher-performance interfaces, including the faster transfer modes of both IDE/ATA and SCSI, require an interface over a high-speed local bus. On modern systems, this means PCI. Older systems that use VLB for the hard disk interface still offer acceptable performance. Extremely old systems that still use ISA for the hard disk controller will be severely limited in performance.

The speed of the system bus is also important; the faster the bus, the faster the interface (simplistically speaking). The standard speed for the PCI bus on Pentium-class systems is 33 MHz, which is fast enough for current hard disks but will need to be replaced within a few years as hard disk performance continues to increase. Some disk subsystems are even now moving to higher-performance versions of PCI, as the limits of conventional PCI are reached. Some older systems used slower versions of the PCI bus, running at 25 or 30 MHz, which wasn't an issue for hard disks of that era.

Most older machines use a dedicated hard disk interface card that goes into a system bus slot and then connects to the drives internally. For the last several years, however, all newer PCs have had the ports for two IDE/ATA channels built into the motherboard itself. In practical terms, there is no real difference except for the cost savings associated with not needing to put a separate hard disk interface card into the PC when it is built into the motherboard. In fact, many people add PCI after-market controller cards to their systems, to enable access to a different interface (such as SCSI) or features not supported at the time their systems were originally designed. For example, inexpensive PCI cards are available to provide support for Ultra DMA modes for newer IDE/ATA drives, which can augment or replace the disk controller built into the motherboard.

Next: Interface Performance

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