Computer systems architecture

partial list of past and current disk buses includes
• AT Attachment (ATA)
*
• Integrated Drive Electronics (IDE)
†
• Enhanced IDE (EIDE)
‡
• Small Computer Systems Interface (SCSI)
§
Some of these standards do not exist anymore, having been replaced due to some of their inherent
limitations. The constantly growing demand for faster and high-capacity disks takes its toll on the
standard buses, which have to be enhanced in order to accommodate the new features and
capabilities. It should be noted that the variety of buses represents the fierce competition between
various manufacturers, in which each one tries to “push” its products and gain a larger market
acceptance. Usually, hard-drive manufacturers are not interested in just developing new standards;
this is done in order to promote their products. In the twenty-first century, the computing industry
prefers open standards that allow sharing devices from many manufacturers. In most cases, the user
prefers a standard product even if a different proprietary product is superior.
One of the buses that was widely used for high-performance systems and is currently being used
mainly for servers is SCSI. Its name reflects the market position its designers were hoping to acquire.
This bus, which was developed several decades ago, has been widely used by IBM to try to
differentiate its products and provide superior performance. It is a parallel channel that connects
and concatenates several devices (
Figure 7.19
).

---

FIGURE 7.19
The SCSI bus.
Generally speaking, the bus is no different from other buses. It has an adapter (SCSI controller)
that converts and bridges between the buses (such as PCMCIA or USB and SCSI). One of its
advantages is its ability to concatenate several devices on the bus. The last device is connected to a
terminator that identifies the end of the bus (
Figure 7.19
, right bottom corner). The bus was
successful due to its relative high throughput compared with other available buses. As with other
devices and buses in the PC market, the standard went through a series of improvements, and over
the years, many variations were developed. In most cases, the variations were for accommodating
features such as faster transfer rates, increasing the maximum number of devices on the bus, and so
on.
The process of defining and developing special buses for connecting mass storage (hard drives)
was not confined only to disks and was done in order to connect a variety of other devices as well.
The more common buses, interfaces, and protocols used in PCs are
• Parallel (line printer or LPT)
*
• Serial (typically RS232C)
†
• PS/2 (for keyboards and mice)
‡
• Universal Serial Bus (USB)