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Differences in SATA and PCI Buses

written by: Debasis Das•edited by: Lamar Stonecypher•updated: 7/29/2009

All kinds of buses can be found in the personal computer. That can get a little confusing. You hear about the SATA and the PCI buses often. When you buy a hard disk or one of the optical disk types, you cannot help hearing about SATA, but the controller is PCI bus! So, what are the differences!

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    What is not different is that both are standard bus systems designed for use in computer systems. Both are meant for interconnecting subsystems. Both buses, like all buses parallel or serial, must have means of addressing the subsystems connecting to them, a means of data exchange, and a method of acknowledging that data has been received by the receiving party, etc. Both can be found inside your PC at the same time. Beyond such general similarities, they are very different- they were designed for different purposes. Another major difference is that the SATA, or the Serial ATA, bus is a serial one and the PCI, or the peripheral component interconnect, bus is a parallel one. The PCI bus was designed for interconnecting high speed peripherals with the processor system. In contrast, SATA has evolved out of the hard disk interface. The image clearly show the interrelationships of different buses in use in your PC today.

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    Buses on a PC

    Buses on a Motherboard
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    All about SATA

    Until SATA came along the mass storage interface was a parallel one known as EIDE. This was named ATA for AT attachment. AT being a reference of how the PC became an advanced technology (AT) PC with the coming of the 80386, 80486, and other improved processors. This ATA bus had 16 data wires and a total of 40 wires. A flat cable of 40 lines became the standard means of cabling the hard drives/optical drives/mass storage to the motherboard. This was redesignated Parallel ATA or PATA and could transfer data at rates up to 133 MB/s (or about 1 Gb/s). With processor speed increasing faster, this transfer rate was turning into a drag on overall system throughput. Besides, the large flat cable, which could not be extended beyond 18 inches, tended to block airflow inside the machine. Yet, with higher powered processors, heat generated inside was increasing.

    That brought in serial ATA or SATA , which carried only a 7 wire cable. It was command compatible with PATA. So that in the compatibility mode you could use the same driver and simply install a SATA interface drive via the cable to the SATA connector on the motherboard. SATA brought in higher speeds. It started with 1.5 GB/s. With a couple of revisions that have been done already, the transfer speed is now at 6 Gb/s. The length of the cable can be up to 1 meter or 3 ft. This much smaller cable then can be organized well easily so that airflow inside was not blocked. As of now, all PCs shipping have the SATA cabling inside them connecting the motherboard with the mass storage devices such as HDD and CD/DVD reader/writers. Some additional features in the interface make the job of the host adapters easier. For example, SATA also supports a hot-plug feature. For this, your hard disk or the optical drive should be capable of supporting it, too. There is an eSATA specification that defines a cable that can support an external SATA drive.

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    The PCI Bus

    While the SATA bus connects a mass storage device to a host adaptor, PCI or PCI Express (PCI-E) is the bus that connects the host adaptor or the drive controller on the motherboard to the processor and the memory systems. The host adaptor may take the form of an integrated circuit (called a planar device in the spec) or a daughter card plugging into a parallel connector built into the motherboard. The PCI bus is a parallel bus. It has replaced the original ISA and the EISA buses that used to interconnect various subsystems to the processor and memory system. However, the processor does not interconnect with the PCI in the current architectures in use. It uses a Front Side Bus or FSB but that’s another story.

    A bridge typically connects the PCI bus to the memory system. PCI peripherals need to communicate with the memory system as data from the peripherals are always transferred to memory directly via DMA operations. It has another bridge that connects to a ISA bus letting you use the older peripherals. The PCI slots have 47 pin connectors on the motherboard. You can have up to five such cards. Speeds a PCI bus can reach in the current form are 1.5 GB/s operating at 133 MHz clock speeds and a 64-bit wide mode. The express version provides multiple lanes thereby multiplying the speed. Many high performance video cards use a 16x lane bus for top performance. Each lane provides a point to point connection between two parties involved in a data transfer.

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