SCSI Controllers

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1. Introduction to SCSI card
2. Why use a SCSI card rather than an enhanced IDE card?
3. Differentiate SCSI-2, SCSI-3, and Ultra SCSI controller cards.
4. Define Fast SCSI, Wide SCSI, and Ultra SCSI.
5. Why were SCSI controller cards developed?
6. What is Auto Termination?
7. Explain the term "bus master."
8. Define transfer rate.
9. Explain ASPI and CAM drivers.
10. Explain the DMA channel.
11. Installing the SCSI card

A SCSI host adapter (the proper name for the SCSI expansion card) actually runs an expansion bus of its own: data is transferred between the host adapter and the SCSI devices (i.e. hard drives) via this bus, which is called the SCSI bus. Each device on the SCSI chain is "intelligent," having its own internal controller, which controls the exchange of data between it and the host adapter. A SCSI device can handle data internally in almost any way – as long as it communicates with the host adapter using the commands specified by the SCSI standard; because of this feature, the SCSI interface offers incredible flexibility. Virtually any kind of device can be attached to a PC through a SCSI host adapter.

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The SCSI controller card is a high performance I/O card that offers performance enhanced IDE cards cannot achieve. The SCSI card’s performance is most beneficial in multi-user or multi-tasking environments (servers, UNIX®, Windows® NT, Windows® 95 systems), applications normal enhanced IDE is not suitable to handle. An 8-bit SCSI card allows connection of up to 7 devices, while Wide SCSI and Ultra SCSI allow up to 15 devices.

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Ultra SCSI is the latest design. It can improve the data transfer rate up to four times the conventional Fast SCSI rate: 20 Mbps for the Ultra version and 40 Mbps for the Ultra Wide version.

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Fast SCSI is defined as the ability to transfer data at 10 Mbps using an 8-bit bus. Wide SCSI refers to a 20 Mbps data transfer rate using a 16-bit bus. Ultra SCSI supports 20 Mbps transfer rate using a 16-bit bus, while the Ultra Wide version supports 40 Mbps transfer rate using a 16-bit bus.

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SCSI controller cards are offered as an alternative to the enhanced IDE controller on today’s mainboards. Unlike the enhanced IDE controllers, which are limited to single tasking, SCSI controller cards are capable of multi-tasking and can match the Pentium-based system’s highest level of performance.

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Many newer host adapters provide automatic termination. You still have to manually terminate the devices at the end of the SCSI chain, but the host adapter itself automatically switches to terminated or unterminated during boot-up, depending on the position of the adapter in the SCSI chain.

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Most adapters on the market include bus mastering on the host adapter. Bus mastering is an intelligent device that when attached to the PCI bus can gain control of the host’s system bus to perform specific tasks. This feature allows the SCSI devices to use the host bus only when they are moving data and commands; they unhook to process data, and can access memory directly, bypassing the system’s standard DMA channels. Bus mastering improves overall system throughput by giving the SCSI devices direct access to memory and freeing the host system’s bus while SCSI devices process commands.

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Data transfer rate is defined as the maximum rate at which data can be transferred from one device to another. Transfer rate is an important indication of SCSI performance because it measures the speed data can be transferred from a SCSI device to the computer’s main memory and CPU for processing. SCSI transfer rates are measured in terms of megabytes per second.

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SCSI host adapters require a device driver for communicating with the operating system. The driver that handles communication between the host adapter and the operating system is called the Universal Driver. Windows® 95 and Windows® NT have built-in universal driver support for almost all SCSI host adapters on the market. Over the years three brands of universal drivers emerged. Microsoft® developed the Layered Device Driver Architecture standard, which never became a significant presence in the market. The two other standards are ASPI (Advanced SCSI Programming Interface) and CAM. ASPI has become the most popular type of device driver, and CAM has gone by the wayside; CAM-to-ASPI converters are available. Almost every SCSI device on the market includes an ASPI device module, which allows it to work with host adapters supported by an ASPI universal driver.

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DMA is useful for making quick backups and for real-time applications. Some expansion boards, such as CD-ROM cards, are capable of accessing the computer’s DMA channel. When you install the board, you must specify which DMA channel is to be used, which sometimes involves setting a jumper of DIPswitch.

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1. Ground yourself by touching a metal object.
2. Make sure that the computer system is powered off and unplugged.
3. Open the chassis by removing the cover.
4. Remove the I/O card from the anti-static packaging.
5. Locate an available ISA slot on the motherboard. Remove the bracket and screw.
6. Position the I/O card over the slot and press down firmly, making sure that the edge connector is seated properly.
7. Screw down the I/O card with the removed screw.

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