SCSI Controllers
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- Introduction to SCSI card
- Why use a SCSI card rather than an enhanced IDE
card?
- Differentiate SCSI-2, SCSI-3, and Ultra SCSI
controller cards.
- Define Fast SCSI, Wide SCSI, and Ultra SCSI.
- Why were SCSI controller cards developed?
- What is Auto Termination?
- Explain the term "bus master."
- Define transfer rate.
- Explain ASPI and CAM drivers.
- Explain the DMA channel.
- Installing the SCSI card
Introduction to SCSI card
SCSI is the abbreviation for Small Computer Systems Interface. The SCSI card,
also called a SCSI host adapter, is a PC interface card that is installed in an
expansion slot and used to connect storage devices and other peripherals to the
system. Devices typically connected to the system using a SCSI cared include
tape drives, scanners, CD-ROM drives and hard disks. SCSI devices are considered
superior to most other similar peripheral devices because of the high data
transfer rate SCSI offers, the large capacity of SCSI devices (hard drives up to
4GB, for instance) and the ease of installation. The number of devices that can
be connected to the system using one card is another attractive feature of SCSI,
with some cards allowing up to 21 hard drives on a single card, although the
typical number of devices attached to one card is 7.
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.
Why use a SCSI card rather than an enhanced IDE card?
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.
Differentiate SCSI-2, SCSI-3, and Ultra SCSI controller cards.
The SCSI-2 card is an improved version of the SCSI-1 card. It supports a
transfer rate of 10 Mbps for Fast SCSI and 20 Mbps for Wide SCSI. It also offers
improved command support for robust compatibility.
The SCSI-3 card is a more enhanced version than SCSI-2. It includes
recommendations for robust active termination and high-density, 16-bit
connectors.
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.
Define Fast SCSI, Wide SCSI, and Ultra SCSI.
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.
Why were SCSI controller cards developed?
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.
What is Auto Termination?
In order to function properly, the devices at either end of the SCSI chain
must be terminated. That means that if you are attaching only internal or
external devices (no matter how many) the host adapter must be terminated. If
you are attaching both internal and external devices to a single host adapter,
it will be in the middle of the chain; therefore, you must disable termination.
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.
Explain the term "bus master."
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.
Define transfer rate.
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.
Explain ASPI and CAM drivers.
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.
Explain the DMA channel.
DMA is short for direct memory access, a technique for transferring data from
main memory to a device without passing it through the CPU. Computers that have
DMA channels can transfer data to and from devices much quicker than computers
without DMA.
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.
Installing the SCSI card.
In order to install an I/O card, you need the following items:
- Phillips screwdriver
- Available IDA slot
- Ground yourself by touching a metal object.
- Make sure that the computer system is powered off and unplugged.
- Open the chassis by removing the cover.
- Remove the I/O card from the anti-static packaging.
- Locate an available ISA slot on the motherboard. Remove the bracket and
screw.
- Position the I/O card over the slot and press down firmly, making sure
that the edge connector is seated properly.
- Screw down the I/O card with the removed screw.
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