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This article discusses a type of hard disk that is more popular in high-end
machines (servers) than in personal desktop computers. First, I will discuss SCSI
and some of its benefits. Then I will take a look at the steps involved in configuring
SCSI devices.
SCSI overview
SCSI is an acronym for Small Computer System Interface. The important part of this
term is “small computer.” SCSI environments use a SCSI controller that is responsible
for managing all SCSI devices and controlling the conversation on the SCSI
chain.
SCSI technology has many advantages over IDE technology, such as:
Types of devices supported
Number of devices supported in a single SCSI chain
Performance of SCSI over IDE devices
SCSI supports a multitude of devices, including hard drives and CD-ROMs.
Remember that IDE devices typically only support hard drives and CD-ROMs. SCSI
devices come in the following flavors:
Hard drives
CD-ROMs
Scanners
Printers
Tape drives
You could have a SCSI chain that contains any of those devices, whereas IDE could
probably support only two types of devices.
The other major benefit of SCSI is the number of devices that are supported in the
SCSI chain, or what is known as the SCSI bus. Original SCSI supports up to eight
devices in the chain, but one of those devices is the SCSI card that’s added to the
computer to give you the capability to use SCSI. Remember that IDE only allowed
two devices in the chain, and EIDE supports four devices, so with SCSI you are not
only allowed to have more types of devices, but you are also allowed to have more
of those devices!
Original SCSI supports up to eight devices in the chain, but if the exam asks how
many devices can be attached to a SCSI bus, the answer is seven (the test makers
assume that the SCSI card is already there). Remember to watch the wording
of the questions closely.
The last benefit of SCSI is performance. Original SCSI devices do not compare in the
performance category with EIDE devices, but some of the latter SCSI
technologies—such as SCSI-2 and SCSI-3—can outperform IDE and EIDE.
Host adapter
Assume that we are dealing with original SCSI, also
known as SCSI-1.
When you install SCSI devices, you first need to install the SCSI host adapter. The
SCSI host adapter is an expansion card that you add to the computer so you can
chain SCSI devices off the adapter. In essence, the SCSI host adapter is the brains of
the SCSI bus; it acts as the controller for the SCSI bus.
The SCSI controller (adapter) is responsible for sending and receiving all information
to and from the SCSI bus, just like the IDE controller. When the system has
information for one of the devices in the SCSI bus, the system will hand the information
over to the SCSI controller, which will then hand the information to the appropriate
device in the chain.
The beauty of the entire setup is that you have installed a SCSI adapter in the computer,
which is a card that will be assigned resources like an IRQ and an IO address.
Each device in the SCSI chain is not assigned these resources, because the processor
will pass information to the SCSI controller and the controller passes the information
to the device. This means that the system never talks to the devices
directly, which means each device does not require an IRQ and an IO address.
When you go out to purchase the SCSI adapter, you will first have to look inside
your system to figure out what type of expansion slots are free. Today, you will typically
have some PCI slots, but you may have an ISA or an EISA slot, as well. The difference
between these expansion slots is performance—PCI runs at 33 MHz, while
ISA and EISA run at only 8 MHz. Also, PCI and EISA are 32-bit technologies, while ISA
is only 16-bit. The bottom line is that if you have some PCI slots free, you will probably
end up purchasing a PCI SCSI adapter.
When the controller receives information for a particular device, how does it send
the information to that device?
Addressing
Each device is assigned an internal address in the SCSI bus, and the SCSI controller
knows the address of each device. When the SCSI controller receives information
for a particular device, the controller references that device by its ID in the SCSI
bus. This way, there’s no confusion as to who the data is destined for.
You assign the SCSI IDs when you connect each device to the SCSI chain. You will
assign an ID either by jumpers or DIP switches if the device is an internal device, or
by a “ticker” (yes, I made that term up) if the device is external. A ticker is an indicator
on the back of the external SCSI device whose value you can change by pressing
the button to increase or decrease the SCSI ID.
If you are installing an internal SCSI device, you will most likely need to assign the
SCSI ID by using jumpers. The internal SCSI devices will have a jumper set with
three pairs of jumper pins. The decimal values of these jumpers, although probably
not shown on the drive, are 4, 2, and 1 (from left to right).
Jumper Block Settings
J2 J1 J0
Decimal Value 4 2 1
SCSI ID 0 0 0 0
SCSI ID 3 0 1 1
SCSI ID 6 1 1 0
If you would like to assign the device a SCSI ID of zero, then you would not jumper
any of the jumper pins. This is shown in Table 9-3 by having the off state (represented
by zero) at each jumper location. Another example of setting a jumper ID
would be if you wanted to assign the device a SCSI ID of 3, you would jumper the J0
pins and the J1 pins, but not the J2 pins. This would enable decimal values of 1+2.
Table 9-3 also shows what would happen if you wanted to set the SCSI ID to six. You
would enable jumper J1 and J2, which enables the decimal values of 2+4, while the
first jumper has an off state.
The IDs you assign to each device are completely up to you, but note that the
higher the number, the more important the device is to the SCSI bus. For example, if
two devices need to send information through the bus at the same time, which
device will the SCSI controller service first? The device with the higher SCSI ID is
always given priority. This is why the SCSI host adapter is usually assigned the
highest number in the chain (usually seven if the bus supports eight devices—the
ID numbers start with zero).
So the host adapter is assigned an ID of seven, and a SCSI bootable hard drive is
assigned a SCSI ID of zero. Outside of that, you decide what the ID numbers are for
each device. As a rule of thumb, we give slower devices higher ID numbers so that
they may get serviced when they need to.
Cabling
Different types of cabling are used to chain SCSI devices to the SCSI adapter. If the
device is an internal device, a 50-wire ribbon cable will be used, which is very similar
to the 40-wire ribbon cable used for an IDE device.
If the device is an external device, you will use a thick Centronics cable to connect
to the Centronics 50 (typically used by SCSI-1) or the Centronics 68 (typically used
by SCSI-2) connector on the back of the device.
Termination
Both ends of the SCSI bus must be terminated so that when a signal is sent down
the SCSI bus, it is absorbed at the end of the bus by the terminator, which is acting
as a resistor. The first device in the chain must be terminated along with the last
device in the chain, the first device being the host adapter. If the device is an internal
device, terminating usually involves playing around with jumpers. If the device
is external, a terminator will be added to the back of the device.
Types of SCSI
Over the last number of years, SCSI has increased the performance of the devices
over other types of devices, such as IDE and EIDE. This is one of the reasons why
you will find network servers using SCSI hard drives instead of EIDE devices.
Original SCSI (SCSI-1), introduced in 1986, was an 8-bit technology and had a transfer
rate of 5 Mbps—a slow speed compared to today’s IDE devices, which run at
about 10Mbps. A few flavors of SCSI-2 were introduced when SCSI-2 was developed
as a standard in 1994. The first flavor was called fast SCSI-2, which increased the
transfer rate from 5Mbps to 10Mbps, but kept it as an 8-bit technology. The other
version of SCSI-2 was called wide SCSI-2, where the 8 bits were increased to 16 bits
and the transfer rate is also increased to 10Mbps.
When developing the standards for SCSI-2 not only was there a Fast SCSI-2 and a
wide SCSI-2 but a third version of SCSI was implemented that combined the features
of fast SCSI-2 and wide SCSI-2. Fast/wide SCSI-2 is a 16-bit technology that has a
transfer rate of 20Mbps. There is also an Ultra SCSI, which has an 8-bit bus width
and a transfer rate of up to 20Mbps. Ultra SCSI has a wide version, which increases
the bus width to 16-bit; as a result, Ultra-Wide SCSI can transfer information at 40
Mbps.
Types of SCSI
Description Bus Width Transfer Rate (Mbps) Number of Devices
SCSI-1 8 5 8
Fast SCSI-2 8 10 8
Wide SCSI-2 16 10 16
Fast/Wide SCSI-2 16 20 16
Ultra SCSI-2 8 20 8
Fast/Wide SCSI-3 16 40 16
Installing SCSI devices
Once you understand the issues with SCSI, installing a SCSI bus is fairly simple.
First, you want to assign a unique ID number to each device. I usually perform this
step at the beginning so that when everything is connected, you won’t have to play
around figuring out how to change the ID of the devices.
When assigning the ID numbers, remember that you want to assign the bootable
drive the ID of zero. You also want to enable the SCSI BIOS on the SCSI controller if
you are booting off a SCSI hard disk. When you enable the SCSI BIOS, you will not be
required to install a driver for the card, because the PC will recognize the device on
startup. If you are booting off an IDE drive and using the SCSI disk as an additional
drive, you should have the SCSI BIOS disabled and install a driver in the operating
system.
Now that you have the ID numbers assigned to each device, insert the SCSI host
adapter into the expansion slots of the PC. After inserting the SCSI card, chain all of
the devices together.
Once you have the devices chained together, you want to make sure that each end
on the SCSI bus is terminated. If the last device is an external device, you will need
to put the terminator on the end of the device. If you are installing internal devices,
you need to check the documentation on the internal devices to find out what
jumpers to set.
At this point, the IDs are configured for each device, the SCSI card is inserted into
the PC, and the devices are connected to the card to create a SCSI bus. We also terminated
the SCSI bus at either end. Before installing the driver for the SCSI card in
the operating system, I’ll review the steps to install a SCSI device one last time.
To install a SCSI bus:
1. Assign unique IDs to each device.
2. Install the SCSI host adapter into the expansion slots.
3. Chain devices to the SCSI host adapter.
4. Terminate the SCSI bus at both ends of the chain.
5. Install the driver for the SCSI card if you are not booting off the first hard disk.
Since SCSI is a technology that is usually implemented in servers, the following sections
outline the steps to install the driver in Windows NT 4.0 and Windows 2000.
Windows NT 4.0
Windows NT 4.0 is a non–plug and play type environment, so installing the device is
a little different than in Windows 95 or Windows 98. With Windows 95 and 98, you
could detect new hardware and take a chance on whether or not the operating system
will see the SCSI card. With Windows NT 4.0, there is no hardware wizard that
will install the device; you have to do it manually.
To install a SCSI host adapter in Windows NT 4.0, follow the steps outlined below.
STEP BY STEP: Installing a SCSI host adapter in Windows NT 4.0
1. Select Start - Settings - Control Panel.
2. Double-click the SCSI adapters.
3. In the SCSI Adapters dialog box, choose the Drivers tab.
4. Choose the Add button to add a driver.
5. In the Install Driver dialog box, choose the manufacturer of the host adapter
and then choose the adapter on the right side. If the adapter is not in the list,
choose the Have Disk button and provide the manufacturer’s disk.
6. Click the OK button.
7. Restart the computer.
Installing the SCSI host adapter in the Windows 2000 environment is a little bit different
than installing the driver in Windows NT 4.0. A wizard will start up, asking
whether you want to detect the device or install the driver. The following Step By
Step shows you how to install a SCSI adapter in Windows 2000.
STEP BY STEP: Installing a SCSI host adapter in Windows 2000
1. Select Start - Settings - Control Panel.
2. Double-click the Add Hardware icon. The Add Hardware Wizard appears displaying
the welcome screen to the wizard.
3. Choose Next.
4. Select Add or Troubleshoot A Device and then click the Next button.
5. In the Choose Hardware Device dialog box, select Add A New Device and then
click the Next button.
6. If you know the host adapter you are installing, select “No, I want to select the
hardware from a list.” If you are unsure of the host adapter you are installing,
then choose “Yes, search for new hardware.” For these steps, you will choose
the adapter to install, so select “No, I want to select the hardware from a list”
and click Next.
7. The next screen will ask you what type of device you are installing. Choose
SCSI and RAID controllers, then click Next.
8. In the list of devices on the left side select the manufacturer, and on the right
side select the adapter to install. Click Next.
9. Click Finish.
10. Restart the computer.
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