The Small Computer Systems Interface SCSI
The SCSI-Bus is an extension of your existing bus.
A controller card, called
adapter, is placed into one of your expansion slots. A ribbon cable that
contains both data and control signals then connects the host
adapter to your
There are several advantages to having SCSI
in your system. If you have a
limited number of bus
slots, adding a single SCSI
adapter enables you to
add up to seven more devices by taking up only one slot with older SCSI
and up to 15 devices with Wide-SCSI. SCSI
has higher throughput
than either IDE
also supports many more different types of devices.
There are several different types of SCSI
devices. The original SCSI specification is commonly referred to as SCSI-1. The newer
specification, SCSI-2, offers increased speed and performance, as well as new
commands. Fast SCSI increases throughput to more than 10MB
per second. Fast-Wide SCSI provides a wider data path and throughput of up to
40MB per second and up to 15 devices. There there are Ultra-SCSI and
The last type, SCSI-3, is still being developed as of this writing and will
provide the same functionality as Fast-Wide SCSI as well
as support longer cables and more devices.
device has its own controller and can send, receive, and execute SCSI commands.
As long as it communicates with the host adapter using
proper SCSI commands, internal data manipulation is not an issue. In fact, most
SCSI hard disks have an IDE controller with a SCSI
interface built onto them.
Because there is a standard set of SCSI
commands, new and different kinds of devices can be added to the SCSI family
with little trouble. However, IDE and
ESDI are limited to disk-type devices. Because the SCSI
commands need to be "translated" by the device, there is a slight overhead,
which is compensated for by the fact that SCSI devices are intrinsically faster
than non-SCSI devices. SCSI devices also have higher data integrity than
non-SCSI devices. The SCSI cable consists of 50 pins, half of which are ground.
Because every pin has its own ground, it is less prone to interference and
therefore it has higher data integrity.
On each SCSI
adapter are two connectors. One connector is at the top of the card (opposite
the bus connectors) and is used for internal devices. A
flat ribbon cable is used to connect each device to the
host adapter. On internal SCSI devices, only one
connector is on the
device itself. Should you have external SCSI devices, there is a second
connector on the end of the card (where it attaches to the chassis). Here SCSI
devices are "daisy chained" together.
The SCSI-Bus must be closed to work correctly. By this I mean that
each end of the bus must be terminated. There is usually a
set of resistors (or slots for resistors) on each device. The devices at either
end of the SCSI-Bus must have such resistors. This process is referred to as
terminating the bus and the resistors are
called terminating resistors.
Its fine to say that the SCSI-Bus needs to be terminated. However, that
doesn't help your understanding of the issue. As with other kinds of devices,
SCSI devices react to the commands sent along the cable to
them. Unless otherwise impeded, the signals reach the end of the cable and
bounce back. In such cases, there are two outcomes, both of which are
undesirable: either the bounced signal interferes with the
valid one, or the devices react to a second (unique in its mind) command. By
placing a terminator at the end of the bus, the signals
are "absorbed" and, therefore, don't bounce back.
The following two figures show examples of how the
SCSI-Bus should be terminated. Note that Figure 0-6 says that it is an example
of "all external devices." Keep in mind that the principle is still
the same for internal devices. If all the devices are internal, then the host
adapter would be still be terminated, as would the last device in the chain.
Image - Example of SCSI Bus with All External Devices (interactive)
Example of SCSI Bus with Both External and
Internal Devices (interactive)
If you don't have any external devices (or only have external devices), the
host adapter is at one end of
the bus. Therefore, it too must be
terminated. Many host adapters today have the ability to be terminated in
software, so there is no need for terminating resistors
(also known as resistor packs).
device is "identified" by a unique pair of addresses,
which are the controller addresses that are also referred to as the
This pair of addresses is usually set by jumpers or dip switches on the device
itself. Keep in mind that the ID is something that is set on the device itself
and is not related to location on the bus.
Note that in the figures
above, the SCSI
ID of the devices are ordered ID 0, 6, and 5. Also the SCSI
ID is often set using a combination of jumpers with no 1:1 relationship. (That
is a pair of pins labeled ID 0 through ID 7) Therefore, you should always read
the hardware documentation to determine how to set the ID.
This sounds pretty obvious, but some people don't make sure. They make
assumptions about what they see on the device regarding how the
ID is set and do not fully understand what it means. For example, I have an
Archive 5150 SCSI
tape drive. On the back are three jumpers, labeled 0, 1, and
2. I have had customers call in with similar hardware with their SCSI
set at 2. After configuring the tape drive and rebooting, they still couldn't
access the tape drive. Nothing else was set at ID 2, so there were no conflicts.
The system could access other devices on the SCSI-Bus, so the host
probably okay. Different SCSI
devices can be plugged into the same spot on the
SCSI cable, so it wasn't the cable. The SCSI-Bus was terminated correctly, so
that wasn't the problem.
Rather than simply giving up and saying that it was a hardware problem, I
suggested that the customer change the SCSI
ID to 3 or 4 to see if that worked.
Well, the customer couldn't, because the jumpers on the back only allowed him to
change the SCSI
ID to 0, 1, or 2. It then dawned on me what the problem was: the
jumpers in the back are in binary! To set the ID to 2, the jumper needs to be on
jumper 1, not jumper 2. Once the customer switched it to jumper 1 and
rebooted, all was well. (Note: I helped this customer before I bought
the Archive tape drive. When I got my drive home and wanted to check the SCSI
ID, I saw only three jumpers. I then did something that would appall most users:
I read the manual! Sure enough, it explained that the jumpers for the SCSI
An additional problem to this whole SCSI
ID business is that manufacturers are not consistent among each other. Some
might label the jumpers (or switches) 0, 1, and 2. Others label them 1, 2, and
4. Still others label them ID0, ID1, and ID2. I have even seen some with a dial
on them with 8 settings, which makes configuration a lot easier. The key is that
no matter how they are labeled, the three pins or switches are
binary and their values are added to give you the SCSI
Lets look at Figure 0-8, which represents the jumper settings on a
SCSI device. In the first example, none of the jumpers is
set, so the SCSI ID is 0. In the second example, the jumper labeled 1 is set.
This is 21 or 2, so the ID here is 2. In the last example, the
jumpers labeled 2 and 0 are set, which is 22 + 20 = 4 + 1,
On an AT-Bus, the number of devices added is limited only by the number of
slots (granted, the AT-Bus is limited in how far away the slot can be from the
CPU and therefore is limited in the number of slots). On a
SCSI-Bus, however, there can be only seven devices in addition to the
host adapter. Whereas devices on the AT-Bus are
distinguished by their base addresses, devices on the SCSI-Bus are distinguished
by their ID number.
ID numbers range from 07 and, unlike base addresses, the higher the ID, the
higher the priority. Therefore, the ID of the host adapter
should always be a 7. Because it manages all the other devices, it should have
the highest priority. On the newer Wide SCSI-Buses, there can be up to 15
devices, plus the host adapter, with
SCSI IDs ranging from 0 to 15.
Now back to our story...
The device address
is known as the logical unit number (LUN). On devices with embedded controllers,
such has hard disks, the LUN is always 0. All the
SCSI devices directly supported by Linux have embedded
controllers. Therefore, you are not likely to see devices set at LUNs other than
In theory, a single-channel SCSI
adapter can support 56 devices. Devices called bridge
adapters connect devices without embedded controllers to the SCSI-Bus. Devices
attached to the bridge adapter have LUNs between 0 and 7. If there are seven
bridge adapters, each with eight LUNs (relating to eight devices), 56 total
devices are therefore possible.
The original SCSI-1 spec only defined the connection to hard disks. The
SCSI-2 spec has extended this connection to such devices as CD-ROMS, tape
drives, scanners, and printers. Provided these devices all adhere to the SCSI-2
standard, they can be mixed and matched even with older SCSI-1 hard disks.
One common problem with external SCSI
devices is that the power supply is external as well. If you are booting your
system with the power to that external device turned off, once the
kernel gets past the initialization routines for that
device (the hardware screen), it can no longer recognize that device. The only
solution is to reboot. To prevent this problem, it is a good idea to keep all
your SCSI devices internally. (This doesn't help for scanners and printer, but
because Linux doesn't yet have drivers for them, it's a moot point.)
Although the number of host
adapter manufacturers has steadily decreased in the past few years, Adaptec, the
premier name in host adapters, has bought up both Trantor and Future Domain.
Adaptecs biggest competitor for years, Buslogic, was no longer able to compete
and was taken over by Mylex (a motherboard manufacturer, among other things).
Despite the decrease in number of manufacturers, the number of models is still
adapter manufacturers provide more than just a single model. Many provide models
for the entire spectrum of buses and SCSI types.
Fast SCSI, Wide SCSI, and Ultra-Wide SCSI are part of the alphabet soup of SCSI
devices. You can connect Wide SCSI disks onto a Fast SCSI adapter, although it
will still only get 8 bits instead of the Wide SCSIs 16 bits, so it therefore
only gets 10Mbytes per second compared to 20Mbytes per second of Wide SCSI.
disks can also be connected with the same limitations (it is an 8-bit bus). It
can also handle Ultra-Wide SCSI and get 40Mbps. This is not too big of an issue,
as most of the devices available today can only handle 10Mbps.
When looking at the performance of a SCSI
device, you need to be careful of the manufacturers test results. They can be
deceiving. If a test reads 200MB from the disk in 10 seconds, you get an average
of 20MB per second. What if those 100MB are all from the same track? The disk
hardware reads the track and keeps it in its own cache.
When the host adapter requests a new block from that track,
the hard disk doesn't need to find the block on the disk,it delivers it from the
cache. This decreases the access time and increases the apparent transfer
rate of the drive dramatically. The manufacturer can say, in all honesty, that
the host adapter has a transfer rate of 20Mbps, though the
drive can only do half of this at most. Again, the chain is only as strong as
its weakest link.
This does not mean that Wide SCSI
or Ultra SCSI are only useful for the companies marketing departments. SCSI has
the advantage of being able to talk to multiple devices. For example, it can
request data from one drive and, rather than waiting for the data, free the
SCSI-Bus (disconnect). When the drive (or other device) is ready, it requests
the bus again (reconnect) and the data is transferred.
While the drive searches for the data, the host adapter can
request data from another device. While this device is looking for the data, the
first device can transfer the data to the host adapter.
Being able to read or write devices like this means that a host adapter could
get a sustained transfer rate of more that what individual devices
can handle. (Note that both the host adapter and device must support
gets its performance gain by the fact it is wide (16 bits versus 8 bits). Ultra
SCSI, on the other hand, gets the increase through a shorter cycle time. This is
an important aspect because this makes for a steeper edge on the
signal (the time from a low to high signal is much shorter,
and vice versa). This means that the SCSI-Bus has higher requirements regarding
Internal devices usually are connected by flat cable ribbons and present few
new problems with Fast SCSI. The maximum length of the
cable is half of what it could be with older SCSI devices and you must follow
the specs exactly. Round cables for external devices have to be created
specifically for Ultra SCSI and are therefore more expensive. Although the
actual data transfer rate between the host adapter and the
device is only as high as the device can handle, the steepness of the edges is
the same. This means that if you connect Fast SCSI devices to Ultra SCSI host
adapters, you still need the special Ultra SCSI cables.
Another consideration is that Ultra SCSI
requires active termination. On the host
adapter side, this isn't a problem because the host adapters are designed to
give active termination. However, many older devices support only passive
termination and therefore cant work on Ultra SCSI host adapters. This really
comes into play when larger amounts of data are being transferred.
PCI devices can generally behave as either masters or slaves. For slave
devices, the CPU is responsible for all the activity. This
is a disadvantage for slave devices because the CPU is often busy transferring
data and issuing commands instead of doing other work. This is really an issue
in multitasking operating systems (like Linux) that have "better" things to do.
Master devices, on the other hand, have an advantage here. The CPU only needs to
tell them where to transfer the data, and they do the work themselves.
Regardless of whether a device acts as a master or slave, it will take up an
interrupt line. Single function devices, such as
host adapters, are given the interrupt INT-A. This means
that the actual IRQ (between 5 and 15) will be determined
by the system BIOS.
Generally, you can say that a higher throughput
is required on a file server
as compared to a workstation. Although there are applications like CAD or video
processing, which require more throughput
on a workstation than other kinds of
applications, the major of the work is done by the server. Therefore, it is
extremely important to consider the performance of your hard disk and similar
devices on the server.
Despite reaching comparable prices and sizes, ATA
harddisks as suited for work in a server because they do not have the throughput
As we discussed previously, SCSI has the advantage of being able to
have two devices communicate with each other directly with the need to go
through the CPU.
In addition, while waiting for one device to find the data, it
can "disconnect" itself and you can make a request of another device on the same
This is especially important on servers as they usually have multiple
disks, as well as other kinds of devices.
Here again, the chain to your
harddisk is only as weak as the weakest link. Therefore, you need to have a SCSI
host adapter that can keep up with the hard disk.
Let's take my Adaptec
adapter which I have in my primary workstation as an example. This
is an Ultra2 SCSI
device, which gives me a maximum transfer rate of 80
Mbyte/second. One neat aspect of this host
adapter, is that it uses a technology
Adaptec calls "SpeedFlex.". Internally, there are three connectors. One for
(80Mb/s), one for Wide Ultra SCSI (40Mb/s) and one for Ultra SCSI
(20Mb/s). Externally, you have two connectors. One 68-pin for Ultra2
one 50-pin for Ultra SCSI.
Therefore, you get the maximum throughput
what kind of device is connect to the host
adapter. In addition, one aspect of
the SpeedFlex technology is that it can operate the different buses at the
different speeds simultaneously, so there no performance lost on one
of a slow device on another bus.
The "W" at the end of the host
means that it has the Wide Ultra SCSI
connector. Adaptec also produces the
2940U2, which has the same specifications, but without the internal Wide Ultra
SCSI connector and without the external Ultra2 connector. Note that in each
case, devices supporting older SCSI
standards can still be connected.
In one one of my machines,
I have an Adaptec 3940U2, which is also an Ultra2 SCSI
key difference is that this adapter is twin-channel. That means I have two SCSI
buses running off of the same host
adapter. This is extremely useful when you
have more device that will fit on a single bus,
plus added speed. Both of this
aspects make this the perfect adapter for a Linux server (although I use it in
my Workstation). Another important difference is that the Adaptec 3940U2
supports 64-bit PCI,
although it is still capable with 32-bit PCI.
One thing to note is that all of the devices support
up to 15 devices (plus the host
adapter per channel). This means that the
twin-channel 3940U2 can connect up to 30 devices. In addition, all devices
support cables up to 12 meters long.
On one machine I have an Adaptec 1542 CF
host adapter. The Adaptec 1540 a SCSI-1 and SCSI-2 compliant ISA
which has an 8-bit data bus
and a maximum transfer rate of 10 Mbyte/sec. This is
a perfect host
adapter for workstations which require less performance.
Initially, I wanted to say "low end", but that tends to create false
impressions of something of lesser quality. This definitely does not apply to
the Adaptec 1540 family (or any of the Adaptec products for that matter). A few
months ago, I replaced the 1542CF I had after six years because the built-in
floppy controller was no longer working correctly. The host
adapter was working
fine. I just couldn't access my floppy drive.
A few weeks later, I put it
back into the machine as it as the only one with the external connector my
CD-ROM changer had. Rather than buying an adapter for the connector, I put in
my old Adaptec 1542 CF and it has run perfectly ever since.
All of the
adapters support what is called "scatter gather." Here, requests
for data on the harddisk which are "scattered" all over the drive are "gathered"
together, in order that they be more efficiently process.
This is similar to
way an elevator works. Image that four people get into an elevator. The first
one presses the button for floor 12, the next one for floor 16 the next one for
floor 3, and the last one wants floor 8. Although the person wanting to go to
floor 12 was there first, the elevator stops at floor 3 first, then floor 8
before continuing to floor 12. This is much more efficient than going to each
floor in the order the buttons were pressed.
Accessing a hard disk is
similar. On active systems, there will be several requests waiting to
processed. Adaptec host
adapters will sort the requests based on their physical
location on the hard disk. Interestingly enough, Adaptec refers to this as an