how scsi works

8/9/2019 How SCSI Works

1/7

How SCSI Works

Controllers, Devices and CablesA SCSI controller coordinates between all of the other devices on the SCSI bus and the computer. Also

called a host adapter, the controller can be a card that you plug into an available slot or it can be builtinto the motherboard. The SCSI BIOS is also on the controller. This is a small ROM orFlash memory

chip that contains the software needed to access and control the devices on the bus.

Each SCSI device must have a unique identifier (ID) in order for it to work properly. For example, if

the bus can support sixteen devices, their IDs, specified through a hardware or software setting, range

from zero to 15. The SCSI controller itself must use one of the IDs, typically the highest one, leaving

room for 15 other devices on the bus.

Internal SCSI devices connect to a ribbon cable.

Internal devices connect to a SCSI controller with a ribbon cable. External SCSI devices attach to thecontroller in a daisy chain using a thick, round cable. (Serial Attached SCSI devices use SATA

cables.) In a daisy chain, each device connects to the next one in line. For this reason, external SCSI

devices typically have two SCSI connectors -- one to connect to the previous device in the chain, andthe other to connect to the next device.

External SCSI devices connect using thick, round cables.
http://computer.howstuffworks.com/motherboard.htmhttp://computer.howstuffworks.com/bios.htmhttp://computer.howstuffworks.com/rom.htmhttp://computer.howstuffworks.com/flash-memory.htmhttp://computer.howstuffworks.com/bios.htmhttp://computer.howstuffworks.com/rom.htmhttp://computer.howstuffworks.com/flash-memory.htmhttp://computer.howstuffworks.com/motherboard.htm


2/7

The cable itself typically consists of three layers:

Inner layer: The most protected layer, this contains the actual data being sent.

Media layer: Contains the wires that send control commands to the device.

Outer layer: Includes wires that carry parity information, which ensures that the data is

correct.

Different SCSI variations use different connectors, which are often incompatible with one another.These connectors usually use 50, 68 or 80 pins. SAS uses smaller, SATA-compatible connectors.

68-pin Alternative 3 SCSI connector

50-pin Centronics SCSI connector

Once all of the devices on the bus are installed and have their own IDs, each end of the bus must be

closed. We'll look at how to do this next.

TerminationIf the SCSI bus were left open, electrical signals sent down the bus could reflect back and interfere with

communication between devices and the SCSI controller. The solution is to terminate the bus, closing

each end with a resistor circuit. If the bus supports both internal and external devices, then the lastdevice on each series must be terminated.

Types of SCSI termination can be grouped into two main categories: passive and active. Passive

termination is typically used for SCSI systems that run at the standard clock speed and have a distance


3/7

of less than 3 feet (1 m) from the devices to the controller. Active termination is used for Fast SCSI

systems or systems with devices that are more than 3 feet (1 m) from the SCSI controller.

Some SCSI terminators are built into the SCSI device, whileothers may require an external terminator like this one.

SCSI also employs three distinct types ofbus signaling, which also affect termination. Signaling is the

way that the electrical impulses are sent across the wires.

Single-ended (SE): The controller generates the signal and pushes it out to all devices on

the bus over a single data line. Each device acts as a ground. Consequently, the signal quicklybegins to degrade, which limits SE SCSI to a maximum of about 10 ft (3 m). SE signaling iscommon in PCs.

High-voltage differential (HVD): Often used for servers, HVD uses a tandem approach

to signaling, with a data high line and a data low line. Each device on the SCSI bus has a signaltransceiver. When the controller communicates with the device, devices along the bus receive

the signal and retransmit it until it reaches the target device. This allows for much greater

distances between the controller and the device, up to 80 ft (25 m).

Low-voltage differential (LVD): LVD is a variation on HVD and works in much the

same way. The big difference is that the transceivers are smaller and built into the SCSI adapter

of each device. This makes LVD SCSI devices more affordable and allows LVD to use lesselectricity to communicate. The downside is that the maximum distance is half of HVD -- 40 ft

(12 m).

An active terminator

Both HVD and LVD normally use passive terminators, even though the distance between devices and

the controller can be much greater than 3 ft (1 m). This is because the transceivers ensure that the signal

is strong from one end of the bus to the other.

For more information on SCSI and other busses, check out the links on the following page.


4/7

SCSI BasicsSCSI is based on an older, proprietary bus interface called Shugart Associates System Interface(SASI). SASI was originally developed in 1981 by Shugart Associates in conjunction with NCR

Corporation. In 1986, the American National Standards Institute (ANSI) ratified SCSI (pronounced

"scuzzy"), a modified version of SASI. SCSI uses a controller to send and receive data and power to

SCSI-enabled devices, likehard drives andprinters.

SCSI connector

SCSI has several benefits. It's fairly fast, up to 320 megabytes per second (MBps). It's been around formore than 20 years and it's been thoroughly tested, so it has a reputation for being reliable. Like Serial

ATA and FireWire, it lets you put multiple items on one bus. SCSI also works with most computer

systems.

However, SCSI also has some potential problems. It has limited system BIOSsupport, and it has to be

configured for each computer. There's also no common SCSI software interface. Finally, all the

different SCSI types have different speeds, bus widths and connectors, which can be confusing. Whenyou know the meaning behind "Fast," "Ultra" and "Wide," though, it's pretty easy to understand. We'll

look at these SCSI types next.

RAIDSCSI is often used to control a redundant array of

independent discs (RAID). Other technologies, like serial-
http://computer.howstuffworks.com/hard-disk.htmhttp://computer.howstuffworks.com/hard-disk.htmhttp://computer.howstuffworks.com/inkjet-printer.htmhttp://computer.howstuffworks.com/inkjet-printer.htmhttp://computer.howstuffworks.com/firewire.htmhttp://computer.howstuffworks.com/bios.htmhttp://computer.howstuffworks.com/bios.htmhttp://computer.howstuffworks.com/hard-disk.htmhttp://computer.howstuffworks.com/inkjet-printer.htmhttp://computer.howstuffworks.com/firewire.htmhttp://computer.howstuffworks.com/bios.htm


5/7

ATA (SATA), can also be used for this purpose. Newer

SATA drives tend to be faster and cheaper than SCSI drives.

A RAID is a series ofhard drives treated as one big drive.These drives can read and write data at the same time, known

as striping. The RAID controller determines which drive gets

which chunk of data. While that drive writes the data, the

controller sends data to or reads it from another drive.RAID also improves fault tolerance through mirroring and

parity. Mirroring makes an exact duplicate of one drive's dataon a second hard drive. Parity uses a minimum of three hard

drives, and data is written sequentially to each drive, except

the last one. The last drive stores a number that represents the

sum of the data on the other drives. For more information onRAID and fault tolerance,

SCSI TypesSCSI has three basic specifications:

SCSI-1: The original specification developed in 1986, SCSI-1 is now obsolete. It featured

a bus width of 8 bits and clock speed of 5 MHz.

SCSI-2: Adopted in 1994, this specification included the Common Command Set

(CCS) -- 18 commands considered an absolute necessity for support of any SCSI device. It alsohad the option to double the clock speed to 10 MHz (Fast), double the bus width from to 16 bits

and increase the number of devices to 15 (Wide), or do both (Fast/Wide). SCSI-2 also added

command queuing, allowing devices to store and prioritize commands from the host computer. SCSI-3: This specification debuted in 1995 and included a series of smaller standards

within its overall scope. A set of standards involving the SCSI Parallel Interface (SPI), whichis the way that SCSI devices communicate with each other, has continued to evolve within

SCSI-3. Most SCSI-3 specifications begin with the term Ultra, such as Ultra for SPI variations,

Ultra2 for SPI-2 variations and Ultra3 for SPI-3 variations. The Fast and Wide designations

work just like their SCSI-2 counterparts. SCSI-3 is the standard currently in use.

Different combinations of doubled bus speed, doubled clock speed and SCSI-3 specifications have led

to lots of SCSI variations. The chart below compares several of them. Many of the slower ones are no

longer in use -- we've included them for comparison.

Name Specification # ofDevices BusWidth BusSpeed MBps

AsynchronousSCSI

SCSI-1 8 8 bits 5 MHz4

MBpsSynchronous

SCSISCSI-1 8 8 bits 5 MHz

5MBps

Wide SCSI-2 16 16 bits 5 MHz10

MBps

Fast SCSI-2 8 8 bits 10 MHz10

MBpsFast/Wide SCSI-2 16 16 bits 10 MHz 20
http://computer.howstuffworks.com/hard-disk.htmhttp://computer.howstuffworks.com/hard-disk.htmhttp://computer.howstuffworks.com/hard-disk.htm


6/7

MBps

UltraSCSI-3

SPI8 8 bits 20 MHz

20MBps

Ultra/WideSCSI-3

SPI8 16 bits 20 MHz

40MBps

Ultra2SCSI-3SPI-2

8 8 bits 40 MHz40

MBps

Ultra2/Wide

SCSI-3

SPI-2 16 16 bits 40 MHz

80

MBps

Ultra3SCSI-3SPI-3

16 16 bits 40 MHz160

MBps

Ultra320SCSI-3SPI-4

16 16 bits 80 MHz320

MBps

In addition to the increased bus speed, Ultra320 SCSI uses packeted data transfer, increasing itsefficiency. Ultra2 was also the last type to have a "narrow," or 8-bit, bus width.

All of these SCSI types are parallel -- bits of data move through the bus simultaneously rather than one

at a time. The newest type of SCSI, called Serial Attached SCSI (SAS), uses SCSI commands buttransmits data serially. SAS uses a point-to-point serial connection to move data at 3.0 gigabits per

second, and each SAS port can support up to 128 devices or expanders.

SCSI controller

All the different SCSI varieties use controllers and cables to interface with devices. We'll look at this

process next.


7/7

how scsi works

Documents