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CompTIA A+ Core TechNote: SCSI
SCSI
The SCSI (pronounced "scuzzy") interface is mostly used to connect mass-storage devices such as hard disk drives, tape devices and CD-drives but is also often used to connect scanners and other optical devices. SCSI devices can be internal or external.
The advantages are that it is fast, reliable, it allows you to connect multiple devices in a chain and it is easily expandable.

Termination
SCSI chains must be terminated using terminators on both internal and external ends to prevent the signal from echoing.
Most internal SCSI devices have built-in termination, external devices often have to be terminated using a terminator.

SCSI IDs and LUNs
Every device in the chain must have a unique ID assigned so the host adapter will know with who it's communicating.
(When using Logical Unit Numbers (LUNs) a single SCSI ID can support multiple devices, this is often used in hardware-raid arrays.)The lowest ID is 0 (often preset on host adapter to be used for bootable hard disk drive.) and the highest is 7 or 15 (depending on the flavor, see table below.)
The host adapter often is preset to ID 7.

The higher the SCSI ID , the higher the priority when negotiating for the SCSI bus. 7 is highest for all SCSI types, priority sequence for wide SCSI is: 7, 6, 5, 4, 3, 2, 1, 0, 15, 14, 13, 12, 11, 10, 9, 8.

On older devices SCSI IDs are configured by setting jumpers or switches on the device. This is a fairly simple process:
For example, if you have 3 jumpers to configure the ID and you want the device to use ID 4 you will need to think in 'bits'.
3 jumpers (bits) allow for a maximum value of 7:
(jumpers) on on on = (bits) 1 1 1 = (ID) 4+2+1 = 7
If you would set the first and third jumper only, the ID 5 will be used (4+0+1=5).
You need to know this for the exam, see the exam objectives at the bottom of this page.

SCSI ID SCSI-2 Jumper or switches
0 0 0 0 off / off / off
1 0 0 1 off / off / on
2 0 1 0 off / on / off
3 0 1 1 off / on / on
4 1 0 0 on/ off / off
5 1 0 1 on / off / on
6 1 1 0 on / on / off
7 1 1 1 on / on / on
 
SCAM (SCSI Auto-Magically), found on SCSI-3 and many SCSI-2 host adapters is a feature that assigns SCSI IDs dynamically and resolves conflicts automatically.

Cabling
SCSI uses different types of cabling and connectors.
- Internal devices use a flat ribbon cable with a 50 or 68-pin connector.
- External devices use a rather thick cable with a 50 or 68-pin connector.
Narrows SCSI (8-bit) uses the 50-pin connector and Wide SCSI (16-bit) uses the 68-pin connector.

Expansion slots
SCSI host adapters have been made for all of the common I/O buses, including ISA, EISA, VLB, MCA and PCI.
PCI is the most common used today.

SCSI Types
There are many different flavors of SCSI, the most important differences are max through-put, bus width and speed, and maximum supported number of devices. Refer to the table below and notice their differences.
Mode Specification # of Devices* Bus Width Bus Speed Through-put**
Asynchronous
SCSI
SCSI-1 8 8 bits 5 MHz 4 MB/s
Synchronous
SCSI
SCSI-1 8 8 bits 5 MHz 5 MB/s
Fast
SCSI
SCSI-2 8 8 bits 10 MHz 10 MB/s
Wide
SCSI
SCSI-2 16 16 bits 5 MHz 10 MB/s
Fast/Wide
SCSI
SCSI-2 16 16 bits 10 MHz 20 MB/s
Ultra
SCSI
SCSI-3
SPI
8 8 bits 20 MHz 20 MB/s
Ultra/Wide
SCSI
SCSI-3
SPI
16 16 bits 20 MHz 40 MB/s
Ultra2
SCSI
SCSI-3
SPI-2
8 8 bits 40 MHz 40 MB/s
Ultra2/Wide
SCSI
SCSI-3
SPI-2
16 16 bits 40 MHz 80 MB/s
Ultra3
SCSI
SCSI-3
SPI-3
16 16 bits 40 MHz 160 MB/s
* The number of devices includes the host adapter, so you'll have 7 or 15 devices left ;)
** This is the maximum through-put for the whole chain, not per device.
Fast: Refers to bus speeds of 10 MHz, Ultra: Refers to bus speeds of 20 MHz, Ultra2: Refers to bus speeds of 40 MHz
Wide: Refers to the bus width (16-bits). As you can see in the table below 'Wide' always supports 16 devices. (Since there is no 'narrow' version of Ultra3 SCSI the word Wide is omitted)

LVD: Low Voltage Differential Signaling is a technology first introduced in the SCSI-3 SPI-2 (Ultra2) specification to overcome cable length limitations when the bus speed was increased to 40 MHz and to allow for higher transfer speeds.

 
Current SCSI related exam objectives for the 2002 A+ Core exam.
1.6 Identify proper procedures for installing and configuring SCSI devices.
Content may include the following:
- Address/Termination conflicts
- Cabling
- Types (example: regular, wide, ultra-wide)
- Internal versus external
- Expansion slots, EISA, ISA, PCI
- Jumper block settings (binary equivalents)

4.3 Identify the most popular type of motherboards, their components, and their architecture (bus structures and power supplies).
- SCSI (Wide, Fast, Ultra, LVD(Low Voltage Differential))

Click here for the complete list of exam objectives.

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