The Small Computer System Interface (SCSI)
By Stephen Bucaro
With SCSI, you install a SCSI controller card (called the host adapter) in your
computer and then daisy-chain devices from the controller. Some motherboards have
an on-board SCSI controller.
Each device must have a unique ID number. The controller itself usually uses ID 7.
The first device has ID 0. Each device has a connector to plug in the next device in
the chain. A terminator, which is a plug containing a resistor, is plugged into the
last device in the chain.
SCSI is not normally used with a work station or home computer. It's used with a
server to implement a fault tolerance scheme called RAID (Redundant Array of Independent
Disks). RAID is fault tolerant because if one disk fails, the data from that disk can
be recreated from data stored on the other disks.
An Aside: I have seen texts that say RAID stands for all kinds of things. The most
laughable (and I won't mention the author) was Random Array of Inexpensive Disks.
There is nothing random about RAID, but the data is stored redundantly, and I
would like to know where they get those inexpensive disks. RAID is a fault
tolerance scheme, not a type of cheap disk. But the disks are independent. If one disk
fails you can replace it without losing data.
The first version of SCSI featured an 8-bit bus and a 5 MBps transfer rate. It
supported up to seven devices, eight including the host adapter. The chart below lists
newer versions of SCSI.
|Type||Bus bits ||Speed MBps ||Devices ||Max cable length meters|
The chart below lists the types of cables used with SCSI
|Connector ||Pins ||Use|
SCSI is complicated to configure and troubleshoot with incompatibility between devices
being a common problem. To setup a SCSI system, you need to know which SCSI standard the
devices support. The host adapter must match the devices.
Even the names for the interfaces are inconsistent; for example, Ultra SCSI was called
SCSI-3 but now we have Ultra3. We might have Fast and Fast-Wide, or Fast-Narrow and Fast-Wide.
On the other hand, SCSI allows you to set up a multi-disk RAID configuration, with the
SCSI host adapter using only a single DMA channel and IRQ line.
To ease setup and troubleshooting, most host adapters come with an interrogator
program. SCAM (SCSI Configuration Automatically) compliant host adapters and devices can
use Plug-and-Play to configure automatically.
System administrators often install both ATA and SCSI drives on a server. With this
configuration the system can boot from the master ATA drive. To boot from a SCSI drive, if
your system doesn't support booting from SCSI, the SCSI host adapter card must have an
on-board BIOS chip. You would then have to disable the ATA interface in the system's BIOS.
The best place to find more information about SCSI is at the website of the leading
SCSI controller company, Adaptec
More Computer Anatomy Articles:
• PC Motherboard Expansion Cards
• How Hard Disks Work
• Power Supply Form Factors
• Understanding Graphic Cards
• Anatomy of a Hard Drive
• Solid State Drive (SSD) Basics
• Basic PC Anatomy Inkjet Printers
• Advanced Micro Devices (AMD) Chipsets
• Understanding Your Motherboard
• Rambus DRAM (RDRAM)