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.

The chart below lists the types of cables used with SCSI

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:
• MPEG4, H.264, MJPEG Compression for DVR Recording - What's the Difference?
• Hard Disc Fundamentals
• A Definitive Guide on How to Choose Your New PC
• Hardware Resources Explained
• Understanding Modern PC Specifications
• Tips and Techniques For Building Your Own PC
• The RS-232 Serial Port
• Serial ATA Hard Drive
• Introduction to Operating Systems
• PC Sound Circuitry

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