All About SCSI |
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| Author:
| BiggyTek
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| Submitted: |
12-Sep-2002 17:18:37 |
| Imported From: |
zZine (original author: biggytek)
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| The Small Computer Systems Interface (SCSI) is a collection of interface standards that cover a wide range of peripheral devices, including hard drives, tape drives, CD-ROMs, and disk arrays (RAID). SCSI is pronounced skuzzy.
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SCSI is not as common in small office and home computers primarily because its components cost more and these computers don't need the flexibility and high-end performance of this interface. ATA IDE is by far the most common interface in those environments.
SCSI is actually not an interface. It is more like a system bus structure on which many SCSI devices can connect to a single SCSI controller by sharing a common interface, called the SCSI bus or SCSI chain. Each device connected to the SCSI bus is assigned a unique device number. These numbers are configured to the device with jumpers, DIP switches, or rotary dials located on the device. Most BIOS systems that support Plug and Play include a feature called SCSI Configured Automatically or SCAM that sets SCSI device IDs automatically by software. In order for this to work, the BIOS, the host adapter, and the peripheral device must all support the SCAM.
When the SCSI controller wants to communicate with one of the devices on the bus, it sends a message encoded with the unit's device number. Any reply to the SCSI controller includes the sender's number. Like IDE/EIDE devices, SCSI devices also have their controllers built in and are able to control their own data access and capture activities, as well as interpret requests from the computer that are passed to it from the SCSI controller.
SCSI devices are connected in what is called a daisy chain, which means that each device is connected in series with the next device on the bus. That is, of course, unless the device is the last device, in which case it uses and DIP switch setting or a resistor block to terminate the bus. Internal SCSI devices attach to a ribbon cable that can connect multiple devices. The ribbon cable is connected to a single port that provides service to all of the devices attached to the cable. The internal SCSI cable serves as the common bus media for all internal devices. External devices usually have two ports, one each for the incoming cable and another to connect to the next device in line or for the terminator, if it is the last device on the bus.
A History of SCSI Interfaces
1. SCSI-1: The original SCSI standard, developed in 1986, defined the basic specifications of the SCSI bus structure, including its commands, transfer modes, and cabling. SCSI-1 supported 8 devices on an 8-bit bus that supported up to 5MBps of data transfer. SCSI-1 was not universally accepted and devices from different manufacturers were not always compatible.
2. SCSI-2: The extensive advancements in SCSI-2 solved many of the problems of SCSI-1. SCSI-2 established the foundation of the SCSI bus on which all future enhancements have been built. SCSI-2, which is also called Fast-Wide SCSI, defines two separate protocols:
a. Fast SCSI: Features data transfer speeds of up to 10MBps over the SCSI-1 8-bit cabling.
b. Wide SCSI: Provides for 16-bit and 32-bit SCSI bus structures.
These two protocols can be used together to create a Fast and Wide SCSI bus. SCSI-2 also increased the number of devices that could be supported on the bus to SCSI-2 is also backward compatible with SCSI-1 devices, but the SCSI-1 devices can only operate at their original speed.
3. SCSI-3: Also known as Ultra SCSI, SCSI-3 defines data transfer speeds of up to 20MBps over an 8-bit bus or higher speeds over the Wide SCSI bus.
Serial SCSI (Firewire)
An alternative bus interface that is associated with SCSI-3 is Serial SCSI, better known as FireWire, which defines a high-speed serial bus structure that can connect up to 63 devices. FireWire is also called IEEE 1394, i.Link, and HPSB (High Performance Software Bus). It is capable of data transfer speeds of 100Mbps in its original specification and up to 3200 Mbps in the newer IEEE 1394b specification. FireWire supports hot swapping, multiple data speeds over the same bus, and isochronous data transfers (time dependant data transfers, such as telephony voice, and real-time audio transmissions_, which makes it an excellent bus for the transfer of multimedia content.
SCSI systems connect to the host adapter in a daisy-chain fashion. This means that the devices are connected in series one to the other. SCSI devices installed as internal devices use a 50-pin ribbon cable very much like the floppy disk cable without the twist. External SCSI devices may require one or more of a series of 50-pin and 68-pin cables, depending on the standard in use. The B-cable should not be confused with the SCSI-3 P-cable (68pin) that must be used with a Q-cable (68-pin).
All SCSI devices should be powered on before the PC to allow the SCSI host adapter (usually inside the system) to detect and interrogate each of the devices on the SCSI bus.
RAID
RAID stands for Redundant Array of Independent Disks. It's a storage technology that uses two or more hard drives in combination for high availability, fault tolerance (error recovery), and performance. RAID disk drives are used frequently on servers but generally aren't necessary for a personal computer.
One of the fundamental concepts of RAID drives is data striping. In this process, data files are subdivided and written to several disks. This technique allows the processor to read or write data faster than a single disk can supply or accept it. While the first data segment transfers from the first disk, the second disk is locating the next segment, and so on.
Another common feature of RAID systems is data mirroring. This feature involves writing duplicate data segments or files to more than one disk to guard against losing the data should a hard drive fail.
I hope you find this article informative. Please feel free to leave comments!
This article was originally published by CyberArmy.net in the CyberArmy Library.
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