Integrated Drive Electronics (IDE)

The defacto consumer standard for drive interfaces. It's beaten out by SCSI in almost every way, but it wins because of the price.

Today's IDE interface has two channels which allow for two devices each, whether they be Hard Disks, CD-ROM's, or other storage drives.  Transfer speeds automatically drop to the speed and capabilities of the slowest drive on a channel for compatibility reasons.

The original form of IDE is self named.  It only allows hard disks on the channel, while offering a measly 2-3 MB/s of average transfer rate.  Most IDE boards have only one channel, allowing only two drives (CD-ROM drives of the time used a floppy drive like interface based off of a Sound Card).

EIDE

A substantia improvement of IDE in order to keep SCSI from the mainstream. It provides improvements to drive throughput, capacity, as well as integrating dual channels for up to 4 devices combined. Non-HD support was also added by the first AT Attachment Packet Interface Mode (ATAPI) which added support for devices like CD-ROM and tape drives.

The throughput problem was solved basically by moving the IDE interface from the ISA to PCI/VLB bus. It also adds support for Direct Memory Access (DMA) mode, where the hard disk can transfer to RAM directly without the CPU being involved.  With the PCI bus EIDE allows throughput of 6.66 MB/s, 8.33 MB/s, 13 MB/s, and 16 MB/s.

Ultra DMA (AKA DMA-33, Ultra ATA-33, Fast ATA-2)

The current step in the evolution of IDE.   Ultra DMA doubles the burst transfer rate to 33.3 MB/s, while integrating Cyclical Redundancy Check (CRC) support.  In order for this mode to operate however, it requires the drive, BIOS, Motherboard Chipset and software drivers to support it.  In DOS, it simply reverts to EIDE. There is also an 18 inch cable limit.

Ultra DMA-66 (Ultra ATA-66)

Is the next step in IDE evolution, invented by Quantum Corporation. The maximum theoretical transfer rate rises to 66.6 MB/s. Again, revised BIOS, chipsets, drivers, and DMA-66 hard drives are needed to support this new mode.  It's future viability seems up in the air right now, because the performance gains do not seem to be as extreme as the theoretical throughput assumes.

Small Computer System Interface (SCSI)

SCSI (pronounced 'Scuzzy') is the do-everything high speed bus interface.  It provides support for literally dozens of devices simultaneously along with high speed transfer rates, multithreading, parity checking, and bus mastering.  For the cost of an expansion slot and SCSI hard disk, CPU utilization can be dramatically reduced, especially in Windows NT.

The key to allowing so many devices is termination - the host adapter (beginning of the chain) and last device (end of the chain) must be terminated in order to keep the connection intact.  The general difficulty involved in properly terminating devices (as well as configuring) has kept SCSI as a workstation/server solution.  For most consumers, IDE provides a much cheaper, easier to maintain solution.

SCSI-1

The original SCSI specification. SCSI-1 is rarely used, if at all now, because of the low transfer rates, bus width, and terrible maximum cable length support.  At the time however, it was enough to break through the powerhouse 'Enhanced Small Device Architecture' (ESDI) spec.

SCSI-2

The current 'bottom level' of the SCSI specification - its generally used for Scanners and CD-R drives. The new spec added support for Tag-Queuing, which allowed instruction use regardless of whether data was currently travelling through the bus.  The instructions could also be prioritized for out of order execution.

Fast SCSI-2 allowed for a doubled transfer rate of 10 MB/s through a 50-pin connector, but was meant for differential SCSI in order to combat noise on the bus.

SCSI-3

The current standard is actually a family of different commands:

Fast/Wide SCSI: The Bus width, transfer rate (with differential support), and device support doubled over SCSI-2.

Ultra SCSI: With a doubled clock speed over SCSI-2 and backwards compatibility, it allowed for double the transfer rate over the stale 8bit wide bus. This was the basis for further Ultra SCSI improvements.

Ultra Wide SCSI: Simply put, this is the combination of Ultra SCSI with the 16-bit data path of Fast/Wide.

Ultra 2 SCSI: The most currently used high speed SCSI technology. It is the first to implement LVD signals for less noise and higher transfer rates - as high as 80 MB/s!

Ultra 3 SCSI: Just recently ratified by the SCSI Trade Association. It doubles the transfer rate yet again to an astounding 160 MB/s while adding advanced CRC support and easy hot-plugging technology (Installing devices without reboot).

According to Quantum the Ultra 3 SCSI interface is basically the same improvement thats involved in DDR SDRAM.

Transceivers

A Device which determines how the data will travel between adapter and drive. There are currently 3 levels: Single Ended (SE), High Voltage Differential (HVD), and recently Low Voltage Differential (LVD).  The most popular is also the cheapest - Single Ended. SE signal transmission requires that the current for all devices comes from the same source, which reduces possible speed with "line noise".

HVD provides improved termination abilities, less "noise", and more cable length to work with, but at the cost of increased power usage and a lack of available part types (like CD-ROM).  The power levels also required higher cost parts to cope with the temperatures involved, as most of the voltage is supplied through the cable, rather than from a separate power connector.

The LVD transceiver is the latest electrical signal based technology, using both the powerful differential technology and the low power consumption of SE.  The Ultra-2 specification also allows a bus speed of 80 MB/s and the ability to support SE devices simultaneously (at the cost of speed).  Most companies get around this by bundling multiple buses to handle SE and LVD separately.

Incidentally, there is a 4th standard - but it's not as clear cut as the other technologies.  The Fiber Channel and Signaling Interface (FC-PH) uses fiber-optics to transfer signals (electrical signals are still supported with hybrid cables).  FC-PH offers high bandwidth with almost no problems in signal colliding - heavy duty performance for the network crowd. The performance does come at substantial cost though, and isn't really suited for the consumer crowd.

SCAM Technology

Stands for SCSI Configured automatically.   When SCAM compliant devices are attached, software can automatically allocate IDs for each device.

Redundant Array of Independent Disks (RAID)

A subset of SCSI/IDE technology that allows the combining of two or more hard disks in various fashions to provide redundancy, and additional speed.

Almost all of the levels work off the theory of 'striping', where blocks of data are written across drives. Typical hard disks must read/write data concurrently - one after another to the same disk.  RAID avoids this by writing concurrently to separate disks - in a 4 disk array that would allow 4 blocks to be written/read at once!

There are several levels of RAID, each with their own number. Those numbers are merely for identification - RAID 4 is not necessarily better than RAID 1, or vice versa.

0 - Striped Disk Array without Fault Tolerance - Breaks up files into blocks that run across drives, the hard disks are combined to act like one. Because information is written and read alternately to each drive speed is increased. There is no fault-tolerance however, so if a drive were to fail, all data on the array would be lost.

1 - Mirroring and Duplexing The RAID controller essentially writes to two or more disks simultaneously. Each drive contains the same information at all times, which provides you with a backup in case a drive fails.

2 - Hamming Code ECC - Data is striped across an array of disks by bits rather than blocks.  For each word (2 bytes) written, a connected array with an equal number of disks simultaneously writes a "Hamming Code ECC word".  This provides absolute fault tolerance with on-the-fly correction, but at the cost of a large amount of drives.

3 - Parallel transfer with parity - The Data Blocks are striped across disks, except that a separate drive is used to hold checksums (parity information).

4 - Independent Data disks with shared Parity disk - Entire Data Blocks are striped across disks, except that a separate drive is used to hold checksums (parity information).

5 - Independent Data disks with distributed parity blocks - Much like RAID 0 - except that parity is added to protect the data blocks written to the drives.  The parity is written across with the data blocks, unlike RAID 4.

6 - Independent Data disks with two independent distributed parity schemes - This is basically an extension of RAID 5. Data Blocks + Parity Blocks are written across the drives, but there is also a second set of parity blocks to cross check the disks for errors.  A rarely used solution.

7 - Optimized Asynchrony - A proprietary RAID setup involving the use of a hard coded real time operating system.

10 - Striping + Mirroring - Two or more sets of paired RAID 1 arrays are combined using RAID 0 to provide a single array that is redundant against failure.  This means that if even the entire set of data fails, there is a complete backup.

53 - Striped Array of Parallel Disks with Parity - RAID 0 is used to stripe data across RAID 3 arrays, which means that the Parity Drives are also striped.

IEEE 1394 - FireWire

The next 'consumer-level' bus that is bound for motherboard integration and replacement of IDE. IEEE 1394 is a serial bus that promises to offer transfer rates of up to 50 MB/s with guaranteed or asynchronous transfers. It also supports up to 16 devices per channel, hot-swapping and automatic termination/ID assignment.

IEEE 1394 is geared to support all media drives, digital cameras/video cameras, and  laser printers. It is currently available as a PCI card for digital video camera users, but is not expected to go mainstream for another year or two so the technology can mature. Although it offers small footprint ports (sort of like stunted serial ports) and fairly stable transfers the needed silicon needs to be shrunk further, and another internal channel needs to be added (or so it looks) before Intel plans on integrating it into their core logic chipsets.

Why not just call it 'FireWire'? The term is actually exclusive to Apple and their PowerPC enabled FireWire computers. The name is great slang, but for legal reasons don't expect to see the name on store shelves.

Links to more information

SCSI Trade Organization
http://www.scsita.org/
A good site for SCSI resources and information.

SCSI T10 Committee
http://www.symbios.com/t10/
The group responsible for setting all SCSI standards.

Quantum
www.quantum.com
Hard Disk manufacturer - site also contains good information on Ultra 3 SCSI.

Maxtor
www.maxtor.com
Hard Disk Manufacturer

Seagate
www.seagate.com
Hard Disk Manufacturer

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