Hard Drives And Other Storage Media
Storage media is separated into 2 different forms of equipment; We will discuss the basics of interfaces and continue with storage media functionality.
Storage Media
Any device that stores information in a permanent fashion on a computer is called a storage media. Hard disks, Floppy disks, and CD-ROMs are all common to us, but LS-120, RAID, Tape drives, and optical drives provide other forms of storage media. Without any forms of storage media, our systems of computers could not function.
Interfaces
Interfaces for hard drives are often called drive controllers. This interface can either be an adaptor card or can be integrated into the motherboard itself. The drive controller is used to connect a hard drive to a motherboard, and exists either on the motherboard/adaptor card or the hard drive itself. There are several standard forms of drive controllers, including ST506/ESDI, IDE, and SCSI.
| Drive Controller | Description |
| ST506/ESDI | These were the original drive controllers. They were big, required complex cabling, and required several steps in order to format them. You probably won't see a question regarding these drives, and you probably will never see one of these drives. (I have... They aren't fun) Just make sure you know that they are drive controllers, and were the original systems used. The controller in this case was an adaptor board. |
| IDE | Integrated
Drive Electronics - This is the most popular format of
hard drive controller. It was created as a
replacement for it's more expensive predecessor,
SCSI. IDE places the controller on the hard drive,
making the interface to the motherboard a simple
pass-through. IDE is also referred to as ATA.
(Advanced Technology Attachment)
IDE can support two drives of up to 528MB per drive. E-IDE (Enhanced IDE) can support up to four drives up to 4GB per drive. |
| SCSI | Small Computer Systems Interface - SCSI (Sounds like scuzzy) Is like IDE in that the controller is built into the hard drive itself, but part of the controller is either built into an adaptor card or built into the motherboard. SCSI has many different formats, which we will discuss later. The important information to remember now is that you can connect up to 8 devices to a single SCSI controller. A group of SCSI equipment linked together is called a SCSI chain or SCSI bus. |
Several terms are important for remembering IDE and SCSI terminology;
| Description | ||||||||||||||||||||||||||||||
| IDE | POI Mode | Programmed I/O Mode - The more conventional, standard protocol used for transferring data over the IDE/ATA interface. There are five different rates, each one defining a different maximum transfer rate; these are called PIO modes. This table shows the different modes, along with which IDE/ATA standard defines support for them (meaning not which standard initially introduced support for the mode, but more "if a drive supports this standard, it should support this corresponding mode): | ||||||||||||||||||||||||||||
| Mode 0 -
3.33Mbps
All ATA Modes |
Mode 1 -
5.2Mbps
All ATA Modes |
Mode 2 -
8.3Mbps
All ATA Modes |
Mode 3 -
11.11Mbps
ATA-2, Fast ATA, Fast ATA-2, ATA-3, ATAPI, Ultra ATA, EIDE |
Mode 4 -
16.67 Mbps
ATA-2, Fast ATA, Fast ATA-2, ATA-3, ATAPI, Ultra ATA, EIDE | ||||||||||||||||||||||||||
| ATA 1, 2, 3 | ATA
is the standard specification for performance in hard
drive interfaces.
The original, base specification for the family of interfaces described in this section is called both IDE and ATA. The name IDE is the more popular of the two, even though it is misleading since it stands for integrated drive electronics which refers to putting the logic board with the hard disk much more than it has anything to do with the interface. The more correct name is ATA or AT Attachment, which defines the standard interface as attached to an AT-style machine (which is the generic way of describing every PC since the old 8-bit XT). The IDE/ATA interface is 16 bits wide, just as the original IBM AT's data and I/O buses were. This size persists to this day, even in the higher-performance enhancements defined in ATA-2. The ATA specification has been defined as an ANSI standard. A major reason for defining ATA as a formal standard was to eliminate some of the incompatibility problems that plagued early IDE/ATA drives. This particularly showed up when drives made by different manufacturers were placed as master and slave on the same IDE/ATA channel--often they would not work. ATA 2 and 3 allowed for improvements in the original ATA protocol. | |||||||||||||||||||||||||||||
| Ultra ATA/ Ultra DMA | The
newest ATA-related standard is Ultra ATA, which also
goes by several other names including Ultra DMA, ATA-33,
and DMA-33. It is also sometimes called ATA-3, which it
is not. Ultra ATA is not a formal standard but rather a
term that refers to the use of the higher-speed DMA-33
transfer mode (multiword DMA mode 3), running at 33.3
MB/s. Special error detection and correction logic (CRC)
is used to support the use of this high-speed mode over
a standard IDE/ATA Ribbon Cable (which has not changed
since transfer rates were below 5 MB/s and can now be a
problem in terms of corruption when used at very high
speeds). Ultra ATA maintains backward compatibility with
the older standards upon which it is based (ATA-2 and
ATA). If you use an Ultra ATA hard disk in a system that
does not support Ultra ATA, it will still work, just at
the slower speeds that system can handle.
Drives that support Ultra ATA allow the use of the high-speed DMA-33 transfer mode, but are otherwise the same as other ATA-2/EIDE drives. Ultra ATA requires both a hard disk and a system BIOS/chipset that supports the Ultra ATA protocol. | |||||||||||||||||||||||||||||
| ATAPI | Originally, the IDE/ATA interface was designed to
work only with hard disks. CD-ROMs and tape drives used
either proprietary interfaces (often implemented on
sound cards) or the floppy disk interface (which is slow
and cumbersome). A few years ago it became apparent that
there would be enormous advantages to using the standard
IDE/ATA interface to support devices other than hard
disks, due to its high performance, relative simplicity,
and universality.
However, because of how the ATA command structure works, it wasn't possible to simply put non-hard-disk devices on the IDE channel and expect it to work. Therefore, a special protocol was developed called the ATA Packet Interface or ATAPI. The ATAPI standard is used for devices like CD-ROM and tape drives. It enables them to plug into the standard IDE cable used by IDE/ATA hard disks, and be configured as master or slave, etc. just like a hard disk would be. When you see a CD-ROM or other non-hard-disk peripheral advertised as being an "IDE device" or working with IDE, it is really using the ATAPI protocol. Internally, however, the ATAPI protocol is not identical to the standard ATA (ATA-2, etc.) command set used by hard disks. A special ATAPI driver is used to communicate with these devices. This driver must be loaded into memory before the device can be accessed (some operating systems like Windows 95 support ATAPI internally and in essence, load their own drivers for the interface). The name "packet interface" comes from the fact that commands to ATAPI devices are sent in groups called packets; ATAPI in general is a much more complex interface than regular ATA. The actual transfers over the channel use regular PIO or DMA modes, just like hard disks, although support for the various modes differs much more widely by device than it does for hard disks. For the most part, ATAPI devices will coexist with IDE/ATA devices and from the user's perspective, they behave as if they are regular IDE/ATA hard disks on the channel. Newer BIOSes will even allow booting from ATAPI CD-ROM drives. | |||||||||||||||||||||||||||||
| DMA | Direct memory access or DMA is the
generic term used to refer to a transfer protocol where
a peripheral device transfers information directly to or
from memory, without the system processor being required
to perform the transaction. DMA has been used on the PC
for years, traditionally over the ISA bus for devices
like sound cards, using DMA channels which are a
standard system resource. The floppy disk interface also
uses conventional DMA.
Most newer hard disks support the use of DMA modes over the IDE/ATA interface, which means that the built-in controller on the hard disk can do transfers to memory without the system processor's involvement. This is in direct contrast to the more conventional use of PIO modes, which require the CPU to do the work. However, this is a different type of DMA than that used on the ISA bus. The standard ISA bus DMA channels are an old design dating back to the early days of the PC. They use the DMA controller built into the system chipset, to perform third-party DMA transfers, where the "third party" is the controller, working with the memory and the device. This sort of DMA is not used by modern hard disks for performance reasons. Modern IDE/ATA hard disks use "first party" DMA transfers, where the peripheral device itself does the work of transferring data to and from memory. This is also called bus mastering. Bus mastering allows the hard disk and memory to work without relying on the old DMA controller built into the system, or needing any support from the CPU. It requires the use of the PCI bus (older buses like MCA also supported bus mastering but are no longer in common use). There are several different DMA modes that are used to define transfer rates when DMA is used for transfers over the IDE/ATA interface:
| |||||||||||||||||||||||||||||
| SCSI | SCSI 1 | SCSI 1 is the first standard for SCSI devices. It could transfer up to 5MBps, and connected with either a Centronics 50-pin or a DB-25 connector, and had an 8 bit bus. | ||||||||||||||||||||||||||||
| SCSI 2 | It could transfer between 10 and 20 MBps, and had a 16 bit bus. | |||||||||||||||||||||||||||||
| SCSI 3 | It could transfer between 40 and 80MBps, and had a 16 bit bus. | |||||||||||||||||||||||||||||
| Fast | Fast SCSI refers to the use of a 10 MHz SCSI bus speed on an 8-bit bus. It is also sometimes called Fast SCSI-2. The maximum transfer rate is 10 MB/s. | |||||||||||||||||||||||||||||
| Ultra | Ultra SCSI devices use a 20 MHz SCSI bus speed on an 8-bit bus, for a maximum transfer rate of 20 MB/s. Ultra SCSI is sometimes called Fast-20 SCSI or Ultra SCSI-3, in reference to the interface's speed and the standard that defines it, respectively. | |||||||||||||||||||||||||||||
| Wide | Wide SCSI refers to devices that use a 5 MHz SCSI bus speed on a 16-bit wide bus. It is also sometimes called Wide SCSI-2. The maximum transfer rate is 10 MB/s, and up to 16 devices are supported per bus. | |||||||||||||||||||||||||||||
| Ultra Wide | The highest-performance standard SCSI (using parallel data wires, as opposed to Serial SCSI), Ultra Wide SCSI devices combine a 20 MHz SCSI bus speed and a wide 16-bit bus, for a maximum transfer rate of up to 40 MB/s. Ultra Wide SCSI is sometimes called Fast-20 Wide SCSI or Ultra Wide SCSI-3, in reference to the interface's speed and the standard that defines it, respectively. Up to 16 devices are supported on the bus. | |||||||||||||||||||||||||||||
| Serial SCSI/Firewire | A new protocol that is comparable to Universal Serial Bus. (USB) Every other form of SCSI is parallel, and therefore transfers several bits at a time. Serial SCSI sends one bit at a time, but at much faster rates then parallel SCSI. Parallel signaling is much more difficult to manage then serial signaling, and therefore simpler cabling and interfaces allow for 6 wires to be used instead of 68 and speeds capable of over 1GHz bus speed can one day be attained. | |||||||||||||||||||||||||||||
How hard drives attach to your computer
IDE/ATA can only handle 2 devices per channel, a primary and a slave. E-IDE allows 4 devices to be connected by separating the devices into 2 channels, a primary and a secondary. E-IDE controllers have two cable channels, where original IDE only has 1. Any cable attached to the primary attachment is controlled by the primary channel, and any plugged into the secondary is controlled by the secondary channel.
In each channel, the devices must be identified as either a master or a slave. This is done by the use of jumpers on the back of the drive that identify the drive as either the master or the slave.
All IDE/ATA/E-IDE hard drives come with 40 pin interfaces and use 40 pin cabling. In order to use some of the higher Ultra-DMA functions, a special 80 pin cable can be required, but all drives are compatible with the 40 pin ribbon cable.
SCSI drives use different connectors as stated in the above table. SCSI 1 used Centronics or DB-25 cables. SCSI 2 used a 50 pin A cable and a 68 pin B cable. SCSI 3 uses a 68 pin P cable and a 68 pin Q cable. Finally, Firewire uses a specific 6 wire cable. B, P, and Q cables are not compatible with each other, so before hooking up a SCSI chain ensure you have the right cable for the job. Don't expect lots of questions on this, but it may help you later on in your studies.
SCSI chains MUST be terminated. If you don't terminate a SCSI bus, information can escape the bus or reflect back across the bus. Either way, a SCSI chain will fail without a terminator pack at either end of the SCSI bus.
Lastly, SCSI host controllers must be powered on before other computer elements. Without powering it on first, the BIOS will sometimes bypass this controller, or the SCSI card may not be able to identify all the SCSI components before the POST requires the information. Built-in SCSI controllers do not have this problem.
SCSI vs. IDE
Although SCSI seems to be the best solution, it is extremely expensive. Many new IDE technologies are making IDE almost as fast as some forms of SCSI at a much lower price. SCSI's main advantages come in it's chain-style connection and 8 drive capability. Also, SCSI RAID is more functional than IDE RAID. Remember to weigh the costs of using SCSI before assuming it is the best route to choose.
©2002 COMPUTER INFO All rights reserved.
