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HDD 5: Interface

Interface

• Interface - Group of electric and physical standards defining the way of connection and transmission protocols for the disk and other devices. Basic purpose of this interface is to connect the disk to the computer and to provide fast data access to the remaining components (processor, memory or graphic card)

• We can divide the connection into external and internal. The connection itself is formed of cables and a host adapter or a controller.

• Controller - SCSI controllers are the best known; EIDE controllers are host adapters, in fact. The classical EIDE controller you will only find in older computers. It also can be said that SCSI is rather the host adapter for the connection of disks and devices.  

• Host adapter - an additional card or connector coming out of the motherboard designed for the connection of disks or other host devices to the computer bus. Each host device has its own integrated controller and then, the host adapter does not work as controller, but it just provides the connection. However, most host adapters are called controllers, despite this is not completely accurate.

• Some controllers can be further specified according to the used bus, e. g. the old controllers were connected through ISA, MCA and VLBUS buses; the newer use the PCI bus. The reason is simple - speed and data throughput.

• Besides the known interfaces mentioned in this text, there also exist older types, e. g. ST506, ESDI, which you can meet only rarely. 

• ST-506/ST-412  - One of the older interfaces defining the characteristics of connection in older disks and controllers. Most electronic parts and devices are left to the controller (the controller is not integrated in the disk). The disadvantage is the appearing interference, lower speed and some other disadvantages removed by integrating the controller directly into the disk. 

• ESDI - another one of the older standards that you cannot meet today at all (used before 1990).

• One of the most important characteristics of the interface is the throughput and the data transfer speed. If you buy a fast disk and put it into an old computer, it is possible that the performance will be used only in part. So it is, when you neglect some settings in the operating system or when you use a wrong cable for the connection of disks. 

• In older computers, the problems with the controller were solved by using a stand-alone card. There existed several types from stand-alone controllers for IDE and FDD connection to multi-function cards bringing connection of serial ports, parallel port and also joystick, in some cases. (see the photo).

ATA ( Advanced Technology Attachment) (IDE - Integrated Drive Electronics, EIDE - Enhanced IDE)  Parallel ATA

• article - IDE interface - connection (Master, Slave, CS), cabing, UATA100

Serial ATA

• Potential substitute for today's Ultra ATA - the biggest difference is in the thin round cable contributing to making the motherboards, the disks and the whole computer including price smaller. When building a computer, it is better to ensure good air flow and to reduce the temperature by help of decreasing the voltage to 3.3V. Instead of the parallel transfer there is the serial transfer used (the data do not go one besides the other, but one after the other). The essential strange feature should be the higher speed (150MB/s speed is planned in the first version with the view of 600 MB/s for the future). Probably, the necessity of configurating by help of jumpers will disappear. 

• You can read on some servers about already existing classical round cables, but these are usually articles made by some computer "fanatics" or these cables are classical cables but curved and sticked in a coat. These solutions are not recommended too much for their higher electromagnetic interference liability. 

• Besides the connection of disks, there is also the connection of other internal devices planned (e. g. ZIP drives). To connect the present ATA and ATAPI devices, the possibility of using reduction should be available.

• Dell - www.dell.com/us/en/gen/topics/vectors_2000-sata.htm

• Serial ATA Working Group - www.serialata.org

SCSI

• Fast and quality interface designed for high performance workstations and servers.  

• article - SCSI interface

Fibre Channel (IEEE 1394)

• New interface designed for external devices, high speed up to 400Mbit/s (50MB/s), possible successor of the USB bus. This interface is planned for the future to reach a  higher speed. (100, 200 and 400 MB/s).

• Automatic configuration

• Plug and Play support, hotswap.

• Cable's length up to 4,5m and connection of up to 63 devices

PCMCIA

• Interface for small disks or memory devices designed for portable computers or miniature electronic devices. 

RAID

• This is no interface but rather a way of connection of several disks through SCSI or EIDE.

• RAID - Redundant Array of Independent Disk - Different connection of several disks for the optimal performance or protection from the data loss. There exist cheap RAID solution on the more recent boards in the form of additional RAID IDE controllers. Originally, the possibility of using HW RAID comes from powerful SCSI controllers. Besides the HW solutions, there also exist some less powerful ones, but still quality SW solutions, e. g. under Windows 2000 Server you can create RAID 0, 1, and 5.

• RAID 0

• RAID 1

• RAID 0+1 - Using of RAID0 and RAID1 fields - min. 4 disks are necessary.

• RAID 5

• RAID 3, 4, 6, 7 - another possible solutions provided by HW solutions.

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