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Devices are connected to the interface in a daisy chain and are identified by a jumper designating the drive as master or slave. The same result can be achieved by using cable select. The connection on the drive is 40 pin. No termination is required on this type of interface. IDE - Integrated Drive Electronics. First appeared in 1989. Cheap, simple integration and reasonable performance made these drives very popular. As the name suggests the much of the control circuitry for the drive is already built into the drive.
EIDE - Enhanced Integrated Drive Electronics Enhanced IDE was developed in response to the need for greater storage capacity in the personal computer industry. As the term implies faster transfer rates, larger capacity hard drives, LBA addressing and CD-ROM devices were to be supported by the standard. This is based on two standards - ATA-2 and ATAPI.
A comparison of the two standards is shown below:
These drives are mainly used in desktop systems running DOS, Windows V3.x, Windows 95, Windows NT, OS/2 etc. They have also been known to be used in servers, the total number of storage capacity limited by the number of drives permitted on the IDE port. Some drives are now what is termed as SMART (Self-Monitoring, Analysis and Reporting Technology) drives. With the use of SMART virtually any intelligent component within a computer can communicate its predicted reliability status. The host transfer rate specification for PIO and DMA modes are as follows:
SCSI - Small Computer Systems Interface. The SCSI bus is a parallel bus which comes in number of variants. The oldest and most used is an 8 bit wide bus with single-ended signals carried on 50 wires. Newer designs also use a 16 bit wide bus with differential signals. This allows transfer speeds of 20Mb on cable lengths of 25 metres. Narrow SCSI will support up to 7 devices. Wide will support up to 15 devices. AV (Audio/Visual) drives are geared specifically for professional multimedia applications. These high-end storage devices require to be able to sustain high data transfer rates in line with the demands of flicker free full motion video applications. The term "Hot Swap" refers to the common practice of either inserting, or removing SCSI disk drives in an operating bus, typically used in RAID subsystems or JBOD (just of a bunch of disks) environments. The ability to "Hot Swap" a disk drive is beneficial to customers. It allows them to remove potentially defective drives from the system, or upgrade capacity without having the inconvenience and expense of taking the entire system down to replace the drive. Four distinct levels of functionality are defined in Table A. The term "Hot Swap" is not actually defined in the ANSI standards. It is interpreted as "the very restrictive Level 4 Removal and Insertion of disk drives." The main difference between Level 4 and the easier levels is that the bus is allowed to operate. Since inserting a disk into any powered bus will result in some level of electrical transients, it is necessary to insure that those transients do not interfere with, or corrupt the control of data signals present on the bus.
*Achieved on Quantum products using SCA-2 connector SCSI data rates have increased over time, doubling about every five years.
SCSI: Data transfer rates of 5 Mb/sec (8 bit). Narrow Fast SCSI (SCSI-2): Data transfer rates of 10 Mb/sec (8 bit). Fast Narrow. Ultra SCSI: Data transfer rates of up-to 20Mb/sec on 8-bit bus. Bus length of 1.5 metres. Wide SCSI-2: Data transfer rates of 20 Mb/sec (16 bit). Wide. Bus length of 1.5 metres. Ultra Wide SCSI: Data transfer rates of up to 40 Mb/sec (16 bit). Ultra Wide. Bus length 0.75 metres. Ultra2 SCSI (LVD): One of the more recent developments is Ultra2 SCSI (LVD), which has a low-voltage differential (LVD) interface and transfer rates of up to 80 megabytes per second (MB/sec). Ultra2 SCSI (LVD) is well positioned for the future because it offers forward compatibility and significant room for growth to support faster microprocessors and drives. Other benefits include minimal SCSI software or protocol changes, with the support of 15 years of backward compatibility experience with SCSI. Currently, Ultra2 SCSI (LVD) is capable of achieving transfer rates of up to 80 MB/sec. Ultra2 SCSI (LVD) is low cost, easy to integrate, and an optimal solution for improved performance and increased device connectivity Single-ended: A single-ended SCSI bus carries signals at either 5 volts or 0 volts (TTL level) and are relative to a common ground. Maximum length of the bus is 6 metres however if the same bus is used for Fast SCSI the length is reduced to 3 metres. Differentiall: A differential bus has a maximum length of 25 metre. The idea with this system is that each signal wire has its own return wire, so each signal is carried on its own (twisted) pair of wires. It is the difference between the voltages on the two wires, which determines whether the signal is asserted or de-asserted. SCA (Single Connector Attachment): Through this type of attachment there is no requirement for the user to set-up the drive by using jumpers i.e. set up the drives id, termination etc. Terminators: These are used to ensure that the correct impedance of the signal lines. The reason for the use of terminators is to prevent signal reflection i.e. the signal echoing back along the cable and causing distortion. If a SCSI bus is not terminated correctly then data corruption may occur. SCSI devices may be connected internally, externally or a mixture of both. The devices are connected are connected together using a single cable (if internal) and daisy chained with each device being allocated a different address. The first and last device on the chain must be terminated. If the devices are a mixture of internal and external then the controller must have its termination disabled.
On a narrow SCSI internal device the connector is standard 50 pin and on a wide device it is a 68-pin connector. On external devices the connections refer to that which is presented to the user regardless of the style of connector on the device. The style is as follows:
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