What is SCSI?

The SCSI interface occupies an unattainable position in the world when it comes to the peripherals of computers, especially the interfaces of storage devices. This not only represents its advanced technology and performance, it also implies that it has a high price.
The meaning of SCSI is the small computer system interface (Small Computer system Interface), which was first developed in the late 1970s. After a continuous development, today's SCSI has been divided into SCSI-1 and SCSI-2, as well as the latest SCSI-3 three types. However, the most popular version is still SCSI-2. Apple Computer is the first to support the use of this interface platform, so Apple's performance, application is undoubtedly the best in the industry model.
A brief review of the type of SCSI
1.scsi-1
SCSI-1 is the most original version, the frequency of asynchronous transmission is 3mb/s, the frequency of synchronous transmission is 5mb/s. Although it is now almost obsolete, it will be used in some scanners and internal zip drives with a 25-pin interface. That is, if you connect the SCSI-1 device to your SCSI card, you must have an internal 25 50-pin interface cable, and if you use an external device, you cannot use any of the internal interfaces (that is, the internal interface is not available at this time).
2.scsi-2
The early SCSI-2, known as FASTSCSI, increases the frequency of the synchronous transmission from the original 5mb/s to 10MB/S, supports 8-bit parallel data transmission, and can connect 7 peripherals. Later, the WIDESCSI support 16-bit parallel data transmission, the data transmission rate is also increased to 20mb/s, can connect 16 peripherals. This version of SCSI uses a 50-pin interface, primarily for scanners, CD-ROM drives, and older hard drives.
3.scsi-3
1995, the birth of a more high-speed SCSI-3, known as ULTRASCSI, data transmission rate has reached the 20mb/s. It increases the frequency of synchronous transmission clock to 20mb/s and improves the technology of data transmission rate. If the wide mode of 16-bit transmission is used, the data transfer rate can be increased to 40mb/s. This version of SCSI uses a 68-pin interface that is primarily applied to the hard disk. The typical characteristic of SCSI-3 is that the bus frequency is greatly improved and the interference of signal is reduced, so as to enhance its stability.
Second, SCSI and IDE differences
In addition to Scsi,ide is also a very common interface. From a simple point of view, the IDE is more suitable for ordinary users, plus PC users not only need to configure a few peripherals, and speed requirements are not high, so the IDE interface is more appropriate to use. In addition, the IDE also has a high performance price ratio, the application of a wide range of features. While the SCSI interface has many unmatched features, the interface and its peripherals that use it are priced too expensive to be affordable by the average user, which determines the limits of its actual use.
1.IDE working mode requires the full participation of the CPU, CPU read and write data can no longer do other operations, this situation in the WINDOWS95/NT operating system, natural will cause the system to slow down greatly. The SCSI interface, then completely through the independent high-speed SCSI card to control the data read and write operation, the CPU does not waste time to wait, obviously can improve the overall performance of the system. However, the IDE interface now improves the problem by using DMA mode instead of PIO mode to read and write, and the data exchange is handled by the DMA channel, and the CPU usage can be greatly reduced. Still, comparing SCSI and IDE usage on the CPU, you can still find that SCSI has a considerable advantage.
The 2.SCSI is more scalable than the IDE, with 2 IDE channels per IDE, a total of 4 IDE devices, and a SCSI interface that connects 7-15 devices, much more than the IDE, and a cable that is far longer than the IDE.
3. Although SCSI devices are more expensive, SCSI performance is more stable, durable, and more reliable than the IDE.
Third, the advantages of SCSI
1.SCSI can support multiple devices, SCSI-2 (FASTSCSI) can receive up to 7 SCSI devices, WideSCSI-2 above can connect to 16 SCSI devices. That is, all devices need only one IRQ, while SCSI also supports a wide range of devices such as CD-ROM, DVD, CDR, hard drive, tape drive, scanner, and so on.
2.SCSI also allows the data to be found by another device while it is being transmitted to one device. This allows for higher performance in multitasking operating systems such as Linux and WindowsNT.
3.SCSI occupies a very low CPU, and it does have a significant advantage in multitasking systems. Because the SCSI card itself with the CPU, can handle all SCSI device transactions, the host CPU as long as the SCSI card to issue the work instructions, the SCSI card will work, return to the work after work to the CPU, throughout the process, the CPU can carry out its own work.
The 4.SCSI device is also intelligent, and the SCSI card can queue the CPU instruction itself, thus increasing the efficiency. In multitasking, the hard drive will be in the current head position, the adjacent task first completed, and then processed.
5. The fastest SCSI bus has 160mb/s bandwidth, which requires a 64-bit 66MHz PCI slot, so the maximum speed that can be achieved in a common PC is 80mb/s, which in theory means that the transmission rate of the hard drive can be as high as 80mb/s.
Four, the SCSI disadvantage
The advantages of SCSI are naturally attractive, but it is also hoped that the user will not overlook its shortcomings when deciding on the choice.
1. The conclusion obtained from the data is that, under the same conditions, the SCSI hard drive has a slower internal transfer speed than the IDE. Because SCSI hard drives are more complex than IDE hard drives, SCSI hard drives use a linear concept when identifying hard disk sectors, that is, the hard disk has only the 1th sector, the 2nd sector, unlike the IDE hard disk, which is the three-dimensional format of the cylinder, head, and sector. The current operating system also uses a linear numbered sector, but the BIOS accepts only three-dimensional disk requests, so the operating system must convert the disk request to a three-dimensional format so that the IDE hard disk can be used directly, but SCSI will have to convert three-dimensional disk requests to linear numbers for compatibility with the BIOS. This makes the data transfer rate of the hard disk greatly reduced. This we can see from the factory published data, such as Seagate7200 turn/minute "grizzly bear" internal transmission rate of 193.88mb/s,seagate7200 turn SCSI hard disk "cool fish" internal transmission rate is only 72mb/s. So SCSI hard drives only in the UNIX, WindowsNT and other tasks can really play the advantage, in the WIN95 environment is also good, but in DOS there is no advantage at all (although DOS is now rarely used). Today, Apple's G4, imac and other models are starting to support IDE interfaces.
2. Finally, it is important to emphasize that the SCSI performance price is not high. Things, SCSI a little expensive, a most popular type of ADAPTECAHA-2940SCSI card is enough to change back to a piece of BX host board, the same speed, brand CD-ROM,SCSI than the IDE more than one time, a 4.5gb7200 turn/cent "big grizzly bear" price, Just enough to change a 2gb5400 SCSI hard drive. Before you choose SCSI, consider your own financial strength before considering whether the price is worth it.
V, Terminator
The last SCSI device in the SCSI chain uses a terminator, and the intermediary device does not need a terminator. Once the intermediary device is using a terminator, the SCSI card will not be able to find a later SCSI device. SCSI also does not work if the last device does not use a terminator. The Terminator is made up of resistors, located at the end of the SCSI bus, to reduce the interaction between the signals and maintain a constant voltage on the SCSI chain.
Most SCSI devices are built-in terminators and use a jumper to control on/off. SCSI devices are now highly intelligent and can automatically control Terminator On/off, such as a hard drive and a CD-ROM, regardless of whether the hard disk's terminator on or Off,cd-rom is working. When two hard drives are connected, the situation becomes complicated, and before the two Seagate hard drives are connected, a hard disk terminator must be off, and a hard disk terminator can be used either on or off before a Seagate hard disk is connected to a quantum hard drive.
Six, SCSI ID
Each SCSI device in the system must have its own unique ID (identification number), which is from 1~15. The SCSIAdapter system default ID is 7. This ID can be set by the jumper at the front of the device. For hard disks It is located on the front or back end of the drive.
Vii. Selection of SCSI devices
1.SCSI Card
SCSI cards are key to the entire SCSI device chain, and there are several factors to consider when choosing:
(1) The brand of the SCSI card
Choosing a brand is important, Adaptec is a SCSI card boutique, like CPU Intel, is already the SCSI standard, of course, compatibility is no problem, and expensive price may be the only problem. In other words, if you are financially acceptable, it is recommended that you choose a Adaptec SCSI card.
(2) Bus interface
PCI SCSI card is selected, the ISA SCSI card is now basically no one to buy, it should be noted that some scanners with the ISA SCSI card is not a common SCSI card, although it has a standard SCSI connector, but also can only connect to the scanner. Second, to try to avoid the purchase of SCSI and motherboard integration Board, mainly to take into account the compatibility of the issue, particularly noteworthy.
(3) Types of SCSI
The commonly used 8-bit data channel has a SCSI-2 data transfer rate of 10Mbps, which is the most common type of SCSI. If the overall consideration, the individual or choose 8-bit data channel SCSI-2 more appropriate. On the one hand, only the hard disk supports the 16-bit data channel WIDESCSI, and the 16-bit data channel WIDESCSI equipment price is higher than 8-bit SCSI-2 many; on the other hand, 16-bit data channel WIDESCSI card is generally built-in 8-bit, 16-bit interface each one, But external only a 16-bit SCSI 68-hole D-type interface, and general external SCSI devices such as: CD-ROM, cdr/w, scanners are 8-bit SCSI devices, only 50 holes D-type interface, if you want to receive 16-bit SCSI 68 hole D-type interface requires a 68-50-hole converter, This converter is not only difficult to purchase, but also requires further testing of compatibility.
(4) Price of SCSI
The last thing to be particularly mentioned is the price, the different speed determines the different price. In order to select the appropriate card according to the actual needs, the general 10MB card dc310/390 is more than 100 yuan, 20MB dc390u/dc395u/dc315 is between 200-500 yuan, 40MB Dc390f/dc395uw can meet the high speed application, The price will be in the thousand dollars or more; for the top application when it belongs to the DC390U2 series, two models 390u2w and 390U2B are 80MB, the price is about 2900/2200 yuan, the difference is u2w more than one lsi53c141 chip, and more interfaces than U2B, On the card u2w has 2 68 needles, a 50-pin interface, suitable for a variety of equipment or need to continue to expand the occasion, and U2B only a 68-pin, the price of natural cheap many, high performance price, suitable for the need for further expansion of the independent application of the occasion. So you can pick out the affordable SCSI card.
The following is the model and type of a partial SCSI card for the Adaptec company:
AHA-2910PCI-TO-FASTSCSI (non-bootable)
Aha-2940pci-to-fastscsi
Aha-2940wpci-to-fastandwidesingle-endedscsi
Aha-2944wpci-to-fastandwidedifferentialscsi
Aha-2940ultrapci-to-wideultrascsi
Aha-3940multichannelscsi-to-pci
Aha-3940wmultichannelwidescsi-to-pci
2.SCSI Hard Drive
SCSI hard drive is next to the SCSI card key equipment, the major hard disk manufacturers have SCSI hard drive production, and all have 7200 RPM products, IBM and Seagate already have 10,000 rpm (the recent Seagate has increased the number of RPM to 15,000 rpm). IBM's products are mostly found in high-end markets, and the main brands in the retail market are Seagate and quantum.
The following is a comparison of the performance characteristics of several SCSI hard drives for reference:
Ibm:ultra star18es (18.2GBU2W-SCSI)
Quantum:atlas IV (18.2GBULTRA160/MSCSI)
SEAGATE:BARRACUDA18LP (18.2GBU2W-SCSI)
Western:digital Enterprise (18.3GBU2W-SCSI)
Western:digital Vantage (18.3GBU2W-SCSI)
Manufacturer IBM Quantum Seagate Western Western
Model DNES-31835 0qm31820 0kn-lwst318275 lwwde18300-0048 a2wd183f
RPM 7200RPM 7200RPM 7200RPM 7200RPM 10,000rpm
Single disc capacity 3.6GB 4.6GB 3.6GB 3.0GB 4.6GB
Seek time 7.0ms 6.9ms 6.9ms 6.9ms 6.6ms
Cache size 2MB 2MB 2MB 2MB 2MB
Seagate main SCSI hard drive products are:
(1) Medalistpro Gold 5400 turn/split pc
(2) Hawk Eagle 5400 rpm/min server with
(3) Barracuda Cool Fish 7200 turn/minute medium, high-end server with
(4) Cheetah Jaguar 10000 RPM/min high-end server, workstation
(5) Elite Elite 5400 RPM/minute mass storage, server with
Obviously, the brands have different models for different purposes, and when choosing SCSI hard drives, pay special attention to the SCSI interface type. Note that the 8-bit SCSI50 pin connector and the 16-bit SCSI68 hole D interface on your hard disk are not universal, and the 8-bit SCSI50 pin connector's hard drive can be used on 16-bit SCSI cards, and vice versa. The SCSI interface type can be seen from the model of the hard disk, taking the SEAGATESCSI hard drive as an example, the end of the hard disk model N, representing the 8-bit SCSI interface, the end number W, representing the 16-bit SCSI connector. SCSI hard drives of the same model, the 16-bit SCSI interface is of course more expensive than the 8-bit SCSI connector.

Disk array mode.