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Small Computer System Interface (SCSI), a standalone processor standard for system-level interfaces between computers and smart devices (hard drives, floppy drives, optical drives, printers, scanners, etc.). SCSI is an intelligent universal interface standard.

 The SCSI interface is a universal interface that can be connected to a host adapter and eight        SCSI peripheral controllers on a SCSI bus. The peripherals can include disks, tapes, CD-ROMs, rewritable optical drives, printers, scanners, and communication devices. The

   SCSI is a multitasking interface with bus arbitration. A number of peripherals mounted on a SCSI bus can work at the same time. Devices on SCSI are equal to the bus.

   SCSI interface can be synchronized or asynchronous transmission of data, synchronous transmission rate can reach 10MB / s, asynchronous transfer rate can reach 1.5MB / s.

    When the SCSI interface is connected to an external device. Its connecting cable can be up to 6m.

The initial SCSI standard maximum synchronous transfer rate of 5MB / s (SCSI-1, also known as Narrow

 SCSI, in 1986, the largest support for 7 devices, the clock frequency of 5MHz), and later the SCSI II provides two kinds of speed options. One is to increase the frequency of data transmission, that is, Fast SCSI (1994, the largest support of seven devices), due to the frequency doubled to 10MB / s (10MHz); another increase in speed is the choice of transmission frequency doubled Also increases the width of the data path from 8 bits to 16 bits. Wide SCSI, the maximum synchronous transmission speed of 20MB / s (clock frequency of 10MHz, 1996, the maximum support of 15 devices).

Around the emergence of the third generation of SCSI in 1995, but there is no uniform standard:

1. Ultra SCSI (also known as Fast-20 SCSI, 20MHz clock) with maximum synchronous transmission speed of 20MB / s;

2. Ultra Wide SCSI (same 1) with maximum synchronous transmission speed of 40MB / s;

3. Ultra2 SCSI (also known as Fast-40 SCSI, clock frequency 40MHz, 1997) with maximum synchronous transfer speeds of 40MB / s.

Later, there have been some updated SCSI standards:

1. Ultra2 Wide SCSI with maximum synchronous transmission speed of 80MB / s (clock frequency is 40MHz);

2. Ultra 3 SCSI (also known as Ultra-160 or Fast-80 Wide SCSI, with a clock rate of 40MHz plus double data rate, up to 160MB / s)

3. The maximum synchronous transmission speed of 320MB / s Ultra 320 SCSI (also known as Ultra 4 SCSI, the clock frequency of 80MHz plus double data rate, 2002)

4. The maximum synchronous transmission speed of 640MB / s Ultra 640 SCSI (clock frequency of 160MHz plus double data rate, 2003, is the latest SCSI standard)

This interface is an interface standard that facilitates system integration, reduces costs and improves efficiency. More and more devices will use the SCSI interface standard, so there are many hard drives and SCSI CD-ROM drives with SCSI interfaces, but due to cost issues, Mainly used in high-end servers and workstations.

Types of


SCSI-1 is the most original version, asynchronous transmission frequency of 3MB / S, synchronous transmission frequency of 5MB / s. Although it is almost eliminated, but will also be used in some scanners and internal ZIP drive, using a 25-pin interface. In other words, if the SCSI-1 device to connect to your SCSI card, you must have an internal 25 for 50-pin interface cable; if the use of external equipment, you can not use any of the internal interface (that is, Of the internal interface can not be used).


Early SCSI-2, called FastSCSI, increased the transmission rate from the original 5MB / s to 10MB / s by increasing the frequency of synchronous transmission. It supports 8-bit parallel data transmission and can be connected to 7 peripherals. Later appeared WideSCSI, support 16-bit parallel data transmission, data transfer rate also increased to 20MB / s, can be connected with 16 peripherals. This version of SCSI uses a 50-pin interface, mainly for scanners, CD-ROM drives and older hard drives.


In 1995, the birth of a more high-speed SCSI-3, known as UltraSCSI, data transfer rate has reached 20MB / s. It will be synchronized transmission clock frequency increased to 20MB / s, improve the data transfer rate of technology. If the 16-bit transmission using the Wide mode, the data transfer rate can be increased to 40MB / s. This version of SCSI uses a 68-pin interface, mainly applied to the hard disk. The typical feature of SCSI-3 is to greatly increase the bus frequency and reduce signal interference in order to enhance its stability. [1] 

SCSI-3 has many models, Ultra (fast-20) transmission frequency of 20MHz, 8-bit data bandwidth, transmission rate of 20MBps

Ultra wide transmission frequency of 20MHz, 16-bit data bandwidth, transmission rate of 40MBps

Ultra 2 transmission frequency of 80MHz, 16-bit data bandwidth, transmission rate of 80MBps

Ultra 160 transmission frequency of 80MHz, 16-bit data bandwidth, transmission rate of 160MBps

Ultra 320 transmission frequency of 80MHz, 16-bit data bandwidth, transmission rate of 320MBps

Ultra 640 transmission frequency of 160MHz, 16-bit data bandwidth, transmission rate of 640MBps

the difference

In addition to SCSI, IDE is also a very common interface. From the point of view of easy to use, IDE is more suitable for ordinary users, coupled with personal computer users not only need to configure the peripherals, but also the speed requirements are not high, so the choice of IDE interface is more appropriate. In addition, IDE also has a high cost performance, wide application and so on. The SCSI interface despite a lot of unparalleled features, but no matter from which point of view, the interface and the use of the interface of the external price is too expensive, the general user can not afford, which also determines its actual use of the limitations Sex.

1. IDE work requires the CPU to participate in the whole process, CPU read and write data can no longer be other operations, this situation in the Windows 95 / NT multi-tasking operating system, will naturally lead to a significant slowdown in system response. The SCSI interface, then completely through the independent high-speed SCSI card to control the data read and write operations, CPU will not have to waste time waiting, obviously can improve the overall performance of the system. However, the IDE interface to improve this problem has also been greatly improved, you can already use the DMA mode instead of PIO mode to read and write, the data exchange by the DMA channel is responsible for the CPU occupancy can be greatly reduced. Nevertheless, compare the SCSI and IDE in the CPU occupancy rate, or can still find SCSI has a considerable advantage.

2. iSCSI expansion than the IDE, generally each IDE system can have two IDE channels, a total of four IDE devices, and SCSI interface can connect 7-15 devices, much more than the IDE, and the connection of the cable Much longer than the IDE.

3. Although the SCSI device prices higher, compared with the IDE, SCSI performance is more stable, durable, and reliability is also better.


1. SCSI can support multiple devices, SCSI-2 (FastSCSI) can be connected to up to 7 SCSI devices, WideSCSI-2 or more can be accessed by 15 SCSI devices. In other words, all devices only need to take an IRQ, while SCSI also supports a wide range of devices such as CD-ROM, DVD, CDR, hard drives, tape drives, scanners and so on.

2.SCSI also allows another device to carry out data lookup while transferring data to a device. This can be in the multi-tasking operating systems such as Linux, WindowsNT to get higher performance.

3.SCSI CPU is very low, really in the multi-task system occupies a clear advantage. As the SCSI card itself with a CPU, can handle all the affairs of the SCSI device, at work, the host CPU as long as the SCSI card to issue work instructions, SCSI card will work on their own, after the end of the work results to the CPU, the whole process , CPU can carry out their own work.

4. SCSI devices also have intelligent, SCSI card itself can be queued on the CPU instructions, thus improving the work efficiency. In the multi-task when the hard disk will be in the current head position, the adjacent task first completed, and then one by one for processing.

5. The fastest SCSI bus has 160MB / s of bandwidth, which requires the use of a 64-bit 66MHz PCI slot, so the PCI-X bus standard can achieve the maximum speed of 80MB / s, if with 10, 000rpm or 15,000 rpm speed dedicated hard disk use will bring significant performance improvement.


SCSI chain of the last SCSI device to use the finalizer, the middle of the device is not required finalizer. Once the intermediate device uses the finalizer, then the SCSI card can not find the future of the SCSI device. And if the last device useless finalizer, SCSI is not working properly. The terminator is composed of resistors, located at the end of the SCSI bus, used to reduce the mutual influence of the signal, to maintain the voltage on the SCSI chain constant.

Most SCSI devices are built-in finalizers and use a jumper to control ON / OFF. SCSI device is highly intelligent, can automatically control the finalizer ON / OFF, such as a hard disk and a CD-ROM connected, regardless of the end of the hard disk ON or OFF, CD-ROM can be used normally. And when the two hard drives connected, the situation becomes complicated, and two Seagate hard disk connected before a hard disk finalizer must be OFF, and when a Seagate hard drive and a Quantum hard drive connected before a hard disk finalizer regardless of ON Or OFF, can be used normally.


Each SCSI device in the system must have its own unique ID (identification number) on the 8-bit bus, which is 0 to 7; on a 16-bit bus, this number is from 0 to 15. The SCSIAdapter system defaults to 7. This ID can be set by a jumper located at the front of the device. For hard drives it is located on the front or back of the drive.

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