【 Weak current College 】 SCSI and IDE difference analysis

Current storage device interface there are five broad categories: IDE, SCSI, USB, parallel port, serial port, parallel port and serial port speed is very slow, not to mention, the most important is the IDE, usb, SCSI. IDE (integrated drive electronics IntegratedDriveElectronics) with its high speed of transmission and the usual price, subject to ordinary users, while the USB device has subsequently ' home, as for SCSI (SmallComputerSystemInterface, small computer system interface), speed, performance and stability is better than the IDE, the price of course is much more expensive, mainly for server and workstation market.

In the past few years, the IDE progress rapidly, UltraDMA33 launched less than two years, the UltraDMA66 has just come onto the market. In fact, SCSI development doesn't slow than IDE, except that we have less contact with their understanding is not deep and others. SCSI standards introduced in 1980, but until now has not yet been harmonized, the manufacturers of naming it, easily confusing is the main reason, the following describes the various aspects of the SCSI interface, and they hope to prepare for the purchase of a SCSI device friends help.

First, an overview

SCSI is a link to the host and peripheral interface, support including disk drives, tape drives, optical drives, scanners, and a variety of devices it is SCSI controller for data manipulation, the SCSI controller is equivalent to a small CPU, has its own set of commands and cache. To learn more about SCSI, you must first understand its type, the following is STA (SCSITradeAssociation, SCSI Trade Association) standard classification.

SCSI type, note:

(1): point to point transmission bus length

(2): SCSI, UltraSCSI or Ultra2SCSI are optional

(3): LVD (low score LowVoltageDifferential, differential) did not define its speed, in 12 meters are able to maintain a normal rate. If you have a device within the bus is set to single-ended, the entire bus will switch into a single endpoint, single end does not define its length

(4): HVD (HighVoltageDifferential, score differential) did not define its speed

(5): after the Ultra2, all high speed transmission are based on broadband (Wide) mode.

See the above standards, whether or not to feel a bit overwhelming, in fact, for a new user, see SCSI controller and the data cable of the type is more important than the interface type. The SCSI bus, the controller is a device, that is, the actual maximum number of devices you can connect = theoretical maximum number of supported devices-1.

Then all SCSI specifications common to several standard terminology explained:

SingleEnded (single ended): many legacy devices are single-end equipment, they are restricted to SCSI-1 agreement 6 m length. Note: this distance including device internal cable distance.

Differential (differential): SCSI bus and device you can use it to the distance along the long transmission, additional line can be up to 25 meters. The disadvantage is and is not compatible with the single-end devices.

FastSCSI: the first generation of the SCSI bus speed to increase from 5MHz 10MHz, theoretical data rates also doubled to 10MB/seconds.

UltraSCSI: the first generation of the SCSI bus speed to increase from 5MHz 20MHz, theoretical data rates also doubled to 20MB/seconds.

WideSCSI: it relies on a second data cable or 68-pin data cable for increased performance, data bus width of 16 or 32bits, traditional SCSI performance up to 2 x or 4 x.

WideUltraSCSI: 68-pin cable to bus performance improvement to 40MB/seconds.

Only through the above description, we still cannot accurately determine the type of a SCSI bus, you must also understand its bus width, bus speed, cable type, and the append command sets to achieve.

Second: the type of SCSI connectors

SCSI connector is divided into internal and external.

Built-in cable appearance and IDE data cable, just PIN numbers and specifications differ slightly, mainly used to connect the optical drive and hard disk,

40-pin IDE cable has a 40 wire, 40-pin 80 wire ATA66, SCSI internal is divided into 50-pin, 68-pin and 80-pin.

As for SCSI external cable, you have the following specifications, their density is different, please don't be mistaken.

AppleSCSI: 25-pin, divided into two rows, 8 bit, commonly used in Mac machines and legacy Sun workstation.

SunMicrosystem DD-50SA: a total of 50-pin, divided into three rows.

SCSI-2: 50 pins, divided into two rows, 8-bit.

50-pin Centronics: total, divided into two rows, 8 bit, a bit like a parallel port, it can be connected to the largest number of devices.

80-pin SCA: total, divided into two rows.

SCSI-3 and WideSCSI-2: a total of 68-pin, divided into two rows, 16-bit. Legacy SCSI single-end DEC with 68-pin high density connector.

III. SCSIID and bus terminator

I believe many SCSI users have a similar experience, plug in the device, the operating system would not recognize, then check the bus, only to find the end and ID does not set up. ID (identify) as SCSI devices on the SCSI bus of the unique identifier, absolutely no duplicates, optional range from 0 to 15, SCSI host controller typically consume id7, means that we can use the device ID number 15.

Bus terminator tell SCSI host controller where the whole bus, and issued a final reflection signals to the controller, you must have two physical terminal for a termination signal to use SCSI bus. Common error is to end the set in the ID number of the highest or lowest place, instead of setting the physical terminal SCSI device. In fact, the SCSI device is always connected to the chain, in order to distinguish which one is the end device. In the end there are three ways:

Since the end of the equipment, the physical bus terminator and since the end of the cable. Most of the new SCSI device has since the end of the jumper, so long as the non-end equipment since the end of the jumper is set to OFF to avoid conflict; physical bus terminator is a hardware connector, is divided into two active and passive, proactive use of voltage regulator to operate passive utilization of energy on the bus signal to operations, more accurate than proactive passive; since the end of the cable can replace physical bus terminator, is also a kind of hardware, its price is very expensive, often used for two hosts connections to the same physical device, such as: two servers to access the same physical SCSI hard drives. By checking the SCSIID and bus terminator, we can find out the most conflict solution, this is the SCSI device users must pay attention to the point.

Fourth, IDE and SCSI

In the face of the new SCSI users, the most commonly heard questions: "what a SCSI, or IDE? this is a hard question to answer, it includes performance, price, ease of use, scalability of factors. From a performance, SCSI course than IDE good, after all, the SCSI controller has an equivalent to the CPU chip, able to handle most of the work, reduce the burden on the central processing unit (CPU). The same time with the launch of the hard drive, SCSI, products rotate speed, cache size and data transfer rates are higher than IDE systems, the IDE than speed, but how is better than SCSI.

In terms of price, SCSI is a synonym for expensive commercial-grade applications, the IDE to low prices, oriented to the desktop computer. Ease of use: using SCSI, regular occurrence SCSIID and bus terminator Setup error, cause hardware failure that is not recognized by the IDE device only master, Deputy, in the same data line only two equipment, as long as they are set to Master and Slave will not have conflicts. Scalability: the ability to connect up to 15 devices is one of the advantages of SCSI, and IDE interfaces of a standard PC, up to 4 devices can be connected.

Before buying the same products, mainly to take into account the needs of their own, with this in mind, it is easy to figure out which product is right for you, just say "good" and "bad" does not make much sense. If you use a computer to play games, watch DVD, Internet, IDE hard disk has been able to meet your application, SCSI only will allow to increase the number of frames, Quake3 is definitely not pay.

If you use a computer to the video capture, video editing, etc. require a large amount of disk input/output, believe that SCSI is your top choice, don't on to save some money and buy IDE Oh, otherwise lose.

5. SCSI's future.

SCSI is a progressive technology, has recently joined the specifications have FibreChannelSCSI, IEEE1394 (Firewire, FireWire) and SCSI3 (160MB/sec), soon to be born with SCSI4 (320MB/sec) and SCSI5 (640MB/sec). Starting from SCSI3, SCSI in accordance with the need to rapidly increase performance, and has a nearly perfect backwards compatibility, protecting the user's investment. With the rising of the speed, the SCSI also began to focus on ease of use, the use of CAM (CommonAccessModel, public access model) in many SCSI command set and procedures regulating between joined a control layer, so that the SCSI programming easier. I firmly believe that scientific progress will take on a SCSI tape and a technology Summit, and the future of SCSI will become cheaper and better.