SCSI (Small Computer System Interface), a comparatively special interface bus, is capable of communicating with multiple types of external devices. SCSI adopts ASPI (Advanced SCSI Programming Interface) standard software interface to have driver communicate with SCSI adapter installed on computer internally. SCSI interface is a high speed data transmission technology widely used on minicomputer with advantages of wide range of application, multitask, high broadband, low CPU occupation rate and so on.

General information:
SCSI, a general interface
SCSI bus can be connected to host machine adaptor and eight SCSI external device controllers including disk, magnetic tape, CD-ROM, rewritable CD-ROM drive, printer, scanner and communication apparatus, etc. SCSI is a multitasking interface with bus arbitration function and external devices hung on one SCSI bus could work simultaneously. All devices on SCSI share the bus equally. SCSI interface is able to transmit data synchronously with transmission rate of 10MB/s and asynchronously with transmission rate of 1.5MB/s.

SCSI interface connected to external device
The connection cable reaches a length of 6m. SCSI interface provides powerful yet flexible connection methods for optical storage products and high performance to allow 7 or more drivers to be connected on one SCSI adaptor. However, it enjoys a very high cost. SCSI should be used in cooperation with expensive SCSI card and the installation and setting of SCSI interface driver are relatively complicated. Therefore, SCSI interface driver is far less popular than IDE interface driver. SCSI optical storage products are more applied in some professional fields with special needs and are seldom used within families.
SCSI is an interface connecting host machine and peripheral devices which supports multiple devices including disc driver, tape driver, CD driver and scanner. SCSI controller performs data operations which equals to a small CPU with its own command sets and cache. To understand SCSI, the SCSI types must be comprehended at first and the following are STA (SCSI Trade Association) standard classifications.

SCSI types
　　Note:
　　(1) Overall bus length from point to point
　　(2) SCSI, Ultra SCSI and Ultra2 SCSI are optional
　　(3)LVD (Low Voltage Differential) doesn’t define its speed and can keep its normal transmission speed within 12m. If one device in bus is set as single ended, the entire bus will be converted into single ended.
　　(4) Single ended doesn’t define its length
　　(5)HVD（High Voltage Differential）doesn’t define its speed
　　(6) All high speed transmissions are based on Wide mode after Ultra2.

Are you dazzled after seeing above-mentioned standards? In fact, it is more important for new user to understand SCSI controller and data line than interface types. In SCSI bus, controller is also a device, namely, real maximum number of devices that can be connected is tantamount to maximally supported device number deducts one.

Development process
Specialized SCSI controller
Specialized SCSI controller is entailed to apply SCSI in system to support SCSI device, namely, a SCSI controller card differing from IDE hard disk. There is a CPU-like chip on SCSI controller which controls SCSI device, handles most of work and reduces CPU burden (CPU occupancy rate). Of contemporaneous hard disks, SCSI enjoys higher rotation speed, buffer memory size and data transmission rate than IDE hard disk, therefore, it is more popular in commercial field.
SCSI was developed by Shugart (predecessor of Seagate), 1979, recognized by ANSI and named as SAAI, namely SCSI-1 in 1986. SCSI-1 is the first SCSI standard supporting synchronous SCSI peripheral devices with rate of 5MB/s and asynchronous SCSI peripheral devices of 3MB/s, employing 8 bit passage width, allowing 7 devices connection maximally and supporting WORM peripheral devices. Moreover, it adopts 25-pin interface, so an external 25-pin to 50-pin interface cable is required while connected to SCSI card (50-pin interface). However, such device has been eliminated and only can be seen on some old devices or individual scanner.

SCSI-2 enhanced reliability
SCSI-s, also called as Fast SCSI, had been greatly enhanced with improved reliability and raised data transmission rate of 10MB/s on the on the basis of SCSI-1 but kept 8 bit parallel data transmission and maximally 7 supported devices. Afterwards, it has been improved again by releasing 16 bit parallel data transmission WIDE-SCSI-2 and FAST-WIDE-SCSI-2 with the former remaining previous data transmission rate but altered into 16 bit transmission and the latter increased data transmission rate to 20MB/s.
SCSI standard edition, issued in 1995 and known as Ultra SCSI, enjoys 20MB/s synchronous data transmission rate and the data transmission rate can be escalated to 40MB/s when 16 bit Wide mode is applied with a maximum 1.5m interface cable.

Release of Ultra2 SCSI（Fast－40）standard edition
In 1997, Ultra2 SCSI（Fast－40）standard edition was released. It still employs 8 bit data passage but introduces LVD transmission mode with 40MB/s transmission rate, allows 12m maximum cable length to greatly enhance device flexibility and supports 15 coupling devices simultaneously. Later, WIDE ULTRA 2 SCSI interface standard was released which adopts 16 bit data passage bandwidth to reach a highest transmission rate of 80MB/s, supports maximum cable length of 12m and 15 coupling devices to highly improve device flexibility. LVD could utilize lower voltage so as to integrate differential driver and receiving program into hard disk onboard SCSI controller. Old fashioned SCSI requires independent and electricity-consuming high voltage devices. Since LVD uses low voltage and low electric current device, it could integrate differential transceiver on hard disk on board SCSI controller without independent external high voltage differential devices.

Ultra160 SCSI
Ultra160 SCSI, also called as Ultra3 SCSI LVD, is a relatively supplicated SCSI interface standard on the basis of SCSI which adopts clocked double-conversion control, cyclic redundancy check and domain name identification technologies. Clocked double-conversion control increases data transmission rate by one time while not improving interface clock rate, which is the crux for the great enhancement of Ultral60 SCSI interface rate. It is easy and of low risk to use Ultral60 SCSI. Meanwhile, Ultral60 SCSI enjoys transmission rate of 160MB/s which is one time higher than Ultra2 SCSI while enhancing the reliability and ease of management.

Features of Ultra160 SCSI:
Ultra2 andUltra160 devices could be installed on one bus without degrading Ultra160 device performance;
Enhance product reliability by improving error detection and correction capability;
Enjoys monitoring interface performance and comparatively reliable transmission rate;
Reach 25m when used in single device and 12m in 2 devices;
Support as many as 15 SCSI devices in one passageway.

Connection types:
SCSI connector comprises internal and external ones.
The configuration of build-in data link is the same as IDE data link with subtle difference in number of pins and specifications. It is mainly used to connect CD-ROM and hard disk. 40-pin IDE has 40 guide lines and 40-pin ATA66 has 80 guide lines while SCSI comprises 50, 68 and 80 pin.

As for external data link, it has following specifications with different density:
Apple SCSI has 25 pins in all which is divided into two rows with 8 bit and used on Mac and old fashioned Sun work station.
DD-50SA of Sun Microsystem falls into 3 rows including 50 pins.
SCSI-2 is split into 2 rows, 8 bit including 50 pins.
Centronics is split into 2 rows and has 50 pins in all with 8 bit which is similar to parallel port and can connect the most devices.

SCA is divided into 2 rows with 80 pins.
SCSI-3 and Wide SCSI-2 is split into 2 rows with 68 pins, 16 bit. Old fashioned single -ended SCSI employs 68 pin high-density interface.
Interfaces and length of SCSI Connectors/SCSI

DB-25, Male External DB-25,	DB-25,Female External DB-25,
Low-Density, 50-pin, Male External	Low-Density, 50-pin, Female External
High-Density, 50-pin, Male External	High-Density, 50-pin, Female External
Low-Density, 50-pin, Male Internal	Low-Density, 50-pin, Female Internal
High-Density, 68-pin, Male External	High-Density, 68-pin, Female External
High-Density, 68-pin, Male Internal	High-Density, 68-pin, Female Internal
VHDCI, 68-pin, Male External	VHDCI, 68-pin, Female External