1. SCSI-1: It is the earliest SCSI interface, launched by Shugart (the predecessor of Seagate) in 1979 and recognized by the American Standards Organization in 1986 as SASI (Shugart Associates System Interface). SCSI-1 defines an 8-bit (parallel) data bus, using a 50-pin data cable (or 25 pairs of twisted cables). It mainly has the following characteristics. A unified bus can connect 8 SCSI devices (including the host computer), providing bus priority arbitration functions, with the highest address (ID7, usually the SCSI adapter) having the highest priority. It provides both synchronous and asynchronous transmission methods, with a maximum synchronous transmission rate of 5Mb/s. It uses Logical Block Addressing (LBA), with LBA size typically set to 512 bytes by default but adjustable according to needs. In terms of drive capacity support, it is clearly superior to the traditional Cylinder/Head/Sector (CHS) addressing method. It provides two electrical specifications for data line configuration - Single-End (SE) type data lines can reach up to 6 meters, while Differential type can reach up to 25 meters. Devices with these two configurations cannot be connected simultaneously on the same bus or interchanged. It provides multiple I/O connection configurations - from single initiator/target device to multiple initiators/(multiple) target devices. In PC systems, the initiator device is usually a SCSI adapter card, and the target device can be considered as hard drives, tape drives, and optical drives among other storage devices. Due to many commands and features being optional in the original SCSI-1 text, and rarely or not guaranteeing that a specific peripheral supports the expected command, this led the entire industry to define a common command set (Common Command Set, CSS) consisting of 18 basic SCSI commands, which became the minimum command set supported by all peripherals and formed the basis for the SCSI-2 standard.
2. SCSI-2: It is the successor interface to SCSI-1, approved by the X3T9 committee in 1990 and ANSI in 1994 (ANSI X3.131), also known as Fast SCSI. If the original 8-bit parallel data transfer mode is used, it is called "Fast SCSI", with a data transfer rate of 10Mb/s and supporting up to 7 connected devices. Later, a 16-bit parallel data transfer mode emerged, known as "Fast Wide SCSI", increasing the data transfer rate to 20Mb/s and supporting up to 15 connected devices. Besides CSS, the major new functions added in SCSI-2 compared to SCSI-1 are as follows:
1. 8-bit data bus with a transmission rate of 10Mb/s for Fast SCSI-2.
2. Data bus widened to 16 bits, with Fast Wide SCSI-2 using 68-pin connectors and cables reaching a transmission rate of 20Mb/s.
3. Command Queuing, allowing a device to receive multiple commands simultaneously (up to 256 for SCSI), and execute them in the order deemed most efficient, which is particularly beneficial for multitasking operating systems sending several requests to the SCSI bus simultaneously.
4. High-density connectors reduced in size by half, enabling smaller peripheral devices to be used, and switching to automatic terminators powered by the initiating device, improving signal stability during Fast SCSI transfers.
5. Single Connector Attachment (SCA) proposed jointly by workstation manufacturer Sun Microsystems and disk drive manufacturers Seagate Technology and Conner Peripheral, integrating the 50-pin or 68-pin interface along with power, SCSI ID, LED, spindle motor synchronization, and other functional connectors into the initially launched 80-pin D-shaped connector by AMP company, providing hot-swappable capabilities suitable for backplane environments.
3. SCSI-3: It is a collection of multiple standards, each defining a very specific part of SCSI: physical interface, transport interface, command interface, and system. Within this large category, two subcategories were introduced based on different data bit widths. If the original 8-bit parallel data transfer mode is used, it is called "Ultra SCSI", with a data transfer rate of 20Mb/s and supporting up to 8 connected devices. After increasing the parallel data transfer bus bandwidth to 16 bits, "Ultra Wide SCSI" emerged, doubling the transfer rate to 40Mb/s and supporting up to 15 connected devices. SCSI-3 supports up to 16 devices (15 excluding the SCSI adapter), introducing the Low Voltage Differential (LVD) signal transmission mode, increasing the data line length from SE's 3 meters for 4 devices/1.5 meters for 8 devices to point-to-point 25 meters for 16 devices/12 meters. Note that although LVD supports 8-bit data buses, no hard drive manufacturer has launched corresponding products.
4. Ultra2 SCSI: It is a SCSI interface type introduced based on Ultra SCSI in 1997. It uses Low Voltage Differential (LVD) transmission mode, allowing the maximum length of interface cables to be 12 meters, greatly increasing device flexibility. Like the previous SCSI interfaces, it is divided into Narrow mode using 8 bits and Wide mode using 16 bits. The Narrow mode using 8 bits is called "Ultra2 SCSI", with a transfer rate of 40Mb/s and supporting up to 7 connected devices; while the Wide mode using 16 bits is called "Ultra2 Wide SCSI", increasing the transfer rate to 80Mb/s and supporting up to 15 connected devices.
5. Ultra3 SCSI: It is the updated interface of Ultra2 SCSI, introduced in September 1998. In addition to supporting existing SCSI specifications and using the same interface cables and terminators as Ultra2 SCSI, it includes some new functions. First, Ultra3 SCSI uses double-edge transition frequency (Double Transition Clocking), whereas Ultra2 SCSI uses single-edge transition frequency, making the transfer rate of Ultra3 SCSI twice that of the former, i.e., 160Mb/s. Additionally, Ultra3 SCSI provides domain validation (Domain Validation), cyclic redundancy check (CRC), packetization (Packetized Protocol), and quick arbitration selection (Quick Arbitrate & Select). To accelerate the introduction of new Ultra3 SCSI technologies, many manufacturers first launched Ultra160/M SCSI, whose technology is similar to Ultra3 SCSI except for lacking quick arbitration selection and packetization, meaning Ultra160/m SCSI is a subset of Ultra3 SCSI.
6. Ultra320 SCSI: It is the full technical specification named "Ultra320 SCSI SPI-4". Ultra320 SCSI can achieve a maximum data transfer rate of 320Mb/s per channel, reaching 640Mb/s if a dual-channel SCSI controller is used. From the perspective of infrastructure development, the upgrade from 160Mb/s to 320Mb/s is not technically complex or costly, making the technological transition from SCSI Ultra160 to SCSI Ultra320 very easy for system integrators. SCSI controller interfaces typically come in 50-pin, 68-pin, and 80-pin varieties, with 50-pin and 68-pin being the most commonly used.
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