1. SCSI-1: It is the earliest SCSI interface, introduced by Shugart (the predecessor of Seagate) in 1979 and officially recognized as SASI (Shugart Associates System Interface) by the American Standards Organization in 1986. SCSI-1 defines an 8-bit (parallel) data bus, using a 50-pin data cable (or 25 pairs of twisted cables). It has several key characteristics:
- Up to 8 SCSI devices (including the computer host) can be connected on a single bus, with bus arbitration for priority. The highest address (ID7, usually the SCSI adapter) has the highest priority.
- Provides both synchronous and asynchronous transfer modes, with a maximum synchronous transfer rate of 5Mb/s.
- Uses Logical Block Addressing (LBA), which defaults to 512 bytes but can be adjusted as needed. This method offers significant advantages over the traditional Cylinder/Head/Sector (CHS) addressing system in terms of drive capacity support.
- Offers two electrical specifications for data cable configurations: Single-End (SE) cables can reach up to 6 meters, while Differential cables can extend up to 25 meters. Devices with these two configurations cannot be mixed on the same bus or swapped.
- Supports multiple I/O connection configurations—from single initiator/target devices to multiple initiators/(multiple) target devices. In PC systems, the initiator is typically a SCSI adapter card, while target devices include hard drives, tape drives, and optical drives.
Due to many optional commands and features in the original SCSI-1 specification, there was little or no guarantee that a specific peripheral would support expected commands. This led to the industry defining a Common Command Set (CSS) consisting of 18 basic SCSI commands, forming the minimum command set supported by all peripherals and becoming the basis for the SCSI-2 standard.
2. SCSI-2: It is the successor to SCSI-1, approved by the X3T9 committee in 1990 and ratified by 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 supporting up to 7 devices. Later, a 16-bit parallel data transfer mode emerged, known as "Fast Wide SCSI," doubling the data transfer rate to 20Mb/s and supporting up to 15 devices.
In addition to CSS, SCSI-2 introduced the following important new features:
- 8-bit data bus with a transfer rate of 10Mb/s for Fast SCSI-2.
- Data bus expanded to 16 bits, with Fast Wide SCSI-2 using 68-pin connectors and cables reaching a transfer rate of 20Mb/s.
- Command Queuing, allowing a device to receive multiple commands simultaneously (up to 256 for SCSI) and execute them in the most efficient order, which is especially beneficial for multitasking operating systems sending multiple requests to the SCSI bus.
- High-density connectors reduced in size by half, enabling smaller peripheral devices and automatic terminators powered by the initiating device, improving signal stability during Fast SCSI transfers.
- Single Connector Attachment (SCA), proposed jointly by workstation manufacturer Sun Microsystems and disk drive manufacturers Seagate Technology and Conner Peripheral, integrating 50-pin or 68-pin interfaces, power, SCSI ID, LED, spindle motor synchronization, and other functions into an 80-pin D-shaped connector initially introduced by AMP Company, providing hot-swapping 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. Two subcategories were launched based on different data 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 supporting up to 8 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 devices.
SCSI-3 supports up to 16 devices (15 excluding the SCSI adapter). The newly introduced Low Voltage Differential (LVD) signal transmission mode increases the data line length from 3 meters for 4 SE 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 disk drive manufacturer has released corresponding products.
4. Ultra2 SCSI: It is a SCSI interface type introduced in 1997 based on Ultra SCSI. It uses Low Voltage Differential (LVD) transmission, allowing interface cables up to 12 meters long, greatly increasing device flexibility. Like previous SCSI interfaces, it is divided into Narrow mode using 8 bits and Wide mode using 16 bits. The Narrow mode is called "Ultra2 SCSI" with a transfer rate of 40Mb/s supporting up to 7 devices; the Wide mode is called "Ultra2 Wide SCSI" with a transfer rate of 80Mb/s supporting up to 15 devices.
5. Ultra3 SCSI: It is an updated interface of Ultra2 SCSI, introduced in September 1998. Besides supporting existing SCSI specifications and using the same interface cables and terminators as Ultra2 SCSI, it includes some new features. First, Ultra3 SCSI uses Double Transition Clocking, whereas Ultra2 SCSI uses Single Transition Clocking, making the transfer rate of Ultra3 SCSI twice that of the former, i.e., 160Mb/s. Additionally, Ultra3 SCSI provides Domain Validation, Cyclic Redundant Check (CRC), Packetized Protocol, and Quick Arbitrate & Select. To accelerate the introduction of Ultra3 SCSI technology, many manufacturers first launched Ultra160/M SCSI, whose technology is similar to Ultra3 SCSI except for the absence of Quick Arbitrate & Select and Packetized Protocol features. Thus, Ultra160/M SCSI can be considered a subset of Ultra3 SCSI.
6. Ultra320 SCSI: It is the technical standard fully named "Ultra320 SCSI SPI-4." Ultra320 SCSI achieves a maximum data transfer rate of 320Mb/s per channel, reaching 640Mb/s with dual-channel SCSI controllers. From the perspective of infrastructure development, the upgrade from 160Mb/s to 320Mb/s is not technically complex or expensive, making the 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 versions, with 50-pin and 68-pin being the most common.
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