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Parallel SCSI (formally, SCSI Parallel Interface, or SPI) is the earliest of the interface implementations in the SCSI family. SPI is a parallel bus; there is one set of electrical connections stretching from one end of the SCSI bus to the other. A SCSI device attaches to the bus but does not interrupt it. Both ends of the bus must be terminated.
SCSI is a peer-to-peer peripheral interface. Every device attaches to the SCSI bus in a similar manner. Depending on the version, up to 8 or 16 devices can be attached to a single bus. There can be multiple hosts and multiple peripheral devices but there should be at least one host. The SCSI protocol defines communication from host to host, host to a peripheral device, and peripheral device to a peripheral device.[a] The Symbios Logic 53C810 chip is an example of a PCI host interface that can act as a SCSI target.
SCSI-1 and SCSI-2 have the option of parity bit error checking. Starting with SCSI-U160 (part of SCSI-3) all commands and data are error checked by a cyclic redundancy check.
History
The first two formal SCSI standards, SCSI-1 and SCSI-2, described parallel SCSI. The SCSI-3 standard then split the framework into separate layers which allowed the introduction of other data interfaces beyond parallel SCSI. The original SCSI-1 version of the parallel bus was 8 bits wide (plus a ninth parity bit). The SCSI-2 standard allowed for faster operation (10 MHz) and wider buses (16-bit or 32-bit). The 16-bit option became the most popular.
At 10 MHz with a bus width of 16 bits it is possible to achieve a data rate of 20 MB/s. Subsequent extensions to the SCSI standard allowed for faster speeds: 20 MHz, 40 MHz, 80 MHz, 160 MHz and finally 320 MHz. At 320 MHz x 16 bits there is a theoretical maximum peak data rate of 640 MB/s.
Due to the technical constraints of a parallel bus system, SCSI has since evolved into faster serial interfaces, mainly Serial Attached SCSI and Fibre Channel. The iSCSI protocol doesn't describe a data interface but uses any IP network, usually run over Ethernet.
Standards
Parallel SCSI is not a single standard, but a suite of closely related standards. There are a dozen SCSI interface names, most with ambiguous wording (like Fast SCSI, Fast Wide SCSI, Ultra SCSI, and Ultra Wide SCSI); three SCSI standards, each of which has a collection of modular, optional features; several different connector types; and three different types of voltage signaling. The leading SCSI card manufacturer, Adaptec, has manufactured over 100 varieties of SCSI cards over the years. In actual practice, many experienced technicians simply refer to SCSI devices by their bus bandwidth (i.e., SCSI 320 or SCSI 160) in Megabytes per second.
As of 2003[update], there have only been three SCSI standards: SCSI-1, SCSI-2, and SCSI-3. All SCSI standards have been modular, defining various capabilities that manufacturers can include or not. Individual vendors and the SCSI Trade Association have given names to specific combinations of capabilities. For example, the term Ultra SCSI is not defined anywhere in the standard, but is used to refer to SCSI implementations that signal at twice the rate of Fast SCSI. Such a signaling rate is not compliant with SCSI-2 but is one option allowed by SCSI-3. Similarly, no version of the standard requires low-voltage-differential (LVD) signaling, but products called Ultra-2 SCSI include this capability. This terminology is helpful to consumers because Ultra-2 SCSI device has a better-defined set of capabilities than simply identifying it as SCSI-3.
Starting with SCSI-3, the SCSI standard has been maintained as a loose collection of standards, each defining a certain piece of the SCSI architecture, and bound together by the SCSI Architectural Model. This change divorces SCSI's various interfaces from the SCSI command set, allowing devices that support SCSI commands to use any interface (including ones not otherwise specified by T10), and also allowing the interfaces that are defined by T10 to be used in alternate manners.
No version of the standard has ever specified what kind of SCSI connector should be used. See § External connectors.
Comparison table
| Interface | Alternative names | Specification document[A] | Connector | Width (bits) | Clock (MHz) | Maximum | Electrical | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Throughput | Length (m) | Devices[B] | Impedance (Ω) | Voltage (V) | |||||||||
| (MB/s) | (Mbit/s) | Single ended[C] | LVD[D] | HVD | |||||||||
| SCSI-1 | Narrow SCSI | SCSI-1 (1986)[E] | IDC50; Amphenol C50 | 8 | 5 | 5 | 40 | 6 | N/A | 25 | 8 | SE 90 ± 6[3] | SE 5 |
| Fast SCSI | SCSI-2 (1994) | IDC50; Amphenol C50 | 8 | 10 | 10 | 80 | 3 | N/A | 25 | 8 | SE 90 ± 6[3] | SE 5 HVD ≥5 | |
| Fast-Wide SCSI | Wide SCSI | SCSI-2; SPI-5 (INCITS 367-2003) |
68-pin | 16 | 10 | 20 | 160 | 3 | N/A | 25 | 16 | SE 90 ± 6[3] | SE 5 HVD ≥5 |
| Ultra SCSI | Fast-20 | SPI-5 (INCITS 367-2003) | IDC50 | 8 | 20 | 20 | 160 | 1.5 | N/A | 25 | 8 | SE 90 ± 6[3] | SE 5 HVD ≥5 |
| 3 | N/A | N/A | 4 | ||||||||||
| Ultra Wide SCSI | SPI-5 (INCITS 367-2003) | 68-pin | 16 | 20 | 40 | 320 | N/A | N/A | 25 | 16 | SE 90 ± 6[3] | SE 5 HVD ≥5 | |
| 1.5 | N/A | N/A | 8 | ||||||||||
| 3 | N/A | N/A | 4 | ||||||||||
| Ultra2 SCSI | Fast-40 | SPI-5 (INCITS 367-2003) | 50-pin | 8 | 40 | 40 | 320 | N/A | 12 | 25 | 8 | LVD 125 ± 10[3] | LVD 1.2 HVD ≥5 |
| Ultra2 Wide SCSI | SPI-5 (INCITS 367-2003) | 68-pin; 80-pin (SCA/SCA-2) | 16 | 40 | 80 | 640 | N/A | 12 | 25 | 16 | LVD 125 ± 10[3] | LVD 1.2 HVD ≥5 | |
| Ultra3 SCSI | Ultra-160; Fast-80 wide | SPI-5 (INCITS 367-2003) | 68-pin; 80-pin (SCA/SCA-2) | 16 | 40 DDR | 160 | 1280 | N/A | 12 | N/A | 16 | LVD 125 ± 10[3] | LVD 1.2 |
| Ultra-320 SCSI | Ultra-4; Fast-160 | SPI-5 (INCITS 367-2003) | 68-pin; 80-pin (SCA/SCA-2) | 16 | 80 DDR | 320 | 2560 | N/A | 12 | N/A | 16 | LVD 125 ± 10[3] | LVD 1.2 |
| Ultra-640 SCSI[F][G] | Ultra-5; Fast-320 | SPI-5 (INCITS 367-2003) | 68-pin; 80-pin | 16 | 160 DDR | 640 | 5120 | N/A | 10 | N/A | 16 | LVD 125 ± 10 | LVD 1.2 |
- ^ Specifications are maintained by the T10 subcommittee of the International Committee for Information Technology Standards.
- ^ Including any host adapters (i.e., computers count as a device).
- ^ For daisy-chain designs, length of bus, from end to end; for point-to-point, length of a single link.
- ^ LVD cabling may be up to 25 m when only a single device is attached to the host adapter, 20 m for Ultra-640.
- ^ The SCSI-1 specification has been withdrawn and is superseded by SCSI-2. The SCSI-3 SPI specification has been withdrawn and is superseded by SPI-2. The SCSI-3 SPI-3 and SPI-4 specifications have been withdrawn and are superseded by SPI-5.[2]
- ^ Ultra-640 substantially increases the requirements for cabling and backplanes, hampering a smooth transition.[4]
- ^ Ultra-640 was specified but no devices were produced.[5]
SCSI-1
The original SCSI standard, SCSI-1, was derived from the Shugart Associates System Interface (SASI) and formally adopted in 1986 by ANSI. SCSI-1 features an 8-bit parallel bus (with parity), running asynchronously at 3.5 MB/s, or 5 MB/s in synchronous mode, and a maximum bus cable length of 6 metres (20 ft), significantly longer than the 18 inches (0.46 m) limit of the ATA interface also popular at the time. A rarely-seen variation on the original standard featured high-voltage differential signaling and supported a maximum cable length of 25 metres (82 ft).[citation needed]
SCSI-2
SCSI-2 was introduced in 1994 and gave rise to the Fast SCSI and Wide SCSI variants. Fast SCSI doubled the maximum transfer rate to 10 MB/s while retaining the same 50-pin cables, while Wide SCSI doubled the bus width to 16 bits on top of that to reach a maximum transfer rate of 20 MB/s, using new 68-pin cables. However, these improvements came at the cost of reducing the maximum cable length to three meters. SCSI-2 also specified a 32-bit version of Wide SCSI, which used two 16-bit cables per bus. The 32-bit implementation was largely ignored because it was deemed expensive and unnecessary, and was officially retired in SCSI-3.
SCSI-2 expanded the command set with the Common Command Set (CCS) for better support of devices other than disk drives, introduced command queueing (up to 256 commands per device) and tightened up the requirements on some features that were optional in SCSI-1; parity was now mandatory and the host adapter was required to provide termination power in order to support active termination. SCSI-1 devices would generally remain compatible while simply ignoring the new features.[6]
A high-voltage differential (HVD) mode that was incompatible with standard single-ended (SE) was introduced to accommodate longer bus lengths.
SCSI-3
Before Adaptec and later the SCSI Trade Association codified the terminology, the first parallel SCSI devices that exceeded the SCSI-2 capabilities were simply designated SCSI-3. These devices, also known as Ultra SCSI[7] or Fast-20 SCSI,[8] were introduced in 1996. SCSI-3 itself is not as much a single document as a collection of various standards that have received updates at different points in time.
The bus speed was doubled again to 20 MB/s for narrow (8-bit) systems and 40 MB/s for wide (16-bit). The maximum cable length remained 3 meters but single-ended Ultra SCSI developed an undeserved reputation for extreme sensitivity to cable length and condition (faulty cables, connectors or terminators were often to blame for instability problems).
Unlike previous SCSI standards, SCSI-3 (Fast-20 speed) requires active termination.
Ultra-2
This standard was introduced c. 1997 and featured a low-voltage differential (LVD) bus. For this reason, Ultra-2 is sometimes referred to as LVD SCSI. LVD's greater resistance to noise allowed a maximum bus cable length of 12 meters. At the same time, the data transfer rate was increased to 80 MB/s. Mixing earlier single-ended devices (SE) and Ultra-2 devices on the same bus is possible but connecting only a single SE device forces the whole bus to single-ended mode with all its limitations, including transfer speed. The standard also introduced very-high-density cable interconnect (VHDCI), a very small connector that allows placement of four wide SCSI connectors on the back of a single PCI card slot. Ultra-2 SCSI actually had a relatively short lifespan, as it was soon superseded by Ultra-3 (Ultra-160) SCSI.
Ultra-3
Ultra-3 includes five new optional features:
- Doubling the transfer rate to 160 MB/s through the use of double-transition clocking
- CRC, a robust error-correcting process more suited for high-speed operation than the parity checking used previously
- Domain validation for negotiating maximum performance for each device on the chain
- Packetization protocol with a reduced number of bus communication phases for less command and protocol overhead
- Quick arbitration and selection reduces arbitration time by eliminating bus free time
First introduced as Ultra-160 toward the end of 1999, this iteration improved on the Ultra-2 standard adding the first three improvements.[9]
Devices supporting all five features were marketed as Ultra-160+ or Ultra-3 (U3). 8-bit bus width as well as HVD operation were eliminated starting with Ultra-3.[6]
Ultra-320
Ultra-320 included the Ultra-160+ features as mandatory, doubled the clock to 80 MHz for a maximum data transfer rate of 320 MB/s, and included read/write data streaming for less overhead on queued data transfers, as well as flow control.[6] The latest working draft for this standard is revision 10 and is dated May 6, 2002. Nearly all SCSI hard drives being manufactured at the end of 2003 were Ultra-320 devices.
Ultra-640
Ultra-640 (otherwise known as Fast-320) was promulgated as a standard (INCITS 367-2003 or SPI-5) in early 2003. It doubles the interface speed yet again, this time to 640 MB/s. Ultra-640 pushes the limits of LVD signaling; the speed limits cable lengths drastically, making it impractical for more than one or two devices. Because of this, manufacturers skipped over Ultra-640 and developed for Serial Attached SCSI instead.
SCSI signals
In addition to the data bus and parity signals, a parallel SCSI bus contains nine control signals:[10]
| Signal name | Meaning when asserted | Meaning when deasserted |
|---|---|---|
| BSY Busy | Bus in use | Bus free |
| SEL Select | Asserted by the winner of an arbitration, during selection by an initiator or reselection by a target | No device has control of the bus |
| RST Reset | Initiator forces all targets and any other initiators to do a warm reset | No reset requested |
| C/D Control/Data[A] | Bus contains control information | Bus contains data |
| I/O Input/Output[A] | Transfer is from target to initiator. Also asserted by a target after winning arbitration to indicate reselection of an initiator | Transfer is from initiator to target |
| MSG Message[A] | Bus contains a message | Bus contains data or command/status |
| REQ Request | Target requests initiator to transfer the next unit of information on the bus, as indicated by the 3 phase signals | Target not requesting transfer |
| ACK Acknowledge | Initiator acknowledges target request, completing the information transfer handshake | No acknowledge |
| ATN Attention | Asserted by an initiator after winning arbitration to select a target | No target selection in progress |
There are also three DC-level signals:
- TERMPOWER
- See § Termination
- DIFFSNS
- Grounded in single-ended buses, otherwise floats to a positive voltage
- GROUND
- Most spare pins in the connector are designated as grounds.
There are three electrically different variants of the SCSI parallel bus: single-ended (SE), high-voltage differential (HVD), and low-voltage differential (LVD). The HVD and LVD versions use differential signaling and so they require a pair of wires for each signal. So the number of signals required to implement a SCSI bus is a function of the bus width and voltage: