Parallel SCSI
Parallel SCSI 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 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. 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 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. The SCSI-2 standard allowed for faster operation and wider buses. 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 which, unfortunately, have confusing names. There are a dozen SCSI interface names, most with ambiguous wording ; three SCSI standards, each of which has a collection of modular, optional features; several different connector types; and three different types of voltage signalling. 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 in Megabytes per second.SCSI has evolved since its introduction. Before summarizing the evolution, a distinction should be made between the terminology used in the SCSI standard itself, as promulgated by the T10 committee of INCITS, and common parlance, as codified by the SCSI Trade Association.
, there have only been three SCSI standards: SCSI-1, SCSI-2, and SCSI-3. All SCSI standards have been modular, defining various capabilities which 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 signalling 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 signalling, 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 command set, allowing devices that support SCSI commands to use any interface, and also allowing the interfaces that are defined by T10 to develop on their own terms. This change is also why there is no "SCSI-4".
No version of the standard has ever specified what kind of connector should be used. See "Connectors," below.
Comparison table
SCSI-1
The original standard that was derived from the Shugart Associates System Interface and formally adopted in 1986 by ANSI. SCSI-1 features an 8-bit parallel bus, running asynchronously at 3.5 MB/s, or 5 MB/s in synchronous mode, and a maximum bus cable length of, significantly longer than the limit of the ATA interface also popular at the time. A rarely-seen variation on the original standard included a high-voltage differential implementation whose maximum cable length was 25 meters.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 CCS for better support of devices other than disk drives, introduced command queueing 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.
An HVD mode that was incompatible with standard SE was introduced to accommodate longer bus lengths.
SCSI-3
Before Adaptec and later SCSITA 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 or Fast-20 SCSI, 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 systems and 40 MB/s for wide. The maximum cable length stayed at 3 meters but single-ended Ultra SCSI developed an undeserved reputation for extreme sensitivity to cable length and condition.
Unlike previous SCSI standards, SCSI-3 requires active termination.
Ultra-2
This standard was introduced c. 1997 and featured a low-voltage differential 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 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 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 SCSI.Ultra-3
This standard featured five new optional features, which lead manufacturers to decide on two substandards to ensure consistent performance. First introduced as Ultra-160 toward the end of 1999, this iteration improved on the Ultra-2 standard by doubling the transfer rate to 160 MB/s through the use of double transition clocking, and offered new features such as CRC, a robust error correcting process more suited for high-speed operation than the parity checking used previously, and domain validation for negotiating maximum performance for each device on the chain.However, Ultra-160 merely represents a subset of Ultra-3, lacking its two other new features, packetization and QAS. Packetization refers to a protocol with reduced number of bus communication phases for less command and protocol overhead, while QAS reduces arbitration time by eliminating bus free time. Devices supporting these features were marketed as Ultra-160+ or U3. 8-bit bus width as well as HVD operation were eliminated starting with Ultra-3.
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. 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 was promulgated as a standard 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 Ultra640 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:| 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 | Bus contains control information | Bus contains data |
| I/O Input/Output | 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 | 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:
| Signal name | Usage |
| TERMPOWER | See the Termination section for details |
| 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, high-voltage differential, and low-voltage differential. 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:
| Bus Width | Voltage | Data | Parity | Control | TERMPOWER | DIFFSNS | GROUND | Reserved | Total |
| 8-bit | SE | 8 | 1 | 9 | 1 | 1 | 30 | 0 | 50 |
| 8-bit | LVD/HVD | 16 | 2 | 18 | 1 | 1 | 12 | 0 | 50 |
| 16-bit | SE | 16 | 2 | 9 | 4 | 1 | 34 | 2 | 68 |
| 16-bit | LVD/HVD | 32 | 4 | 18 | 4 | 1 | 7 | 2 | 68 |
SCSI IDs
All devices on a parallel SCSI bus must have a SCSI ID, which may be set by jumpers on older devices or in software. The SCSI ID field widths are:| Bus-width | ID width | IDs available |
| 8-bit | 3-bit | 8 |
| 16-bit | 4-bit | 16 |
Bus operation
The parallel SCSI bus goes through eight possible phases as a command is processed. Not all phases will occur in all cases:| Phase | Comments |
| Bus-free | This is the state in which no device communication is in process. |
| Arbitration | One or more devices attempt to obtain exclusive control of the bus by asserting /BSY and a single bit corresponding to the device SCSI ID. For example, a device with a SCSI ID of 2 would generate the inverted bit pattern 11111011 on the bus. |
| Selection | The arbitrating device with the highest ID takes control of the bus by asserting /BSY and /SEL. "Highest" on an eight bit bus starts from 7 and works downward to zero. On a 16 bit bus, the eight bit rule applies, followed by 15 and working downward to 8, thus maintaining backward compatibility on a bus with a mix of eight and 16 bit devices. The controlling device is now the "initiator." |
| Command | The initiator sends the command descriptor block to a "target," which is another device on the bus. The CDB tells the target what to do. |
| Reselection | During a transaction, the target device may be required to execute an operation that is slow in wall-clock time terms relative to the speed of the bus. In such a case, the target may temporarily disconnect from the bus, causing the latter to go to the bus-free condition and allowing other unrelated operations to take place. Reselection is the phase where the target reconnects to the initiator to resume the previously suspended transaction. |
| Data | In this phase, data is transferred between initiator and target, the direction of transfer depending on the command that was issued. For example, a command to read a sector from a disk would result in a transfer from the disk to the host. Or, if an error occurred, the initiator could send a "request sense" command to the target for details, the latter which would be returned during the data phase. |
| Message | A message code is exchanged between initiator and target for the purposes of interface management. |
| Status | A status code is sent to the initiator to report the success or failure of the operation. |
The above list does not imply a specific sequence of events. Following a command to a target to send data to the initiator and a receipt of a command complete status, the initiator could send another command or even send a message.
External connectors
No version of the standard has ever specified what kind of connector should be used. Specific types of connectors for parallel SCSI devices were developed by vendors over time. Connectors for serial SCSI devices have diversified into different families for each type of serial SCSI protocol. See the SCSI connector article for a more detailed description.Although parallel SCSI-1 devices typically used bulky Blue Ribbon Amphenol connectors, and SCSI-2 devices typically used Mini-D connectors, it is not correct to refer to these as "SCSI-1" and "SCSI-2" connectors. One valid rule is that connectors for wide SCSI buses have more pins and wires than those for narrow SCSI buses. An Amphenol-50 or HD-50 connector is for narrow SCSI, while an Amphenol-68 or HD-68 connector is for wide SCSI. On some early devices, wide parallel SCSI buses used two or four connectors and cables while narrow SCSI buses used only one.
The first parallel SCSI connectors were the Amphenol type. They then evolved through two main stages, High-Density and most recently SCA - 80 pin.
With the HD connectors, a cable normally has male connectors while a SCSI device has female. A female connector on a cable is meant to connect to another cable.
Termination
Parallel SCSI buses must always be terminated at both ends to ensure reliable operation. Without termination, data transitions would reflect back from the ends of the bus causing pulse distortion and potential data loss.A positive DC termination voltage is provided by one or more devices on the bus, typically the initiator. This positive voltage is called TERMPOWER and is usually around +4.3 volts. TERMPOWER is normally generated by a diode connection to +5.0 volts. This is called a diode-OR circuit, designed to prevent backflow of current to the supplying device. A device that supplies TERMPOWER must be able to provide up to 900 mA or 600 mA.
Some early disk drives included internal terminators, but most modern disk-drives do not provide termination which is then deemed to be external.
Termination can be passive or active. Passive termination means that each signal line is terminated by two resistors, 220 Ω to TERMPOWER and 330 Ω to ground. Active termination means that there is a small voltage regulator which provides a +2.85 V supply. Each signal line is then terminated by a 110 Ω resistor to the +2.85 V supply. Active termination provides a better impedance match than passive termination because most flat ribbon cables have a characteristic impedance of approximately 110 Ω. Forced perfect termination is similar to active termination, but with added diode clamp circuits which absorb any residual voltage overshoot or undershoot. There is a special case in SCSI systems that have mixed 8-bit and 16-bit devices where high-byte termination may be required.
In current practice most parallel SCSI buses are LVD and so require external, active termination. The usual termination circuit consists of a +2.85 V linear regulator and commercially available SCSI resistor network devices.
Terminators themselves must be matched to the SCSI bus. Using an SE terminator on an LVD bus causes the bus to drop back to SE speeds, even if all other devices and cables are capable of LVD operation – the same effect any other SE device has. Passive terminators may cause Ultra speed communication to be unreliable. Generally, and reflecting the order in which each type of terminator was introduced, unmarked terminators are passive, those marked only "active" are SE, and only those marked LVD will correctly terminate an LVD bus and allow it to operate at full LVD speeds.
Compatibility
For purposes of discussing compatibility, remember that SCSI devices include both host adapters and peripherals such as disk drives. When you ask whether you can cable a certain host adapter to a certain disk drive, you are asking whether you can attach those two SCSI devices to the same SCSI bus.Different SCSI transports, which are not compatible with each other, usually have unique connectors
to avoid accidental mis-plugging of incompatible devices. For example, it is not possible to plug a parallel SCSI disk into an FC-AL backplane, nor to connect a cable between an SSA initiator and an FC-AL enclosure.
Mixing different speeds
SCSI devices in the same SCSI transport family are generally backward-compatible. Within the parallel SCSI family, for example, it is possible to connect an Ultra-3 SCSI hard disk to an Ultra-2 SCSI controller albeit with reduced speed and feature set.Mixing Single-Ended and Low Voltage Differential
However, there are some compatibility issues with parallel SCSI buses. Ultra-2, Ultra-160 and Ultra-320 devices may be freely mixed on the parallel LVD bus with no compromise in performance, as the host adapter will negotiate the operating speed and bus management requirements for each device. Single-ended and LVDS devices can be attached to the same bus, but all devices will run at a slower, single-ended speed.On some host adapters, this problem is solved by using a SCSI bridge to electrically split the bus into an SE and an LVD half, so LVD devices can operate at full speed. Other adapters may provide multiple buses.
The SPI-5 standard deprecates single-ended devices, so some devices may not be electrically backward compatible.
Mixing Wide and Narrow
Both narrow and wide SCSI devices can be attached to the same parallel bus. All the narrow SCSI devices must be placed at one end and all the wide SCSI devices at the other end. The high half of the bus needs to be terminated in between because the high half of the bus ends with the last wide SCSI device. You can get a cable designed to connect the wide part of the bus to the narrow part which either provides a place to plug in a terminator for the high half or includes the terminator itself. This is sometimes referred to as a cable with high-9 termination. Specific capability commands allow the devices to determine whether their partners are using the whole wide bus or just the lower half and drive the bus accordingly.As an example of a mixed bus, consider a SCSI wide host adapter with an HD-68 male connector connected to a SCSI narrow disk drive with an HD-50 female connector. You might make this connection with a cable that has an HD-68 female connector on one end and an HD-50 male connector on the other. Inside the cable's HD-68 connector, there is termination for the high half of the bus and the cable contains wires for only the low half. The host adapter determines that the disk drive uses only the low half of the bus, so talks to it using only the lower half. The converse example—a SCSI narrow host adapter and SCSI wide disk drive also works.
Alternatively, each narrow device can be attached to the wide bus through an adapter. As long as the bus is terminated with a wide - internal or external - terminator, there is no need for special termination.