Parallel SCSI

History

The first two formal SCSI standards, SCSI-1 and SCSI-2, included parallel SCSI as a central part of the protocol. The SCSI-3 standard then split the framework into separate layers so parallel SCSI is now just one of a number of available implementations. See the main SCSI article for a complete list. As with all types of SCSI bus, parallel SCSI communication takes place between an initiator and a target.

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, as the 32-bit option was more expensive and was thus hardly ever used.

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 even have a physical definition at all but uses any IP network, usually based on Ethernet.

As of 2012, SCSI interfaces had become impossible to find for laptop computers. Adaptec had years before produced PCMCIA parallel SCSI interfaces, but when PCMCIA was superseded by the ExpressCard Adaptec discontinued their PCMCIA line without supporting ExpressCard. Ratoc produced USB and Firewire to parallel SCSI adaptors, but ceased production when the integrated circuits required were discontinued. Drivers for existing PCMCIA interfaces were not produced for newer operating systems.

Since 2013, with the release of various ExpressCard and Thunderbolt-to-PCI Express adapters, it is again possible to use SCSI devices on laptops, by installing PCI Express SCSI host adapters using a laptop's ExpressCard or Thunderbolt port.

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 (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 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 (i.e., SCSI 320 or SCSI 160) 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 (SCSITA).

As of 2003, 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 (LVD) 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 (including ones not otherwise specified by T10), 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.

SCSI-1

The original standard that 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 meters (just under 20 feet—compared to the 18 inch (0.45 meter) limit of the ATA interface). A rarely seen variation on the original standard included a high-voltage differential (HVD) 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 and Wide SCSI doubled the bus width to 16 bits on top of that to reach a maximum transfer rate of 20 MB/s.  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 expensive and unnecessary, and was officially retired in SCSI-3.

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 and fast-20 SCSI, were introduced in 1996. The bus speed doubled again to 20 MB/s for narrow (8 bit) systems and 40 MB/s for wide (16-bit). 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 (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. Ultra-2 SCSI actually had a relatively short lifespan, as it was soon superseded by Ultra-3 (Ultra-160) SCSI.

Ultra-3

Also known as Ultra-160 SCSI and introduced toward the end of 1999, this version was basically an improvement on the ultra-2 standard, in that the transfer rate was doubled once more to 160 MB/s by the use of double transition clocking. Ultra-160 SCSI offered new features like cyclic redundancy check (CRC), an error correcting process, and domain validation, a way to negotiate maximum performance for each device on the chain.

Ultra-320

This is the Ultra-160 standard with the data transfer rate doubled to 320 MB/s. 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 have skipped over Ultra640 and are developing for Serial Attached SCSI instead.

SCSI signals

In addition to the data bus and parity signals, a parallel SCSI bus contains nine control signals:

Notes: * One of 3 signals which are driven by a target during information transfer to indicate the Bus Phase

There are also three DC levels:

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:

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:

Parallel SCSI bus operation

The parallel SCSI bus goes through eight possible phases as a command is processed. Not all phases will occur in all cases:

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.

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(s). 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 (single-ended SCSI) or 600 mA (differential SCSI).

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 +3.3 V supply. Each signal line is then terminated by a 110 Ω resistor to the +3.3 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 (FPT) 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 +3.3 V linear regulator and commercially available SCSI resistor network devices (not individual resistors).

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 busses. 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 the slower single-ended speed. The SPI-5 standard (which describes Ultra-640) deprecates single-ended devices, so future 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.

SCA adapters

Single Connector Attachment (SCA) parallel SCSI devices may be connected to older controller/drive chains by using SCA adapters. Although these adapters often have auxiliary power connectors, caution is recommended when connecting them, as it is possible to damage devices by connecting external power.

Device IDs and termination

Each parallel SCSI device (including the computer's host adapter) must be configured to have a unique SCSI ID on the bus. Another requirement is that any parallel SCSI bus must be terminated at both ends with the correct type of terminator. Both active and passive terminators are in common use, with the active type much preferred (and required on LVD busses and Ultra SCSI). Improper termination is a common problem with parallel SCSI installations. In early SCSI busses, one had to attach a physical terminator to each end, but several generations' SCSI devices often have terminators built in, and the user simply needs to enable termination for the devices at either end of the bus (typically by setting a DIP switch or moving a jumper). Some later SCSI host adapters allow the enabling or disabling of termination through BIOS setup. Advanced SCSI devices automatically detect whether they are last on the bus and switch termination on or off accordingly.

SCAM

SCSI Configured Automatically (initially Automagically) was an optional method to configure the SCSI ID without requiring user intervention for easier installation and to avoid problems. It was dropped from later standards.