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SCSI connector
SCSI connector
SCSI connector
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A SCSI connector (/ˈskʌzi/ SKUZ-ee) is used to connect computer parts that communicate with each other via the SCSI standard. Generally, two connectors, designated male and female, plug together to form a connection which allows two components, such as a computer and a disk drive, to communicate with each other. SCSI connectors can be electrical connectors or optical connectors. There have been a large variety of SCSI connectors in use at one time or another in the computer industry. Twenty-five years of evolution and three major revisions of the standards resulted in requirements for Parallel SCSI connectors that could handle an 8, 16 or 32 bit wide bus running at 5, 10 or 20 megatransfer/s, with conventional or differential signaling. Serial SCSI added another three transport types, each with one or more connector types. Manufacturers have frequently chosen connectors based on factors of size, cost, or convenience at the expense of compatibility.

SCSI makes use of cables to connect devices. In a typical example, a socket on a computer motherboard would have one end of a cable plugged into it, while the other end of the cable plugged into a disk drive or other device. Some cables have different types of connectors on them, and some cables can have as many as 16 connectors (allowing 16 devices to be wired together). Different types of connectors may be used for devices inside a computer cabinet, than for external devices such as scanners or external disk drives.

Nomenclature

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Many connector designations consist of an abbreviation for the connector family, followed by a number indicating the number of pins. For example, "CN36" (also written "CN-36" or "CN 36") would be a 36-pin Centronics-style connector. For some connectors (such as the D-subminiature family) use of the hyphen or space is more common, for others (like the "DD50") less so.

Parallel SCSI

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Main article: Parallel SCSI
A stack of external SCSI devices displaying various SCSI connectors

Parallel SCSI (SCSI Parallel Interface SPI) allows for attachment of up to 8 devices (8-bit Narrow SCSI) or 16 devices (16-bit Wide SCSI) to the SCSI bus. The SCSI Host controller takes up one slot on the SCSI bus, which limits the number of devices allowed on the bus to 7 or 15 devices respectively. SCSI Host Controllers may have multiple SCSI buses (e.g. Adaptec AHA-2940) to allow more SCSI devices to be attached.

Internal

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IDC header

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Early generations of SCSI hard drive assemblies generally had two connectors (power and communication). Some very early 16-bit units used two data connectors, with three connectors in total. The power connector was typically the same 4-pin female Molex connector used in many other internal computer devices. The communication connectors on the drives were usually a 50 (for 8-bit SCSI) or 68 pin male (for 16-bit SCSI) "IDC header" which has two rows of pins, 0.1 inches (2.54 mm) apart. This connector has no retaining screws to secure the connectors together, and ribbon cables are both inconveniently wide and somewhat delicate, so this connector style was primarily used for connections inside of a computer or peripheral enclosure (as opposed to connecting two enclosures to each other). Thus it is often called an "internal SCSI connector." This type of header was used in a typical desktop PC until around 2010, including the 40-pin (two rows of 20) version used for ATA fixed and optical disk drives.

While the female connector is slotted such that a cable with a matching keyed male connector can not be inserted upside-down, some manufacturers (including Sun Microsystems) supplied internal cables with male connectors that did not have the key, allowing for incorrect (and possibly damaging) connections.

In most cases, the host adapter would have a similar header-style connection. In some cases, though, the host adapter end of the cable would use a different connector. For example, in the Sun 260 series chassis (used for the Sun 3/260 and Sun 4/260 computers), the connector was the same 3-row 96-pin connector used to attach peripheral cards to the VMEbus backplane.

SCA

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Eventually, there was a desire to combine power and data signals into a single connector. This allows for quick drive replacement, more reliable connections, and is more compact. Most late parallel SCSI disk-drives utilize an 80-pin SCA (Single Connector Attachment) connector.[citation needed] This connector includes a power connection and also has long and short pins which enable hot swapping. Note that this connector is primarily found on disk drive HDAs (and of course the mating enclosure backplane connector).

External

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From top to bottom: VHDCI, HD50, HD68, CN50

Most typically, external drive enclosures will have female connectors, while cables will have two male connectors. As with everything SCSI, there are exceptions.

First generation

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Standardization was perhaps less consistent in the early days of SCSI manufacture.

Early SCSI interfaces commonly used a 50-pin micro ribbon connector. This connector is similar to the 36-pin connector used by Centronics for the parallel interface on their printers, thus the connector became popularly known as "Centronics SCSI" or "CN-50". It is also referred to as a "SCSI-1 connector"; since many connectors have been used for SCSI-1, this can be confusing.

SCSI-1 card with an external Centronics port which requires a terminator, from an Acorn computer.
Old Macintosh DB-25 SCSI port (narrow)

Apple used DB-25 connectors, which, having only 25 pins rather than 50, were smaller and less expensive to make, but decreased signal integrity (increasing crosstalk)[citation needed] and cannot be used with differential signaling. Furthermore, DB-25s were commonly used for RS-232 serial cables and also to connect parallel printers, meaning that users might accidentally try to use completely inappropriate cables, since the printer and serial cables would fit the connector properly and be hard to visually distinguish.

Sun Microsystems and Data General used a 50-pin 3-row DD-50 connector, which was sometimes incorrectly called a "DB-50" or "HDB-50". Sun also used DB-25s on a few products.

Digital Equipment Corporation mostly used the CN-50, but the VAXstation 3100 and DECstation 3100/2100 made use of a MALE 68-pin connector on the rear of the workstation. This connector looks like it would be a high density Wide SCSI-2 connector, but is actually 8-bit SCSI-1.

Macintosh HDI-30 SCSI Male Connector (narrow)

Apple Macintosh laptops used a squarish external SCSI connector called an HDI-30 (High Density Interconnect) on the laptop itself (not on the peripheral end of the cable, unless two laptops were being connected). These machines also had the interesting ability to become "SCSI slaves" (officially known as "SCSI Disk Mode" in Apple documentation), meaning that they could appear to be disk drives when attached to another computer's SCSI controller (a feature later reimplemented over FireWire and Thunderbolt for later, non-SCSI Mac hardware).

IBM's early RS6000 workstations sometimes used a "High Density Centronics" connector,[citation needed] more correctly known as a Mini Delta Ribbon (MDR) connector which was a Centronics-style connector with smaller pins and shell. It had 60 pins and is thus known as the "HDCN60".

Certain Japanese digital camera manufacturers wanted to put SCSI into their equipment, but conventional connectors would have been too large. Like IBM, they used a miniaturized Centronics connector, but this one had 50 pins and was called the "HPCN50".[citation needed]

Some manufacturers used a DC-37 connector, often incorrectly referred to as a DB-37. These will most commonly be seen on three-cable systems, which are typically 16-bit or 32-bit "Wide SCSI" systems. Extra confusion is generated here since this connector was also frequently used with SMD disk drives, which are completely incompatible with SCSI drives.

SCSI-2

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With the arrival of SCSI-2, the situation was a bit less chaotic. For narrow SCSI, most manufacturers used the Amplimite .050 connector, also sometimes referred to as a High Density or HD50. This connector has two rows of 25 pins and a trapezoidal (D-shaped) shell, and is about 1 3/8” (36mm) wide.[1]

A few vendors did use the Micro Centronics 50, also known as Mini Delta Ribbon,[2] and IBM continued to use the HDCN60 on some RS-6000 systems.

The Amplimite and MDR connectors are similar in shape and size, but can be distinguished by the former using pin contacts and the latter using wipers.

For Wide SCSI-2, the most common connector was the larger 68-pin sibling of the HD50, known as the HD68, MiniD68, HPDB68, and sometimes as "SCSI-3". This is about 1 7/8” (47mm) wide.[3] IBM used the HDCN68 on some RS-6000 systems, and it seems likely that a few other manufacturers used other alternatives.

Post SCSI-2

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SCA-2 connector on Fujitsu MAP3735NC

As time went on, some manufacturers desired connectors even smaller than the SCSI-2 connector. One such in somewhat common use was the VHDCI (Very High Density Cable Interconnect) connector, also known as an "AMP HPCN68M", and sometimes as "SCSI-5". There are 68 pins on the connector in two rows; the pins are 0.8 mm apart. This connector is reputed to suffer fewer bent pins than the 68-pin SCSI-2 connector despite its minuscule pins.

Interoperability

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There are adapters between most types of parallel SCSI connector, and some companies will manufacture custom cables to guarantee having the correct connectors. An adapter from narrow to wide must include termination to work properly.

Different SCSI standards use the same SCSI connectors as in HVD and LVD SCSI (High-Voltage Differential and Low-Voltage Differential) . HVD uses 15V while LVD uses 3.3V, so connecting an HVD device to an LVD host bus adaptor can blow the line drivers on the HBA, likewise an HVD HBA connected to an LVD device.

Similarly, connecting a single-ended device (SE) onto a LVD SCSI chain will cause the bus to fall back to single-ended mode, removing the ability to run faster than Ultra speed (20 MHz) and possibly causing an unstable bus for exceeding SE limits.

While interconnectivity of a number of devices may look straightforward, there are many pitfalls, and with older SE devices the cabling length becomes an issue as signal degrades.

Drive caddies

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Many manufacturers have devised systems in which a SCSI disk drive or other device was placed in a small "caddy" container (also called a "drive sled"), which carried connections for both power and data. The caddy or canister would be placed in a larger enclosure. Some of these systems allowed for hot swap (drives could be replaced with the system running), while others allowed "warm swap", in which the SCSI bus was "quiesced" (meaning all drive activity was stopped) but remained powered on with devices ready.

SCA 80 pin Connector – on hot-plug drive (HP/Compaq or DELL)

Digital Equipment Corporation's StorageWorks products were one system of this type. DEC briefly allowed third parties to license this system, but reversed the decision after less than a year; as a result, third-party StorageWorks products are quite rare. Compaq also made a drive caddy system for the Proliant line of servers. Compaq purchased DEC, and Hewlett-Packard later purchased Compaq, and the Proliant and StorageWorks names were reused on other storage products, including later hot-swap systems.

Some of these caddy systems were OEM manufactured, which means that the same product could appear with numerous brand names and model identifications. These Hot-Plug drives in caddies generally use 80 pin SCA connectors (HP, Compaq, DELL from SCSI-3 to Ultra-320)

Single Connector Attachment

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SCSI hard drives showing 80-pin SCA connector (top), and separate 68-pin and power connectors plus configuration jumpers (bottom)
SCSI backplane with 80-pin SCA connectors. Hard Drives are mounted on proprietary hot-swappable caddies.

Single Connector Attachment, or SCA, is a type of connection for the internal cabling of Parallel SCSI systems. There are two versions of this connector: the SCA-1, which is deprecated, and SCA-2, which is the most recent standard. In addition there are Single-Ended (SE) and Low Voltage Differential (LVD) types of the SCA.

SCA is no longer in widespread use, having been superseded by Serial Attached SCSI (SAS).

Since hard disk drives are among the components of a server computer that are the most likely to fail, there has always been demand for the ability to replace a faulty drive without having to shut down the whole system. This technique is called hot-swapping and is one of the main motivations behind the development of SCA. In connection with RAID, for example, this allows for seamless replacement of failed drives.

Normally, hard disk drives make use of two cables: one for data and one for power, and they also have their specific parameters (SCSI ID etc.) to be set using jumpers on each drive. Drives employing SCA have only one plug which carries both data and power and also allows them to receive their configuration parameters from the SCSI backplane. The SCA connector for parallel SCSI drives has 80 pins, as opposed to the 68 pin interface found on most modern parallel SCSI drives.

Some of the pins in SCA connectors are longer than others, so they are connected first and disconnected last. This ensures the electrical integrity of the whole system. Otherwise, the angle at which the plug is inserted into the drive could be the reason for damage because, for instance, the pin carrying the voltage could get connected before its corresponding ground reference pin. The additional length also provides what is known as a pre-charge which provides a means whereby the device is alerted to a pending power surge. That allows a slower transition to full power and thereby makes the device more stable.

SPARCstation 20 drive in cradle

To make better use of their hot-plugging capability, SCA drives usually are installed into drive bays into which they slide with ease. At the far end of these bays is the backplane of the SCSI subsystem located with a connector that plugs into the drive automatically when it is inserted.

Full hot-swappable functionality still requires the support of other software and hardware components of the system. In particular the operating system and RAID layers will need hot-swap support to enable hard drive hot-swapping to be carried out without shutting down the system.

Serial SCSI

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Serial SCSI disk-drives use smaller connectors due to the reduced number of signals required. There are three types of physical layer transports specified:

Additionally, there is the iSCSI transport, which is not present on the drives themselves, but is used to connect devices using TCP/IP networks. The drives themselves would use one of the other three connector types.

Connectors on internal drives

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Internal SFF-8482 connector on a SAS HDD
  • Fibre Channel FC-AL disk-drives include a 40-pin SCA-2 connector
  • SSA disk drives include a "unitized" composite connector
  • SAS disk drives have an SFF-8482 connector. This is "form factor compatible" with the connector on SATA disk drives, meaning that a SATA drive may be installed in an SAS drive bay, and the enclosure can use the Serial ATA Tunneling Protocol (STP) to make use of the drive.[4][5]: 16, 17  There are keyed parts to the connector on an SAS drive that will prevent it from being inserted into a SATA drive bay.[5]: 15 
  • iSCSI isn't used for connecting disk drives internally

External connectors

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  • Fibre Channel
    • FC-AL cables initially used DE-9 connectors (electrical) or SC connectors (optical)
    • More recent FC-AL cables use HSSDC connectors (electrical) or LC connectors (optical).
    • Many FC-AL products now use an intermediate device called a GBIC (GigaBit Interface Converter) which allows more flexibility. GBICs can interconnect with a range of Small Form-factor Pluggable (SFP) connectors.
  • SAS interconnections use either
    An SFF-8484 connector.
    • SFF-8484 multilane unshielded serial attachment connector (internal connector)
    • SFF-8470 multilane copper connector, also known as an Infiniband connector (external connector)
    • SFF-8087 Molex iPASS unshielded mini-multilane, reduced width internal connector
    • SFF-8088 Molex iPASS shielded mini-multilane, reduced width external connector
  • SSA cables are terminated with 9-pin micro-D connectors
  • iSCSI may be interconnected by any means used to build a TCP/IP network, since the SCSI commands are simply being carried over TCP/IP. Ethernet is the predominantly used physical layer.

Drive caddies

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The situation is fundamentally similar to that of Parallel SCSI drive caddies; there have been a range of manufacturers, and the caddies themselves contain a generic device (with one of the standard internal connectors) which can be removed and replaced.

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

SCSI connector

View on Grokipedia
from Grokipedia
The SCSI connector is a type of electrical interface used within the Small Computer System Interface (), a set of standards originally using a parallel bus architecture developed for connecting computers to peripheral devices such as hard disk drives, tape drives, scanners, printers, and drives, allowing for efficient data transfer and device control. Defined by ANSI standards starting with X3.131-1986 (also known as SCSI-1), these connectors facilitate daisy-chaining up to eight devices (or 16 in wide configurations) on a shared bus, with support for both single-ended and differential signaling to manage over distances up to 25 meters in differential mode. SCSI connectors evolved alongside the interface's versions, beginning with the original SCSI-1 specification in 1986, which supported data transfers at up to 5 MB/s (synchronous) using 8-bit buses, and progressing through SCSI-2 (1990) and SCSI-3 (1996) to include enhanced and faster synchronous modes, wider 16-bit buses, and enhanced command sets for multi-device environments. Early connectors were predominantly 50-pin designs for 8-bit systems, while later iterations introduced 68-pin variants for wide SCSI to double throughput to 10–40 MB/s or more. Common physical forms include the Centronics 50-pin (external, unshielded), DB-25 (compact external for Macintosh systems), high-density 50-pin (external shielded), and high-density 68-pin (for internal and external wide applications), with pin assignments handling signals like data bits (DB0–DB15), control lines (e.g., REQ, ACK), and parity for error detection. These connectors adhere to strict mechanical and electrical requirements, such as 2.54 mm pin spacing in non-shielded versions, 100-ohm cable impedance, and termination resistors (e.g., 220Ω pull-up for single-ended) to prevent signal reflections, ensuring reliable operation in enterprise and server settings where parallel SCSI dominated before being largely supplanted by serial interfaces such as SATA (for consumer applications) and SAS (a serial evolution of SCSI for enterprise) in the 2000s. Internal variants like IDC 50-pin ribbon cables and SCA 80-pin (which integrate power and hot-swapping) further supported dense, high-performance storage arrays, while external options emphasized robustness for daisy-chaining. Despite the obsolescence of parallel SCSI in consumer applications, SCSI connectors—including those for serial implementations—remain notable for enabling architectures that influenced modern storage interfaces.

Nomenclature

Naming conventions

SCSI connectors are commonly named using an abbreviation denoting the connector family or type, followed by the pin count to indicate the specific configuration. For instance, the Centronics-style 50-pin connector is often abbreviated as CN50 or C50, while the high-density 68-pin variant is designated HD68. These abbreviations facilitate quick identification in technical documentation and hardware specifications, emphasizing both the physical interface style and the number of pins for compatibility assessment. D-subminiature connectors, frequently used in applications, exhibit variations in hyphenation and lettering based on shell size and pin arrangement. The standard 25-pin version is typically named DB-25, incorporating a and "B" to reflect its shell size with two rows of pins. In contrast, the 50-pin three-row D-sub, employed in some narrow setups, is denoted as DD50 without a , where the double "D" signifies the larger D-shell accommodating the additional row. Informal and vendor-specific names further diversify SCSI connector , often reflecting adaptations. Early Apple Macintosh systems utilized a DB-25 connector for external SCSI interfaces, sometimes referred to in documentation simply as the Macintosh SCSI port without additional qualifiers. Similarly, Sun and Data General employed a three-row 50-pin D-sub connector, commonly called the Sun 50-pin or DB34 variant for their workstations, distinguishing it from standard configurations. The evolution of naming parallels the progression of SCSI standards, distinguishing between narrow (8-bit) and wide (16-bit) buses. In SCSI-1, primarily narrow implementations used 50-pin connectors like CN50 for low-density external connections. SCSI-2 introduced high-density options such as HD50 for continued narrow use, while establishing 68-pin HD68 for wide buses to support expanded data paths. Subsequent standards, including SCSI-3, refined these with terms like VHD68 for very high-density 68-pin wide connectors, emphasizing increased performance without altering the core abbreviation-pin count pattern. The Small Computer System Interface (SCSI) began with the ANSI X3.131-1986 standard, known as SCSI-1, which defined an 8-bit parallel bus operating at 5 MHz for transfer rates up to 5 MB/s. This standard established the foundational protocol for connecting computers to peripherals like hard drives and scanners. Subsequent enhancements came with SCSI-2 under ANSI X3.131-1994 (later INCITS 131-1994), introducing Fast SCSI for doubled synchronous transfer rates at 10 MHz and support for a 16-bit wide bus to increase throughput to 10 MB/s in narrow configurations or 20 MB/s in wide setups. These updates also standardized command sets and electrical interfaces to improve reliability and device compatibility. The SCSI-3 family, developed from the mid-1990s, modularized the architecture into separate standards, with the SCSI Parallel Interface (SPI) series focusing on parallel implementations. SPI progressed through five generations: SPI-1 (INCITS 280-1996) for Fast-20 (Ultra) at 20 MB/s narrow or 40 MB/s wide, SPI-2 (INCITS 320-1998) for Ultra2 at 40 MB/s narrow or 80 MB/s wide, SPI-3 (INCITS 336-2000) for Ultra160 at 160 MB/s wide, SPI-4 (INCITS 362-2002) for Ultra320 at 320 MB/s wide, and SPI-5 (INCITS 367-2003) enabling Ultra640 at 640 MB/s wide through double-edge clocking and advanced error correction. These evolutions maintained while pushing parallel bus limits for enterprise storage. Parallel SCSI transitioned to serial standards in the late 1990s and early , with (SAS) emerging as the primary successor under INCITS 417-2006 for SAS-1.1, using point-to-point serial links and connectors defined in the SFF-848x series by the Small Form Factor (SFF) committee. Related serial protocols included Serial Storage Architecture (SSA), an early IBM-developed ring topology standard (ANSI X3.295-1996), and Arbitrated Loop (FC-AL), which adapted SCSI commands over fiber-optic serial channels for higher-speed networking (INCITS 303-1998). By the early , parallel SPI standards were largely deprecated in new designs in favor of SAS, which offered better scalability, longer cable lengths, and compatibility with SATA drives.

Parallel SCSI connectors

Internal IDC connectors

Internal IDC connectors, also known as Insulation Displacement Connectors, are used for internal cabling in parallel SCSI systems, facilitating connections between host adapters and peripheral devices such as hard drives and tape units within computers. These connectors employ a design where the insulation of the cable is displaced to make with the conductors, eliminating the need for . They lack shielding and securing screws, relying instead on friction fit for stability, which made them suitable for compact internal environments but limited their use to shorter cable lengths to minimize signal interference. The 50-pin IDC connector supports 8-bit Narrow configurations as defined in SCSI-1 and SCSI-2 standards, featuring two rows of 25 pins with 0.1-inch (2.54 mm) spacing. Male headers of this type are typically mounted on host adapters or controllers, mating with female IDC ends on the . This setup enabled data transfer rates up to 5 MB/s in early implementations and was widely adopted in personal computers and entry-level servers during the 1980s and 1990s. For higher-performance applications, the 68-pin IDC connector accommodates 16-bit Wide from SCSI-2 onward, including variants like Fast Wide and Ultra Wide, with the same 0.1-inch pin spacing but additional pins for the expanded data bus. These connectors, also with male headers on host adapters, supported transfer rates up to 40 MB/s in Ultra Wide configurations and remained in use through the early , particularly in workstations and mid-range servers. Both 50-pin and 68-pin variants meet ANSI X3.131-1986 specifications for electrical and mechanical reliability, with a current rating of 1.0 A per contact and operating temperatures from -40°C to +105°C. Power delivery to SCSI devices connected via IDC is handled separately from the data signals, using a standard 4-pin providing +5 V and +12 V DC supplies, with pin assignments including two grounds, +5 V, and +12 V to meet drive requirements. This separation allowed flexibility in cabling but required additional connections in system builds. Internal IDC-based SCSI cabling persisted in PCs and servers until around 2010, when serial interfaces like SAS largely supplanted them, though SCA connectors offered an alternative for integrated data and power in designs.

SCA connectors

The Single Connector Attachment (SCA) design for parallel SCSI backplanes integrates data signaling, power delivery, and grounding into a single 80-pin interface, enabling direct attachment of drives without separate cables. Defined under the SCSI-3 Parallel Interface (SPI) standards, the SCA-2 variant specifies 68 pins for wide SCSI data signals (supporting 16-bit transfers), 4 primary power pins (typically +5V and +12V), and 8 pins dedicated to ground and termination power (termdag) functions, with additional grounds distributed across the connector for ESD protection and . This configuration adheres to SFF-8046 specifications, ensuring compatibility with enterprise backplanes in arrays and servers. A key feature of SCA-2 connectors is the use of staggered pin lengths on the host receptacle—longer pins for power and ground to establish connections first, followed by shorter signal pins—to enhance insertion stability and prevent damage during hot-swapping. This power-first sequencing, combined with pre-charge circuitry and alignment guides, supports blind mating and hot-plug operations as mandated by SPI Annex hot-plug cases 1 and 2. SCA-2 supports both single-ended (SE) and low-voltage differential (LVD) signaling, achieving transfer rates up to Ultra- (320 MB/s) over the parallel bus. In contrast, the earlier SCA-1 used uniform pin lengths without these advanced grounding and sequencing features, making it less suitable for reliable hot-plugging and leading to its in favor of SCA-2 for modern implementations. SCA connectors are primarily deployed in high-availability enterprise environments, such as storage subsystems and server s, where their integrated design simplifies cabling and facilitates drive hot-swapping without system interruption. Unlike non-integrated alternatives like internal IDC connectors that require separate power cables, SCA enables seamless backplane attachment for multiple drives, improving density and maintenance in data centers.

External connectors

External connectors for parallel SCSI systems provide interfaces for linking host adapters to peripherals outside the chassis, such as hard disk enclosures, tape drives, and scanners, using shielded cables to maintain and support daisy-chaining up to eight (narrow) or 16 (wide) devices. These connectors accommodate single-ended (SE), low-voltage differential (LVD), and high-voltage differential (HVD) signaling, with maximum lengths of 6–12 meters for SE/LVD and up to 25 meters for HVD, per ANSI SCSI standards (X3.131 and later). Common retention methods include thumbscrews, bails, or latches for secure connections in enterprise and settings. Key types include:
  • Centronics 50-pin (CN50): A 50-pin unshielded connector with two rows of 25 pins, used for narrow (8-bit) SE SCSI-1 and early SCSI-2 external applications. Its large trapezoidal design with bail clips supports asynchronous/synchronous transfers up to 5–10 MB/s over short distances, common on older scanners and controllers.
  • DB-25: A compact 25-pin connector for narrow SE external SCSI, particularly on Macintosh systems, which compresses the 50-pin signals into 25 pins using twisted-pair cabling. It enables connections up to 6 meters at speeds up to 5 MB/s, with screw retention for portability.
  • High-Density 50-pin (HD50 or HPDB50): A shielded 50-pin mini-D connector (two rows of 25 pins) for narrow SE and LVD SCSI-2/3, offering improved protection over CN50. Used for external tape drives and enclosures supporting up to Ultra SCSI (20 MB/s), with thumbscrew fastening and compatibility up to 12 meters in LVD mode.
  • High-Density 68-pin (HD68 or HPDB68): A 68-pin mini-D connector (two rows of 34 pins) for wide (16-bit) SE and LVD SCSI-2/3, facilitating Fast/Wide and Ultra Wide transfers up to 40 MB/s. Shielded design suits external arrays and scanners, with thumbscrews and support for daisy-chaining over 12 meters in LVD.
  • Very High Density Counter Inserted (VHDCI): A compact 68-pin connector with 0.8 mm pitch for wide LVD SCSI-3 (Ultra2 and later), enabling high-speed external connections up to 80 MB/s (Ultra2 Wide) in workstations and servers. Features integrated shielding, latch retention, and dense cabling for tape libraries or enclosures, compatible with 12-meter LVD runs.
For HVD signaling in external setups, D-sub variants like DB-25 or specialized 50-pin differential connectors are used, though SE and LVD predominated in later external applications. Adapters bridge different connector types and signaling modes, but require matching termination to prevent reflections. These connectors were prevalent from the through the early 2000s before serial interfaces displaced .

Interoperability

Interoperability among parallel SCSI connectors often requires adapters to bridge different physical interfaces, enabling connections between legacy and modern devices. For instance, adapters convert DB-25 connectors, common in early external SCSI-1 setups, to CN50 (Centronics 50-pin) for internal narrow SCSI compatibility, while HD50 (High-Density 50-pin) to VHDCI (Very High-Density Counter Inserted, 68-pin) adapters facilitate wide SCSI-3 integration with narrower systems. These adapters ensure proper pin mapping but necessitate careful attention to signaling and termination to avoid data corruption or hardware damage. Signaling incompatibilities pose significant challenges in mixed parallel SCSI environments, particularly between High-Voltage Differential (HVD) and Low-Voltage Differential (LVD) systems. HVD operates at 15-18V differential signaling for longer cable runs up to 25 meters but is electrically incompatible with LVD's 3.3V low-voltage differential, which supports up to 12 meters at higher speeds like 80 MB/s; direct mixing without converters causes LVD devices to shut down on HVD buses or vice versa, potentially damaging transceivers. Converters, such as HVD-to-LVD modules, allow legacy HVD hosts to interface with LVD drives by translating voltage levels, though they add latency and cost. Single-Ended (SE) signaling further complicates when integrated with LVD backplanes, as the bus automatically reverts to SE mode upon detection, capping speeds at 20 MHz (Ultra SCSI) and reducing maximum cable length to 6 meters, while mismatched connections risk or component failure. SE devices on LVD systems require multimode terminators to handle the transition safely, but improper setup can lead to signal reflections and unreliable operation. Proper termination is essential for stable daisy-chain configurations in , with requirements varying by signaling type and position. Active terminators, which use voltage regulators for precise signal clamping, are recommended for LVD and mixed SE/LVD setups to minimize , whereas passive terminators relying on resistors suffice for simpler SE chains but offer less stability over longer distances; terminators must be installed only at the physical ends of the bus, with intermediate devices disabled to prevent reflections. In mixed setups, multimode active terminators support both SE and LVD, automatically switching based on bus detection. Device ID addressing ensures unique identification in chains, with narrow (8-bit) configurations using IDs 0-7 and wide (16-bit) supporting 0-15, allowing mixed narrow and wide devices via adapters that map unused pins. The host adapter typically claims ID 7 for priority, and all devices must have distinct IDs to avoid bus contention; parity checking, enabled across mixed setups for detection on and command lines, verifies transmission but requires consistent to prevent failures in heterogeneous chains.

Auxiliary parallel systems

Drive caddies

Drive caddies for are designed as sleds or trays that house 3.5-inch drives, enabling easy installation and removal in server enclosures. These caddies typically incorporate the SCA-2 connector, an 80-pin interface that provides power, signaling, and configuration pins for reliable data transfer up to 80 MB/s in Ultra320 implementations. They support hot-swapping through long and short pins for sequenced power-up, integrated with Enclosure Services to manage drive insertion or removal without system downtime, a key feature for enterprise storage arrays. In systems like UltraSPARC servers and arrays, these caddies facilitate dense configurations, often converting standard drives for mounting. Common features include activity LEDs for drive status, lockable latches for security, and blind-mate insertion akin to SCA designs, ensuring alignment without manual cabling. These caddies were widely used in data centers for HDDs, supporting speeds from 20 MB/s in SCSI-3 to 80 MB/s in later standards, enabling scalable storage in setups before the shift to serial interfaces in the 2000s. The design emphasized robustness for multi-drive bays, accommodating 8- to 14-drive enclosures in JBOD or modes.

Backplane integration

Backplane integration in parallel SCSI systems primarily utilizes the Single Connector Attachment-2 (SCA-2) 80-pin connector to enable direct mounting of multiple drives onto a shared backplane, eliminating the need for individual cabling between drives and the host controller. This design facilitates daisy-chaining of drives in a multi-drop bus topology, where each drive connects sequentially through the backplane traces, allowing seamless communication across the SCSI bus without external cables for internal configurations. The SCA-2 connector, defined in the SFF-8046 specification, supports hot-swapping capabilities compliant with the SCSI Parallel Interface-2 (SPI-2) standard, enabling drives to be inserted or removed while the system remains powered on. Power distribution is integrated into the SCA-2 connector via four dedicated power pins—two for +5V and two for +12V—allowing the to supply DC power directly to each connected drive, with typical current capacities supporting up to 1.5A per rail per drive. This setup enables backplanes to accommodate 8 to 16 drives per bus, depending on the signaling mode and cabling constraints, as the parallel standard permits a maximum of 16 devices (including the initiator) on a wide bus. In dense storage enclosures, the backplane's power traces are designed to handle the cumulative load, often drawing from the system's power supply unit to ensure stable operation across all slots. Termination in SCSI backplanes employs a multi-drop configuration where active terminators, typically 110-ohm resistors for Low Voltage Differential (LVD) modes, are placed at the bus endpoints to prevent signal reflections. Many designs incorporate fixed active terminators on the board itself, while some support auto-termination on end-positioned drives that detect their location via dedicated sense pins on the SCA-2 connector and enable termination accordingly. This approach ensures in shared bus environments, with the backplane managing termination power distribution to avoid conflicts during hot-plug events. To minimize noise in high-density backplane setups, parallel SCSI supports both Differential (HVD) and LVD signaling across the traces, with LVD being predominant in modern enclosures due to its lower power consumption and reduced over short distances typical of . HVD, while offering longer reach, is less common in compact integrations but can be used for noise-prone environments. Vendors such as and LSI Logic implemented these features in their controllers and JBOD enclosures, like the Adaptec AAR-2410SA series, which used SCA-2 backplanes for scalable storage arrays supporting up to 14 drives per channel in or JBOD modes.

Serial SCSI connectors

Internal connectors

Internal connectors for serial SCSI interfaces facilitate direct board-to-board or backplane-to-device connections within enclosures, enabling high-speed data transfer without external cabling. These connectors support serial protocols such as (SAS), Arbitrated Loop (FC-AL), and earlier standards like Serial Storage Architecture (SSA), prioritizing point-to-point topologies for improved reliability and scalability over the multi-drop daisy-chaining of predecessors. The primary internal connector for SAS is defined by the SFF-8482 specification, featuring a 29-contact unshielded with 14 contacts for dual-lane high-speed differential signals and 15 for power and control functions. This connector supports keying variations to distinguish host and drive roles, allowing SAS hosts to interface with SATA drives in hybrid environments through compatible pinouts, though the reverse is prevented by physical barriers to avoid mismatches. Unshielded right-angle variants are commonly mounted on motherboards or backplanes for compact integration, accommodating speeds up to 12 Gb/s in SAS-3 implementations while integrating power delivery via dedicated pins to simplify cabling. For Fibre Channel drives operating in FC-AL configurations, the 40-pin SCA-2 variant per SFF-8045 provides a single-connector attachment solution tailored for internal racking in cabinets, combining dual-port serial signals, power (+5V and +12V with precharge for hot-plugging), and control lines like loop ID selection and enclosure services interface. This setup emulates SCSI command structures for compatibility in mixed storage arrays, with ground-referenced guide pins ensuring precise alignment on backplanes. Early serial SCSI efforts included SSA, which utilized unitized composite connectors to link drives in loop topologies, offering an IBM-developed interface with serial transmission rates of 20 MB/s full-duplex per port before being superseded by SAS. These connectors supported internal chaining of up to 48 devices per loop, emphasizing fault-tolerant designs for enterprise storage. Unlike parallel SCSI's daisy-chain requirements, serial internal connectors like those in SAS enable dedicated point-to-point links, reducing signal skew and supporting expander-based scaling for dense drive populations.

External connectors

External connectors for serial SCSI interfaces, such as SAS and , enable cabling between host systems and external storage enclosures, supporting high-bandwidth serial links with shielding to minimize and secure latching or thumbscrew mechanisms for reliable connections. These connectors facilitate data rates from early gigabit speeds up to 12 Gb/s for SAS-3 and 128 Gb/s for advanced implementations, often incorporating hybrid designs for electrical or optical transmission. In SAS systems, the SFF-8484 connector serves as a 32-pin unshielded multilane interface primarily for internal-to-external cable transitions, combining four lanes of serial attachment signals with power delivery to support up to 6 Gb/s per lane in configurations linking controllers to external ports. The SFF-8470 connector, a 34-pin external type compatible with both optical and electrical media, uses thumbscrew retention for secure mating and provides hybrid connectivity for four-lane SAS links at speeds up to 6 Gb/s, enabling transitions from internal SAS to external cabling while maintaining through integrated shielding. For higher-density external applications, the SFF-8087 (internal 36-pin, four-lane) and SFF-8088 (external counterpart) form the basis of mini-SAS HD designs supporting 6 Gb/s and beyond, featuring latch mechanisms for quick-release connections and with externals through distinct keying that prevents incorrect insertions while allowing SAS controllers to interface with devices via breakout cables. These SAS-3 capable connectors, including SFF-8644 external variants, sustain up to 12 Gb/s aggregate throughput across lanes with shielding and positive latching to ensure stability in enterprise enclosures. Fibre Channel external connectors emphasize optical and copper options for SAN environments, with the DE-9 (DB-9) serving as a nine-pin copper interface for early FC-AL topologies, supporting short-distance gigabit links in arbitrated loop configurations through shielded cabling up to 10 meters. The HSSDC (High-Speed Serial Data Connector), an eight-pin push-pull design, provides a compact copper alternative for at up to 2.125 Gb/s, featuring hot-pluggable contacts and grounding pins for reduced in external loops. For longer-reach gigabit and beyond applications, SC and LC duplex optic connectors dominate, with SC's 2.5 mm offering push-pull latching for multimode or single-mode cables up to 500 meters, while LC's smaller 1.25 mm enables higher-density ports with similar low-insertion-loss performance in FC-AL and switched fabrics. Modern 128 Gb/s employs QSFP28 transceivers with LC or MPO connectors over OM4/OM5 multimode , achieving 100-meter distances via PAM4 modulation and maintaining with lower-speed FC through adapter mechanisms. Earlier serial SCSI implementations, like Serial Storage Architecture (SSA), utilized nine-pin micro-D connectors for external copper links up to 25 meters, featuring twisted-pair bundles with 75-ohm impedance and thumbscrew or push-pull retention for loop topologies at 20 MB/s speeds, providing a shielded pathway for dual-loop in pre-SAS storage arrays. Across these serial variants, external connectors prioritize rugged shielding against , with latch or thumbscrew options ensuring vibration-resistant mating in rack-mounted enclosures, and keying differences in SAS designs allowing seamless integration of peripherals without cross-compatibility risks.

Drive caddies

Drive caddies for serial SCSI, particularly (SAS), are designed as sleds or trays that house 2.5-inch or 3.5-inch drives, enabling easy installation and removal in server enclosures. These caddies typically incorporate the SFF-8482 connector, a 29-pin interface that provides both power and dual-lane SAS signaling for reliable data transfer up to 6 Gb/s per lane in early implementations. They support hot-swapping through integration with SAS expanders, which manage multiple drive connections and allow drives to be inserted or removed without powering down the system, a critical feature for maintaining uptime in enterprise environments. Hybrid SAS/SATA caddies extend compatibility to both interface types, allowing a single tray design to accommodate either SAS or drives within the same . In systems like servers, these hybrid trays convert 2.5-inch drives for use in 3.5-inch bays, optimizing space in dense configurations. Similarly, HPE servers employ hybrid trays that support SAS and drives interchangeably, facilitating mixed-drive deployments in rack-mounted arrays. Common features include activity LEDs to indicate drive status and access, lockable mechanisms for , and blind-mate insertion similar to legacy SCA designs, ensuring precise alignment with connectors without manual cable handling. These caddies are widely deployed in data centers for SAS SSDs and HDDs, supporting speeds from 6 Gb/s in SAS-2 to 24 Gb/s in SAS-4 as of 2025, enabling high-performance storage in NVMe-compatible hybrid setups. The transition from caddies has emphasized improved airflow through slimmer profiles and higher-density layouts, accommodating 24-bay or larger enclosures for scalable petabyte-scale storage.

Evolution and usage

Historical development

The Small Computer System Interface (SCSI) originated in 1986 with the publication of the ANSI X3.131-1986 standard, known as SCSI-1, which introduced a parallel 8-bit bus operating at a synchronous clock speed of 5 MHz and transfer rates up to 5 MB/s. This initial specification utilized Centronics 50-pin (CN50) connectors for internal connections and DB-25 connectors for external ones, enabling the attachment of up to eight devices including hard drives, tape drives, and scanners to computers. The design built on earlier proprietary interfaces like Shugart Associates System Interface (SASI), providing a standardized, multitasking-capable I/O bus that marked a significant advancement in peripheral connectivity for workstations and early servers. In 1994, the SCSI-2 standard (ANSI X3.131-1994) enhanced performance by introducing Fast SCSI at 10 MHz (10 MB/s for 8-bit) and Fast Wide SCSI at 20 MB/s for 16-bit operations, while also supporting asynchronous modes at lower speeds. Connector evolution included the adoption of 50-pin High-Density (HD50) for narrow buses and 68-pin HD68 for wide configurations, allowing longer cable runs up to 25 meters with High Voltage Differential (HVD) signaling. These improvements addressed growing demands for faster data transfer in expanding storage arrays, with SCSI-2 becoming prevalent in mid-1990s environments. By the late , parallel reached its peak with Ultra SCSI variants, including Ultra-160 (introduced around 1999 at 160 MB/s) and Ultra-320 (around 2002 at 320 MB/s), which employed Low Voltage Differential (LVD) signaling to mitigate noise over distances. Enterprise integration advanced through Single Connector Attachment-2 (SCA-2) 80-pin connectors for hot-swappable drives and Very High Density Counter Insert (VHDCI) 68-pin external interfaces, facilitating dense server storage without individual cabling. These developments optimized for high-availability systems but highlighted parallel limitations like signal skew and cable complexity. The transition to serial interfaces began with interim technologies in the 1990s, such as IBM's Serial Storage Architecture (SSA, developed in 1990 and commercialized by 1995) offering up to 40 MB/s in a ring topology, and Fibre Channel Arbitrated Loop (FC-AL, standardized around 1994) providing 100 MB/s serial links via fiber or copper. Full serial adoption culminated in 2004 with the Serial Attached SCSI (SAS-1) standard, defined by ANSI INCITS 376-2003 and using SFF-8482 connectors for internal point-to-point connections at 3 Gbit/s, directly succeeding parallel SCSI by resolving skew issues and enabling longer, thinner cables while maintaining SCSI command compatibility. During the , became obsolete as manufacturing support waned, with vendors ceasing production by the mid-decade due to SAS's superior and cost-efficiency. SAS-3 (12 Gbit/s, introduced 2013) and SAS-4 (22.5 Gbit/s, around 2017) dominated enterprise storage, powering data centers with multi-lane serial links that supported far more devices than parallel limits allowed.

Modern applications

In contemporary data centers, (SAS-4), operating at signaling rates of 22.5 Gbps per lane (marketed as 24G SAS), remains a cornerstone for connecting arrays of hard disk drives (HDDs) and solid-state drives (SSDs) in high-performance storage environments. This technology supports hyperscale operations across sectors such as banking, healthcare, and , where it handles intensive computational workloads and escalating storage traffic demands. SAS-4 delivers effective bandwidth up to 2.4 GB/s per lane, enabling faster data throughput for mission-critical applications while incorporating features like 128b/150b encoding and to enhance in dense server configurations. A key advantage of SAS-4 is its backward compatibility with earlier generations, including SAS-3 (12 Gbps), SAS-2 (6 Gbps), and SATA (6 Gbps), allowing seamless integration into existing infrastructures without requiring full hardware overhauls. This compatibility facilitates mixed-drive environments, where SAS and SATA devices coexist in the same bays, supporting high-availability setups for enterprise storage. Meanwhile, legacy parallel SCSI connectors persist in niche applications through adapters, primarily in industrial control systems and retro computing setups, though their use has become rare owing to obsolescence and the dominance of serial protocols. For ultra-high-performance storage, the SFF-8639 (U.2) connector standard enables the integration of both SAS and NVMe-based drives in the same form factor, providing a unified physical interface for PCIe/NVMe and SAS protocols in server backplanes. This approach allows coexistence of the protocols, optimizing for demanding workloads like AI training and , with speeds up to 64 Gb/s PAM4 for PCIe lanes and 24 Gb/s for SAS. (FC) continues to underpin storage area networks (SANs) by transporting the protocol over , with the upcoming Gen 8 (128GFC) standard expected to achieve 128 Gbps by late 2025 using LC or SC connectors for distances up to 100 meters. This enables scalable, lossless block-level access in enterprise SANs, with ensuring gradual upgrades. Ethernet-based has seen a marked decline in adoption for new deployments, supplanted by NVMe over TCP (NVMe/TCP) due to the latter's superior performance—offering up to 35% higher , 25% lower latency, and 20% greater throughput over standard Ethernet without specialized hardware. 's limitations in handling modern flash storage's parallel nature have relegated it to legacy maintenance, while SAS drive caddies persist in hybrid cloud environments to facilitate hot-swappable storage.

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