SCSI connector
The SCSI connector is a type of electrical interface used within the Small Computer System Interface (SCSI), 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 optical disc drives, allowing for efficient data transfer and device control.[1][2] 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 signal integrity over distances up to 25 meters in differential mode.[1][3] 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.[2][4] 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.[3] 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.[3][1] 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.[1] 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.[3] 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.[4]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.[5][6] 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.[5] D-subminiature connectors, frequently used in SCSI 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 hyphen and "B" to reflect its shell size with two rows of pins.[7] In contrast, the 50-pin three-row D-sub, employed in some narrow SCSI setups, is denoted as DD50 without a hyphen, where the double "D" signifies the larger D-shell accommodating the additional row.[7][6] Informal and vendor-specific names further diversify SCSI connector nomenclature, often reflecting proprietary 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.[8] Similarly, Sun Microsystems 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.[6] 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.[9] 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.[6] 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.[5][6]Related standards
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 data transfer rates up to 5 MB/s.[10] This standard established the foundational protocol for connecting computers to peripherals like hard drives and scanners.[11] 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.[4] These updates also standardized command sets and electrical interfaces to improve reliability and device compatibility.[12] 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.[13] 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.[14][15] These evolutions maintained backward compatibility while pushing parallel bus limits for enterprise storage.[16] Parallel SCSI transitioned to serial standards in the late 1990s and early 2000s, with Serial Attached SCSI (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.[17] Related serial protocols included Serial Storage Architecture (SSA), an early IBM-developed ring topology standard (ANSI X3.295-1996), and Fibre Channel Arbitrated Loop (FC-AL), which adapted SCSI commands over fiber-optic serial channels for higher-speed networking (INCITS 303-1998).[18] By the early 2000s, 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.[19]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 ribbon cable design where the insulation of the cable is displaced to make electrical contact with the conductors, eliminating the need for soldering. 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.[20] The 50-pin IDC connector supports 8-bit Narrow SCSI 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 ribbon cable. 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.[21][20][3] For higher-performance applications, the 68-pin IDC connector accommodates 16-bit Wide SCSI 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 2000s, 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.[21][20][3] Power delivery to SCSI devices connected via IDC is handled separately from the data signals, using a standard 4-pin Molex connector 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 backplane designs.[22][3]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 signal integrity. This configuration adheres to SFF-8046 specifications, ensuring compatibility with enterprise backplanes in RAID arrays and servers.[23] 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 (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 deprecation in favor of SCA-2 for modern implementations.[23][24] SCA connectors are primarily deployed in high-availability enterprise environments, such as RAID storage subsystems and server backplanes, 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.[25][11]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 signal integrity 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 professional settings.[26][3] 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.[3]
- DB-25: A compact 25-pin D-subminiature 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.[26][3]
- 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 EMI 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.[26][3]
- 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 RAID arrays and scanners, with thumbscrews and support for daisy-chaining over 12 meters in LVD.[26][3]
- 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.[26][3]