SCSI connector

A SCSI connector (pronounced "scuzzy") is used to connect computer parts that use a system called SCSI to communicate with each other. 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 Mbit/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 often makes use of cables to connect devices together; 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. This would mean that four connectors were involved in wiring the disk drive and computer together: the connector on the motherboard, the connectors at each end of the cable, and the connector on the disk drive. It is sometimes possible to have cables which have different types of connectors on them, and some cables can have as many as 16 connectors (allowing 16 devices to be wired together). Some types of connectors are typically used inside a computer or disk drive case, while others are used to connect a computer to a separate device such as a scanner or external disk drive.

Nomenclature

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 "HD50") less so.

Parallel SCSI

A stack of external SCSI devices displaying various SCSI connectors

Parallel SCSI allows for attachment of up to 16 devices to the SCSI bus, thus cables may have up to 16 connectors. It is unusual, however, for external cables (those that run between enclosures) to have more than 2.

Internal

IDC header

Early generations of SCSI hard drive assemblies generally had two connectors (power and communication) for 8-bit units, and either two or three connectors for 16-bit units. 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 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." It is worth noting that this type of header was used for most internal connections in a typical desktop PC until recent times, including the 40-pin (two rows of 20) version used for ATA 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

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 parallel SCSI disk-drives now utilize an 80-pin SCA (Single Connector Attachment) connector. 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 HDA's (and of course the mating enclosure backplane connector).

External

Assorted SCSI Connectors

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

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 DB25 SCSI Port

Apple used DB-25 connectors, which, having only 25 pins rather than 50, were less expensive to make, but compromised functionality. Further, 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. The DB-25's only advantage was that it was smaller than a CN50.

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

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, which was a Centronics-style connector with smaller pins and shell. For some reason 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".

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

With the arrival of SCSI-2, the situation was a bit less chaotic. For narrow SCSI, most manufacturers used the MD50, also sometimes referred to as a Micro DB50, High Density or HD50, Half-Pitch or HP50, or MiniHonda. This connector has two rows of 25 pins and a trapezoidal (D-shaped) shell, and is about 1 3/8” (36mm) wide.

A few vendors did use the Micro Centronics 50, and IBM continued to use the HDCN60 on some RS-6000 systems.

For Wide SCSI-2, the most common connector was the larger sibling of the MD50, known as the MD68, HD68, MiniD68, or HPDB68. This is about 1 7/8” (47mm) wide. IBM used the HDCN68 on some RS-6000 systems, and it seems likely that a few other manufacturers used other alternatives.

Post SCSI-2

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" (a manufacturer part number), 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

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 (slow) SE single-ended device onto a (fast) LVD SCSI chain will cause the HBA to sync down to the lowest speed.

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

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)

Serial SCSI

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

Internal SFF-8482 connector on a SAS HDD

External connectors

Drive caddies

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.

References

  1. ↑ Michael Micheletti (2006-01-07). "SAS and Serial ATA Tunneling Protocol (STP)". serialstoragewire.net. Retrieved 2014-09-22.
  2. 1 2 Willis Whittington (2007). "Desktop, Nearline & Enterprise Disk Drives" (PDF). Storage Networking Industry Association (SNIA). Retrieved 2014-09-22.
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