Electronics News
A introduction to SCSI Connector
2008-08-05
A introduction to SCSI Connector The Small Computer System Interface (SCSI) is a communications protocol standard that has become increasingly popular for interconnecting computers and other input/output devices. Personal computers provide today's users with the power to communicate with other networked users, share information and access data from peripheral devices. To achieve high performance data transfer interfaces between the host computer system and a given peripheral device, computer users typically use hardware that can take advantage of the small computer system interface (SCSI) protocol. A small computer standard interface (SCSI) specification defines a number of parameters to interface peripheral devices to computer systems. Typically, computer systems implementing the SCSI specification contain an internal bus. In addition, computer systems contain a connector so that the internal bus may be expanded external to the computer system. The expansion of the bus permits interfacing of a number of peripheral devices. Once a computer is provided with SCSI capabilities, the user is able to connect either internal or external SCSI peripheral devices to the computer. Internally, SCSI ribbon cables are used to interconnect the peripheral devices to the computers controller card or a motherboard's connector. Externally, SCSI peripheral devices can be coupled to the computer's connector receptacle via an external SCSI cable. SCSI (Small Computer System Interface) connectors provide a standard interface between computers and internal/external SCSI peripheral devices, such as fixed and removable storage drives, scanners, compact discs, etc. Connectors are of course the physical devices that are used to attach a SCSI cable to a SCSI device. Several different types of SCSI connectors are used to construct SCSI cables. Different SCSI connector types have evolved over the years as the SCSI interface has matured. Since external and internal cables generally use different connectors, each has four different "alternatives". Below are the connector types most commonly seen used with SCSI cables in the PC world.
D-Shell (D-Sub, DD): The earliest SCSI standard, SCSI-1, defined a 50-pin D-shell connector for narrow SCSI implementations. The name of this connector comes from the "D-shaped" metal shell that goes around the pins on the male half of the connector. The design is identical to the 25-pin and 9-pin D-shell connectors used for parallel and serial connections on PCs, but bigger. This connector type was very large and cumbersome, never really caught on.
A male DD-50 SCSI connector Getronics: The other external connector type defined by the SCSI-1 standard is a 50-pin connector that is commonly called a Getronics connector, after a formerly-popular printer that first used this type of connector. In Getronics connectors, instead of thin pins, two rows of flat contacts are used. Two latches on either side are used to hold the connector in place. Getronics connectors are still used for PC printer cables, on the end that attaches to the printer; SCSI Getronics connectors are the same, just with a different number of pins. These 50-pin connectors are still present in the current SCSI specification and are called "Alternative 2" external connectors.
Male (above) and female 50-pin Getronics connectors High-Density (HD): The D-shell connectors defined in the SCSI-1 standard were replaced by newer, high-density shielded connectors in SCSI-2. These are really not all that different from the older D-shell connectors, but the space between pins was reduced, making the connectors smaller, cheaper to make and easier to use. The narrow, 50-pin version is called "Alternative 1", and the wide, 68-pin version "Alternative 3". These connectors use a "squeeze to release" latching mechanism instead of Getronics-style latches, and are still used by hardware devices today.
Male 50-pin (above) and 68-pin and external high density connectors Very High Density Cable Interconnect (VHDCI): To further improve the flexibility of SCSI hardware, a new type of external connector was defined as part of the SPI-2 standard. This connector is wide only (68 pins) and is sometimes called a "micro-Getronics" connector, because it uses the same design as the Getronics connectors, only with the contacts much smaller and closer together. This is "Alternative 4" for external connectors and is growing in popularity because of its small size. One way that VHDCI is useful; for example, is that two of these connectors can be squeezed side-by-side within the width of a single SCSI host adapter's back edge (expansion slot insert). This doubles the number of external connectors that can be crammed onto a high-end SCSI host adapter.
A male 68-pin VHDCI connector
OK, now let's look at some internal (unshielded) connectors: Regular Density: The SCSI-1 standard defined a single connector type for internal narrow (8-bit) devices. This is a rectangular connector with two rows of 25 pins. This connector type is very similar to that used for IDE/ATA devices, except that there are five extra pins in each row. It is most often seen in older devices and also some newer, slower drives. It is called unshielded "Alternative 2" in the current SCSI standards.
Male (above) and female 50-pin regular density internal connectors High Density: SCSI-2 defined two new connector types, which are both called high density because their pin spacing is half that of the older SCSI-1 connectors, making them much smaller. These are the most common SCSI connectors used today within the PC box. The narrow, 50-pin version is unshielded connector "Alternative 1" and the 68-pin version is "Alternative 3".
A male, internal, high-density 68-pin connector Single Connector Attachment (SCA): "Alternative 4" in the SCSI standards for unshielded connectors doesn't actually refer to cable connectors, but the connector used for the single connector attachment system for backplane-connection of SCSI drives. See the section on SCA for more details.
A female 80-pin SCA connector