SCSI Cable Pin Types

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    D-Shell, External

    • The earliest SCSI standard, this connector has a D-shaped "shell," or metal boundary, surrounding the pins. It provides thumbscrews for securing to ports when plugged in. Typically, it had 50 pins, but Apple briefly used a 25-pin version, which was physically identical to a parallel port connector. (If you plug an SCSI connector into a parallel port, it could potentially cause damage.)

    Centronics, External

    • Named after a once popular printer brand that used this connector, a centronics connector has two rows of 50 flat contacts and comes in male and female versions that secure together with wire snaps. Centronics connectors are also widely referred to as SCSI-1 and Alternative-2.

    High Density, External

    • A more densely packed version of the D-shell connector, this kind of connector secures together with clamping braces that you can release by squeezing the sides. The 50-pin version is widely called SCSI-2 and "Alternative-1," while the 68-pin version is called SCSI-3 and Alternative-3.

    Very High Density, External

    • Similar in appearance to a centronics connector, this type has a row of 68 flat contacts and is significantly narrower than other SCSI connectors. It is secured by thumbscrews in the same manner as the D-shell connector. It is also called SCSI-4 and Alternative-4.

    Regular Density, Internal

    • The internal counterpart to the D-shell external connector, this connector has two rows of 25 pins and secures to female connectors through "crimping," or the friction between the male connector's shell and the female connector's shell. It looks similar to connectors for IDE/ATA devices, but it has five extra pins in each row. It is typically used for connecting hard drives and CD-ROM drives and is referred to as unshielded Alternative-2.

    High Density, Internal

    • This is the internal counterpart to the external high density connector. Like the internal regular density connector, it secures by crimping. It comes in 50-pin and 68-pin versions like its external counterpart. These are called, respectively, unshielded Alternative-1 and unshielded Alternative 3.

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