Storage Basics: SCSI Part II Page 4
SCSI supports both internal and external devices with an array of connector options for each. Over time, many of these connectors and their associated cable types have fallen into obscurity, but there are plenty of legacy SCSI devices out there using the various connector types.
As far as internal connectors are concerned, there are only a few variants. The original regular density 50-pin SCSI connectors were designed to be used with SCSI-1 devices. These regular density cables resembled the 40-pin connectors used with IDE devices. The SCSI-2 standard brought with it a new internal connector, the high-density connector, which comes in a 50-pin and a 68-pin version. Today's SCSI implementations are likely to use the 68-pin internal connectors.
When it comes to external SCSI connectors, there are a range of possibilities which can be grouped into 2 distinct categories, D-shell connectors and Centronic connectors.
A D-shell connector was first used with the original SCSI-1 standard. This original 50-pin connector was replaced with the high-density shielded connectors in SCSI-2. The newer high-density D-shell connectors are available in both 50-pin and 68-pin versions. The latest and greatest external SCSI D-shell connector is referred to as the Very High Density Cable Interconnect (VHDCI) connector. The VHDCI connector is only available in a 68-pin version and is the one you are most likely going to encounter when working with modern SCSI implementations.
An alternative to the D-shell SCSI connectors are the Centronic connectors. While Centronic connectors are now most often associated with PC printer cables, they were once common for connecting external SCSI devices. Today, these 50-pin SCSI Centronic connectors may still be found for connecting scanners or printers, but they have fallen out of favor for the more versatile VHDCI connectors.