High-Density (HD):
Newer, high-density shielded connectors in SCSI-2 replaced the D-shell connectors that were defined in the SCSI-1 standard. The main difference being the space between pins were reduced. That made the connectors smaller, thereby cheaper to make and easier to use.
Very High Density Cable Interconnect (VHDCI):
This connector has 68 pins (wide only) and is also referred to as a "micro-Centronics" connector, as it uses the same design as the Centronics connectors, with the contacts much smaller and closer. Because of its small size, it is becoming very popular and much sought out.
Regular Density
A single connector type for internal narrow (8-bit) devices was defined by the SCSI-1 standard. This was a rectangular connector with two rows of 25 pins each. This connector type was very much similar to the one that is used for IDE/ATA devices but for the fact that there are five extra pins in each row. This is most often seen in older devices and also in some newer but slower disks
High Density
SCSI-2 defined two new connector types, both called the high density based on their pin spacing which is half that of the older SCSI-1 connectors which makes them comparatively very much smaller. These are the most common SCSI connectors in use today.
Single Connector Attachment "Alternative 4" in the SCSI standards of unshielded connectors basically does not refer to the cable connectors but to the connectors that are used for the single connector attachment system for backplane-connection of SCSI disks.
