SCSI Technical information
Introduction
SCSI Standards
SCSI Connectors
Internal Connectors
IDC50 - Regular Density IDC 50 pin Narrow
This connector was part of the SCSI-I specification. It is used on older narrow hard disks and current lower throughput drives such as CDROM's and SCSI tape drives. Construction is the same as IDE drive connectors except that there are 50 pins instead of 40. In the current SCSI standard they are known as unshielded 'alternative 2'.
HD68 - High Density 68 pin Wide
This is the most common internal SCSI connector in the PC world. It was introduced as part of the SCSI-II standard. The pin pitch is half that of the earlier IDC connectors hence the name 'high density'. In the current SCSI standard they are known as unshielded 'alternative 3'.
SCA80 - Single Connector Attachment 80 pin Wide
SCA connectors carry all the SCSI signals, the drive power lines and configuration jumper connections within a single 80 pin connector. They are used for hot swap configurations and when quick drive replacement is important. The drive connector plugs into a connector on a back plane within the drive housing. Laluna Technology can supply adapters to allow connection of SCA drives to a normal HD68 cable. The current SCSI standard calls SCA80 connections 'alternative 4'.
External Connectors
DB25 - D-Sub 25 pin Narrow 'Apple'
The D-Sub (or D-Shell) 25 pin connector allows connection of narrow (8 bit) external devices. It is widely used by Apple ComputersTM. In order to squeeze the normal 50 wire narrow SCSI into 25 pins, the signal return and ground wires were dispensed with. This results in a loss of cable performance meaning shorter runs and slower speeds. This connection is exactly the same as a PC parallel printer so be very careful not to plug one into the other or damage may result. There was a 50 pin D-Sub connector used for narrow SCSI that had 3 rows of pins which you may come across. It was never popular due to it's large size.
CN50 - Centronics 50 pin Narrow
The Centronics 50 pin connector is widely used for narrow (8 bit) external devices. The connector was defined as part of the SCSI-I Standard and is still included in the current standard where it is called the 'alternative 2' external connector.
HD50 - High Density 50 pin Narrow
The SCSI-II standard added a new high density 50 pin D-Sub connector known as 'Alternative 1'. As the name suggests the pins are closer together than the standard D-Sub. The small size makes it easier to fit them on an expansion card plate. The plug is secured using a quick realease mechanism instead of the usual thumscrews. This connector is used extensively for connection of host adapter cards to external narrow devices.
HD68 - High Density 68 pin Wide
This is the wide 68 pin version of the high density D-Sub. Defined as part of the SCSI-II standard this connector is known as 'Alternative 3'. It is Used to connect wide (16 bit) external devices. The HD68 internal connector is identical in construction.
VHDCI - Very High Density Cable Interconnect 68 pin Wide
Known as 'Alternative 4' in the SCSI specification, this connector is gaining in popularity for high end SCSI and RAID cards. It is wide (16 bit) only. Two of these connectors can be fitted side by side on the back plate of an adapter card allowing up to four wide connections. VHDCI connectors are similar in concept to Centronics using small plates instead of pins and are often known as 'micro Centronics'.