SCSI Interface Types

Hardware Interface

SCSI = Small Computer Systems Interface

Software Interface

ASPI = Advanced SCSI Programming Interface
This was a way to provide a common API after SCSI-1 was introduced. SCSI-2 provided a common subset of programming instructions.

Electrical Specification

SE = Single-Ended
HVD = High Voltage Differential
LVD = Low Voltage Differential
With LVD, cabling must be twisted pairs (e.g. 'twist and flat' ribbon cable) and all devices must be set to use LVD if available (differential wiring is more noise-immune). HVD offers longer distance cabling but is not often used. SE is common among early devices using e.g. flat ribbon cables.

Descriptive Terms

Narrow / Wide = number of bits or data lines.
Fast / Ultra / Ultra 2 / Ultra 3 etc. = speed (data transfer rate) improvements.
SCSI-1 = The first 8-bit hardware and software specification.
SCSI-2 = A family of 8-bit and 16-bit hardware types with a new software standard.
SCSI-3 = A family of 8-bit and 16-bit hardware types.
SCSI-4 = A family of 8-bit and 16-bit hardware types.

The types of SCSI Interface

TYPE	Data width	Transfer rate	Internal disk drive connector	External device connector^[2]	No. of devices	Max. cable length
SCSI-1	8 bits	5 MB/s	50-pin or 68-pin + MOLEX Power Connector	50-pin Centronics^[3]	7	6 metres (SE) or 12 metres (LVD)
Fast SCSI (Narrow) (SCSI-2)	8 bits	10 MB/s	50-pin or 68-pin + MOLEX Power Connector	50-pin high-density	7	3 metres (SE) or 12 metres (LVD)
Fast Wide SCSI (SCSI-2^[1])	16 bits	20 MB/s	68-pin + MOLEX or 80-Pin SCA (Single-Connector-Attachment)	68-pin high-density	15	3 metres (SE) or 12 metres (LVD)
Ultra SCSI (Narrow) (SCSI-3)	8 bits	20 MB/s	50-pin or 68-pin + MOLEX Power Connector	50-pin high-density	7	1.5 metres (SE) or 12 metres (LVD)
Wide Ultra SCSI (SCSI-3)	16 bits	40 MB/s	68-pin + MOLEX or 80-Pin SCA (Single-Connector-Attachment)	68-pin high-density	7 (SE) or 15 (LVD)	1.5 metres (SE) or 12 metres (LVD)
Ultra 2 SCSI (Narrow) (SCSI-4)	8 bits	40 MB/s	50-pin or 68-pin + MOLEX Power Connector	50-pin high-density	7	12 metres (LVD)
Wide Ultra 2 SCSI (SCSI-4)	16 bits	80 MB/s	68-pin + MOLEX or 80-Pin SCA (Single-Connector-Attachment)	68-pin high-density	15	12 metres (LVD)
Ultra 3 SCSI (Narrow) (Ultra160/m)	8 bits	80 MB/s	50-pin or 68-pin + MOLEX Power Connector	68-pin high-density	15	12 metres (LVD)
Wide Ultra 3 SCSI (Ultra160/m)	16 bits	160 MB/s	68-pin + MOLEX or 80-Pin SCA (Single-Connector-Attachment)	68-pin high-density	15	12 metres (LVD)
Wide Ultra 320 SCSI	16 bits	320 MB/s	68-pin + MOLEX or 80-Pin SCA (Single-Connector-Attachment)	68-pin high-density	15	12 metres (LVD)
?	32 bits	?	?	?	31	?

^[1]Fast Wide SCSI-2 is sometimes erroneously referred to as SCSI-3 because of the 68-pin connector.
^[2]Some pictures of the various connectors are available.
^[3]The 50-pin high density connector was sometimes used

E.&O.E.