The Digital Storage Systems Interconnect (DSSI) is a computer bus developed by Digital Equipment Corporation for connecting storage devices and clustering VAX systems. It was designed as a smaller and lower-cost replacement for the earlier DEC Computer Interconnect that would be more suitable for use in office environments. DSSI was superseded by Parallel SCSI.

It was introduced in 1988 and has a bandwidth of 32 Mbit/s (4 MB/sec) and was typically limited to a length of 25 metres (82 ft). DSSI support was later extended to MIPS-based DECsystems and DEC Alpha-based AlphaServer systems.

DSSI peripherals could be distinguished from narrow SCSI devices by their characteristic five-conductor power connector featuring an extra lead intended for Battery/UPS connection to power critical components in the event of a power failure.

Characteristics

The major characteristics of the DSSI bus are:

  • Eight-bit data path
  • Up to eight nodes on the bus including variable numbers of initiators and targets
  • Distributed arbitration
  • Peak bandwidth of 4 megabytes per second
  • All systems connected to the same DSSI bus must have a common power/ground.
  • "The Q-Bus 5-Enclosure Rule": For Q-bus VMScluster configurations, a maximum of five enclosures, excluding SF200/210s, can be configured on a single DSSI bus. Note that only one (1) SF100 is allowed on any DSSI bus.
  • "The VAX 6000 Cabinet Rule": For a VAX 6000 DSSI VMScluster system, a maximum of two or three VAX 6000 systems and one storage cabinet, typically an SF200 or SF210, can be configured on a single DSSI bus.
  • Maximum DSSI bus length cannot exceed 27 meters (89 feet) and ground offset voltage cannot exceed 30mv (dc) or 10.5mv (rms).

Operating system support in VAX/OpenVMS

Third Party Peripherals

Other companies, such as CMD and Symbios Logic made DSSI compatible chipsets or peripherals, for example, CMD manufactured various models of the CDI-4000 which would allow SCSI peripherals to be used on a DSSI bus.

See also

  • DSSI VAXcluster Installation and Troubleshooting manual, EK-410AB-MG
  • DEC Technical article: [ Ultrix ] How To Configure a kfqsa on a DECstation (sic) 5500 (DS5500), Last Technical Review: 6-SEP-1991 Size: 110 lines
  • DEC Technical article: [OpenVMS] Configuring RF Drives Connected To DSSI Adapter In A Cluster, Last Technical Review: 8-OCT-1996 Size: 332 lines
  • DEC Technical article: [OpenVMS] Guide To DSSI VAXcluster Configurations
  • DEC Technical article: How To Initialize An RF72 For High Speed Mode
  • DEC Technical article: [OpenVMS] Chart Overview Of Digital Disk Drive Storage Capacities
  • DEC Technical article: [PERFORMANCE] Disk Drive Performance Specifications
  • "OpenVMS Release Notes Addendum for VMScluster Systems", October 1993, (AA-Q1X7A-TE).
  • "Dual-Host MicroVAX Systems", VAXcluster Systems Quorum, February 1990, (EC-P0623-57), pp. 3–31.
  • "VAX 4000 Systems Info Sheet", (EC-F1424-41).
  • "VAX 6000 Systems Info Sheet", (EC-F1417-46).
  • "KFESA DSSI Adapter Installation and User's Guide", May 1994, EK-KFESA-OP.A01.
  • CDI-4000 DSSI to SCSI Adapter User's Manual, MAN-004000-000