Digital Storage Systems Interconnect

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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 DSSI was superseded by Parallel SCSI.[1]

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.


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).
                                 Bus Length      Allowable Offset
                           Meters/Feet     (DC)    (AC)
                           -----------     ----------------
                           up to 20/65     200mv   70mv (rms)
                           20-25/65-82      40mv   14mv (rms)
           VAX6000 QUAD-Host-->  27/89      30mv   10.5mv (rms)

Operating system support in VAX/OpenVMS[edit]

   OpenVMS VAX V5.0-2:
     o  Single- and Dual-Host support with 2 EDA640s (MicroVAX 3300/3400
     o  Systems Communications Services (SCS) for the VMScluster is run
       over the Network Interconnect (NI) for MicroVAX 3300/3400.
   OpenVMS VAX V5.1:
     o  Standalone support for the KFQSA.
     o  Up to 6 ISEs on a single DSSI bus regardless of whether 1 or 2
        hosts systems are attached to the bus.
   OpenVMS VAX V5.1-1:
     o  Dual host support for KFQSAs (1 KFQSA per system).
   OpenVMS VAX V5.2:
     o  For 3300/3400 series, SCS communication over the DSSI.
     o  Multiple KFQSA's per system, but *not* multiple dual-hosted
        DSSI's per system (here, multiple means 2, since only 2 hosts
        are allowed on the DSSI bus).
     o  DSSI adapter must connect to another like adapter, e.g. EDA640
         to EDA640 or KFQSA to KFQSA.
     o  Up to 7 ISEs on a single DSSI bus when only 1 host system is
        attached to the bus.
   OpenVMS VAX V5.3:
    o  Mixed DSSI adapters, i.e. EDA640 to KFQSA.
    o  Mixed DSSI adapters, i.e. EDA640 to KFQSA.
     o  Double dual-host configurations, i.e. 2 DSSI buses between two
        CPUs. Each bus is terminated at its own pair of adapters.
   OpenVMS VAX V5.3-2:
     o  Support for VAX 4000. (The VAX 4000 has 2 SHACs and can support
        two KFQSAs on the bus, so it can have up to 4 DSSI buses).
   OpenVMS VAX V5.4-2:
     o  Support for the KFMSA, an XMI to DSSI adapter. The KFMSA may
        only be used to single-host a VAX 6000 model or dual-host 2 VAX
        6000 models together.
   OpenVMS VAX V5.4-3:
     o  Tri-host configurations supported.
   OpenVMS VAX V5.5-2:
     o  Quad-host configurations with VAX 6000, VAX 7000/10000
        series systems now supported.
        NOTE:  KFQSAs do not support SCS communication over the DSSI bus,
               they MUST have a NI connection in order to run the cluster
   OpenVMS Alpha V1.5, OpenVMS VAX V6.0:
     o  Quad-host DSSI configurations with DEC 4000 AXP systems supported.
        DEC 4000 systems can be configured only as end nodes on a DSSI
        bus, therefore, quad-host configurations must include at least
        two VAX systems.
     o  KFQSA Q-bus to DSSI configurations supported.
        VAX processors that use the KFQSA adapter can be placed on the
        same DSSI bus as DEC 4000 processors.

Third Party Peripherals[edit]

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[edit]

  • 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


  1. ^ "Digital Storage Systems Interconnect". Gordon Bell.