Digital Storage Systems Interconnect

The Digital Storage Systems Interconnect (DSSI) is a bus developed by Digital Equipment Corporation for connecting storage devices and clustering VAX systems. It was introduced in 1988. The bus has a bandwidth of 32 Mbit/s and was typically limited to a length of 25 meters.

    The major characteristics of the DSSI bus are:

     o  Eight-bit data path
     o  Up to eight nodes on the bus
     o  Distributed arbitration
     o  Peak bandwidth of 4 megabytes per second
     o  All systems connected to the same DSSI bus must have a common power/ground.
     o  "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.
     o  "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.
     o  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)
           VAX7000
           VAX10000

  DSSI SUPPORT IN OpenVMS:

   OpenVMS VAX V5.0-2:

     o  Single- and Dual-Host support with 2 EDA640s (MicroVAX 3300/3400
        series).

     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, and VAX 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
               software.

   OpenVMS Alpha V1.5, OpenVMS VAX V6.0:

     o  Quad-host DSSI configurations with DEC 4000 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.

See also