Digital Storage Systems Interconnect

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 DSSI was superseded by Parallel SCSI.^[1]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.

Characteristics[edit]

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)            VAX7000            VAX10000 

Operating system support in VAX/OpenVMS[edit]

  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 7000/10000         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 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

References[edit]