Digital Storage Systems Interconnect

Digital Equipment Corporation
Industry	Computer hardware
Products	Digital Storage Systems Interconnect (DSSI)

The Digital Storage Systems Interconnect (DSSI) is a computer bus created by Digital Equipment Corporation to link storage devices and facilitate clustering of VAX systems. Developed as a more compact and cost-effective substitute for the earlier DEC Computer Interconnect, it was primarily aimed at office settings. Eventually, DSSI was replaced by Parallel SCSI technology.

Launched in 1988, DSSI provides a data transfer rate of 32 Mbit/s (equivalent to 4 MB/sec) and typically supports cable lengths up to . It was also compatible with MIPS-based DECsystem machines as well as DEC Alpha-based AlphaServer platforms.

Devices using the DSSI interface can be recognized by a unique five-wire power connector that includes an additional lead designed for Battery or UPS connections, ensuring uninterrupted power supply during electrical failures.

Characteristics

The DSSI bus offers several notable specifications:

An eight-bit wide data channel
Support for up to eight nodes per bus, accommodating multiple initiators and targets
Arbitration distributed across devices
Maximum throughput of 4 MB/s
Requirement for common power and ground among all connected components
The "Q-Bus 5-Enclosure Rule": limits the number of enclosures on a single DSSI bus to five (excluding SF200/210 enclosures); only one SF100 enclosure permitted per bus
The "VAX 6000 Cabinet Rule": permits two or three VAX 6000 systems plus one storage cabinet (usually an SF200 or SF210) on each DSSI bus
Maximum cable length capped at 27 meters (89 feet), with ground offset voltage thresholds varying according to length

                                 Bus Length      Permissible 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

Support for DSSI evolved through various OpenVMS versions:

OpenVMS VAX V5.0-2: Provided single- and dual-host functionality via EDA640 adapters on MicroVAX 3300/3400 models; supported SCS over Network Interconnect.
V5.1: Introduced standalone KFQSA support; allowed up to six ISEs per bus regardless of the number of hosts.
V5.1-1: Enabled dual-host KFQSA operation, with one KFQSA adapter per system.
V5.2: Added SCS communication over DSSI for MicroVAX; permitted multiple KFQSA adapters per system but restricted multiple dual-hosted DSSI adapters per system; imposed pairing constraints on adapters; supported up to seven ISEs with a single host.
V5.3: Allowed mixed use of EDA640 and KFQSA adapters; supported dual-bus, dual-host configurations.
V5.3-2: Extended support to VAX 4000 systems, enabling up to four DSSI buses.
V5.4-2: Introduced KFMSA adapter support (XMI to DSSI bridge); limited to single-host or dual-host setups on VAX 6000 systems.
V5.4-3: Added tri-host configuration capabilities.
V5.5-2: Expanded quad-host support to include VAX 6000 and the VAX 7000/10000 series; noted that KFQSA adapters require NI connections for SCS communications.
OpenVMS Alpha V1.5 / VAX V6.0: Supported quad-host configurations with DEC 4000 AXP systems; DEC 4000 machines were restricted to end-node roles, requiring at least two VAX systems within quad-host clusters; enabled compatibility between KFQSA Q-bus to DSSI adapters allowing mixed processor buses.

This development path highlights DEC’s commitment to broadening DSSI’s integration across its hardware lineup while preserving essential cluster communication functions critical to enterprise operations.

Third-Party Devices

Manufacturers such as CMD and Symbios Logic developed chipsets and peripherals compatible with the DSSI standard. For example, CMD provided the CDI-4000 adapter, which allowed SCSI devices to interface with a DSSI bus.

References

Characteristics

Operating system support in VAX/OpenVMS

Third-Party Devices

See also

References