Digital Storage Systems Interconnect

The Digital Storage Systems Interconnect (usually given as DSSI) is a mass storage bus from DEC. The DSSI specification lists the main attributes of DSSI as: 8-bit wide parallel; multi-drop linear bus electrical topology; low-cost, single-ended signalling interface circuitry; DC coupling.

It is an alternate physical layer in the Computer Interconnect system; the CI system had a 'native' physical layer before the creation of DSSI. The relationship between the DSSI and the CI's physical layer is shown in the following diagram:

                                                          <-----+
                        +---------------------+                 |
                        | Port Driver         |                 |
                        | Layer               |                 |
                        | ( SCA Specification)|                 |
                        +---------------------+                 |
                                  |                             |
                                  |                             |
                        .......................                 |
                        . CI Port Adapter     .                 |
                        . ( eg: VAX CI Port ) .                 |
                        .......................                 |
                                  |                             | P
      +------>                    |                             | P
      |                 +---------------------+                 | D
      |                 |     CI Port         |                 |
    C |                 |     Layer           |                 | L
    I |                 |   ( DEC Std 161 )   |                 | A
      |                 +---------------------+                 | Y
    A |                    |               |                    | E
    R |             +------+               +------+             | R
    C |             |                             |             |
    H |  +----------------------+      +---------------------+  |
    I |  | DSSI Datalink        |      |    CI Datalink      |  |
    T |  |    Layer             | .... |      Layer          |  |
    E |  | ( DSSI Spec. )       |      |  ( Dec Std 161 )    |  |
    C |  +----------------------+      +---------------------+  |
    T |             |                             |       <-----+
    U |             |                             |       <-----+ P L
    R |  +----------------------+      +---------------------+  | H A
    E |  |    DSSI Physical     |      |    CI Physical      |  | Y Y
      |  |    Interconnect      |      |    Interconnect     |  | S E
      |  |    ( DSSI Spec. )    |      |   ( DEC Std 161 )   |  | I R
      |  +----------------------+      +---------------------+  | C
      +------>                                            <-----+ A
                                                                  L

                Figure 1-2:  CI PPD Architectural Layers

DSSI thus made it possible to build VAXclusters from MicroVAX and smaller VAX computers, by adding CI-bus functionality to smaller VAX systems. It can also be used to create loosely-coupled multi-processors, by sharing a number of disk drives between two systems. Such shared drives are termed 'Integrated Storage Elements' (ISEs).

A description of the DSSI from the 'Digital's Storage System Interconnect' protocol specification document:

"The DSSI, supporting the needs of low-end and mid-range systems, is one in a family of high-performance computer-to-computer interconnects .. that combine a common host interface and port layer with an implementation-specific datalink and physical interconnect.
CI-class interconnects provide the transmission services required by Digital's System Communication Architecture (SCA) - a four-tiered set of protocols and interfaces.."

DSSI Controllers/Interfaces

• KFQSA QBUS DSSI Adapter
• KFMSA XMI Dual DSSI Adapter
• KFMSB XMI DSSI adapter
• KFESA EISA DSSI Adapter
• KFESB EISA DSSI Adapter
• KFPSA PCI DSSI Adapter
• KFE52 VAXft Multi-Function Interface
• KDXDA VAXft 810 DSSI Interface
• EDA640 MicroVAX 3300/3400 KA640 embedded DSSI Adapter (EDA)
• KFA40 DEC 4000 I/O Module

DSSI Hierarchical Storage Controllers (HSDs)

Hierarchical Storage Controllers:

• HSD05
• HSD10
• HSD30
• HSD50

DSSI Disk Drives

• RF30
• RF31
• RF31F
• RF31T
• RF32
• RF35
• RF36
• RF37
• RF70
• RF71
• RF72
• RF73
• RF74
• RF75

DSSI Tape Drives

• TF30
• TF70
• TF85
• TF86
• TF87
• TF88
• TF89

DSSI Connectors

There are (at least) four types of DSSI connectors:

Two DSSI cable connectors:

• MR (midrange or micro ribbon) connector = "50 Way High Density Micro Ribbon"
• PS (pedestal style or pin socket) connector = "50 Way High Density Honda"

Two DSSI device connectors:

• Single DSSI Device Connector 50-pin (SCSI-like)
• Dual 3.5" / Single 5.25" DSSI Disk Interface Card Assembly e.g. for VAX 4000, BA430/BA440

• BA430 BA440 Enclosure Maintenance.pdf (EK-348AB-MG-002)
• DSSI VMScluster Installation and Troubleshooting Guide (EK-410AB-MG.D01)

DSSI PS Connector female (Cable)

DSSI PS Connector male (CPU)

DSSI PS Terminator female

DSSI Interface Card Assembly Dual RF35

DSSI Device Connector RF30

DSSI MR & PS Connectors

DSSI Interface Card Assembly (5.25" FH disk)

External Links

• DSSI - DSSI and related documents at Bitsavers
  • Digital's Storage System Interconnect - protocol specification document
• MicroVAX Dual-Host Systems (EK-338AC-DH-003) - contains a clear description of how a DSSI can be used to create a loosely-couple multi-processor
• RF Series Integrated Storage Element User Guide (EK-RF72D-UG-008)