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 Cabling

DSSI MR & PS Connectors

The "RF Series Integrated Storage Element User Guide" (EK-RF72D-UG-008) says "The DSSI bus is a 50-conductor cable. Inside an enclosure, the bus may be a flat ribbon cable or a round bundle of twisted pairs. Between enclosures, the bus is a shielded round cable approximately ½-inch in diameter."

The short cables run from the PCBs that implement the interfaces to a standard connector on the back of the enclosure. (Different enclosures use different short cables with a given board; see the "KFQSA Module Installation and User Manual", Section 2.1 for a partial list.)

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 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)