Digital Storage Systems Interconnect

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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. The DSSI's analog characteristics mean that DSSI installations require terminators.

The DSSI 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 Cabling

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

DSSI MR & PS Connectors

The long inter-enclosure DSSI cables are found with two different types of connectors:

  • MR (Micro Ribbon or midrange), with flat contacts = "50 Way High Density Micro Ribbon"
  • PS (Pin Socket or pedestal style), with round pins = "50 Way High Density Honda"
PS Connector female (Cable)

With these two different types of connectors, three different types of long DSSI cables are needed:

  • MR-MR
  • PS-PS
  • MR-PS (can be reversed to provide PS-MR)

Two types of DSSI connectors are found on devices:

  • 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

DSSI Controllers/Interfaces

DSSI Hierarchical Storage Controllers (HSDs)

Hierarchical Storage Controllers:

DSSI Storage Expanders

DSSI Disk Drives

DSSI Tape Drives

Images

PS Connector male (CPU)
PS Terminator female
Interface Card Assembly (5.25" FH disk)
Interface Card Assembly Dual RF35
Device Connector RF30


External Links

Retrieved from "https://gunkies.org/w/index.php?title=Digital_Storage_Systems_Interconnect&oldid=31469"