Digital Storage Systems Interconnect

DDSSI cable

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.."

Advance in DSSI Interface Usage

In the beginning there were DSSI interfaces and DSSI Integrated Storage Elements connected to them. Each individual ISE was a kind of Hierarchical Storage Controller, i.e. a cluster-capable controller, managing just a single storage device. This was a smart but expensive design and limited storage capacity to a maximum of 8 DSSI nodes (i.e. one or two DSSI controllers and seven resp. six DSSI devices) per DSSI bus.

In 1994 a new concept was introduced for DSSI devices: The HSD05 Array Controller as a first of three models of DSSI-to-SCSI bus adapters (HSD05/HSD10/HSD30). From now on an HSD05 could connect up to seven SCSI devices (disk and tape drives) to a DSSI bus as a single DSSI node, thus multiplying the available storage capacity. As an additional effect, the costs were reduced because cheaper SCSI disks (without a complex DSSI interface) could be used.

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"

MR female connector

When found on cables, these exist in both 'straight' and 'right-angle' forms (to allow cables to be neatly dressed, in part so they are less likely to get damaged).

With these two different types of connectors, three different basic types of long DSSI cables are needed (cable names for types available from DEC included):

MR-MR - BC21Q-xx (2x straight); BC29S-xx (right-angle to straight)
PS-PS - BC21M-xx (2x right-angle)
MR-PS - BC22Q-xx (right-angle PS to straight MR); BC29R-xx (right-angle PS to right-angle MR)

(As usual with DEC cables, the '-xx' gives the length, numerically.) The MR-PS cables can be reversed to provide PS-MR.

Two types of DSSI connectors are found on devices: