Digital Storage Systems Interconnect
The Digital Storage Systems Interconnect (usually given as DSSI) is a mass storage bus from DEC. 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).
DSSI is a 8-bit wide parallel bus. 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 DSSI specification lists the main attributes of DSSI as: multi-drop linear bus electrical topology; 8-bit parallel; low-cost, single-ended signalling interface circuitry; DC coupling.
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
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 thus made it possible to build VAXclusters from MicroVAX and smaller VAX computers by adding CI-bus functionality to smaller VAX systems.
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:
DSSI Disk Drives
DSSI Tape Drives
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)
