DSSI Specification 1.0.0 19900327

This document defines the Digital Storage Systems Interconnect (DSSI), a high-performance computer-to-computer interconnect tailored for low-end and mid-range systems, including VAX clusters.

DSSI is fundamentally based on the higher-layer protocols of Digital's Computer Interconnect (CI) (DEC STD 161) but differentiates itself by using a different data-link and physical interconnect designed for reduced cost.

Key characteristics and features of DSSI include:

• Topology: It utilizes a multidrop electrical topology with a single electrical path, contrasting with CI's star topology and dual paths.
• Data Bus: Employs an 8-bit parallel data bus with DC coupling, using low-cost, single-ended drivers and receivers.
• Performance: Supports data transfers up to 4 Mbytes/second (approximately 32 Mbits/second).
• Physical Constraints: Limited to a maximum cable length of 6 meters and supports up to 8 nodes.

• Protocol Layers: The document details the DSSI's architecture, specifying:

  • Datalink Layer: Responsible for reliable, error-free delivery of information frames, including channel control, arbitration, and transmission error recovery.
  • Physical Bus Protocol: Defines the bus phases (e.g., Bus Free, Arbitration, Selection, Command Out, Data Out, Status In), signal definitions (e.g., DATA<7:0>, BSY, SEL, C/D, I/O, REQ, ACK, RST), and precise timing parameters.
  • Electrical and Physical Specifications: Outlines requirements for bus termination, driver/receiver characteristics, power supply, cable types (50-conductor, shielded for external use), connectors (50-pin), and rules for device connection/disconnection (hot-swap is prohibited).

• CI Protocol Adaptations: It clarifies additions and modifications to the CI Port layer, such as using 3-bit multibit sequence numbers (instead of CI's single-bit) and specific handling for unrecognized packets and self-directed commands.

The document serves as the functional specification for the DSSI bus, enabling the design and configuration of DSSI nodes into an operational system.