Digital Storage Systems Interconnect - Characteristics

Characteristics

The major characteristics of the DSSI bus are:

• Eight-bit data path
• Up to eight nodes on the bus including variable numbers of initiators and targets
• Distributed arbitration
• Peak bandwidth of 4 megabytes per second
• All systems connected to the same DSSI bus must have a common power/ground.
• "The Q-Bus 5-Enclosure Rule": For Q-bus VMScluster configurations, a maximum of five enclosures, _excluding_ SF200/210s, can be configured on a single DSSI bus. Note that only _one_ (1) SF100 is allowed on any DSSI bus.
• "The VAX_6000 Cabinet Rule": For a VAX 6000 DSSI VMScluster system, a maximum of two or three VAX 6000 systems and one storage cabinet, typically an SF200 or SF210, can be configured on a single DSSI bus.
• Maximum DSSI bus length cannot exceed 27 meters (89 feet) and ground offset voltage cannot exceed 30mv (dc) or 10.5mv (rms).

Bus Length Allowable Offset Meters/Feet (DC) (AC) ----------- ---------------- up to 20/65 200mv 70mv (rms) 20-25/65-82 40mv 14mv (rms) VAX6000 QUAD-Host--> 27/89 30mv 10.5mv (rms) VAX7000 VAX10000