Technical information

1.5 Digital Storage Systems Interconnect (DSSI)
The KA660 CPU module contains an embedded DSSI chip on the module
which becomes a DSSI bus interface connector that is dedicated to the mass
storage devices in the VAX 4000 Model 200 system. (See Figure 1–1 for the
VAX 4000 Model 200 block diagram.)
The DSSI bus interface connects to a DSSI Integrated Storage Element
(ISE). An ISE is a 5.25-inch disk drive that has special mounting brackets
for simplified enclosure installation and upgrading.
Each DSSI bus has the following characteristics:
• A 4-Mbytes-per-second bandwidth
• Up to eight nodes (one interface and up to seven ISEs)
• Eight data lines
• One parity line
• Eight control lines
DSSI architecture improves system performance as follows:
• The DSSI bus handles all mass storage transactions.
• Mass storage devices can act independently, since each device contains
its own controller. Several devices can work simultaneously.
The DSSI bus interface supports up to eight nodes. These eight nodes
include the DSSI interface and seven ISEs.
An ISE can maintain connections to more than one DSSI interface. In a
dual-host configuration in which multiple CPUs can be connected to the
DSSI bus interfaces. For example, two VAX 4000 Model 200 systems can
have access to each ISE over a shared DSSI bus.
For more information about using VAX 4000 Model 200 systems in a dual-
host configuration, and the advantages of such configurations, see the VAX
4000 Model 200 Dual-Host Systems manual.
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