Technical information

1.5 Digital Storage System Interconnect (OSSI)
The KA670 CPU module contains two DSSI bus interfaces that are
dedicated to the mass storage devices in the VAX 4000 Model 300 system.
(See Figure 1-1 for the VAX 4000 Model 300 block diagram.)
Each DSSI bus interface connects to a DSSI integrated storage assembly
(lSA). An ISA is a 5.25-inch integrated storage element (lSE) that is housed
in a special mounting bracket 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 ISAs)
•
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 two DSSI bus interfaces support up to 16 nodes. These 16 nodes include
the 2 DSSI interfaces and 14 ISAs.
An ISA 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, two VAX 4000 Model 300 systems have access to each
ISA on the DSSI busses.
For more information about dual hosting VAX 4000 Model 300 systems
and the advantages of such configurations, see the section on dual-host
capability in the VAX 4000 Model 300 Operation manual.
Base System Description 1-13