Dual-port scsi sub-system with fail-over capabilities

Abstract

A dual port enclosure monitor for servicing a dual port bus includes four primary components: two enclosure monitors and two bus expanders with isolation circuitry. The sub-system is configured such that an enclosure monitor and an expander are both connected to an external port. The internal bus then connects the two expanders, as well as all of the internal devices (e.g. hard drives, CD-ROMs, tape drives). The enclosure monitors can communicate with various host devices over the external buses. These host devices can instruct the enclosure monitor to either connect or isolate the internal bus, thereby the peripherals attached to it. This is accomplished through a set of independent control signals that run between the monitor and the expander. There are three different methods of control. The first is independent, paired control between enclosure monitor/bus expander pairs. A separate host controls each enclosure monitor/bus expander pair. The second involves the enclosure monitor/bus expander pairs having cross over control between the ports. There, a single host send requests to both pairs, but using the opposite port as the port to be isolated. The recipient enclosure monitor controls the opposite port from its connection port. The third method utilizes two dependent enclosure monitor/bus expander pairs. The pairs are connected through an interconnection bus and can communicated isolation states as well as pass on isolation requests. As a further improvement, the pair of enclosure monitors is replace with a single enclosure monitor, which controls both bus expanders.