Importance

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Abstract

A SCSI bus arbitration controller includes a hub to which each of the SCSI devices having access to the bus is connected. The hub includes a controller that monitors the data lines on the bus, as well as the BUSY and SELECT lines. The hub also includes a plurality of switches located between each of the devices and the data bus. For each set of device connections to a bus, the highest priority ID data line has a switch in it, as does the data line that corresponds to the ID associated with that particular device. During an initial arbitration cycle, the controller monitors the bus data lines, and determines which of the devices participate in the initial arbitration cycle. It then latches the identities of the participating devices in a register. Of the SCSI IDs of the system, the controller reserves the highest priority ID for itself. When the initial arbitration cycle is complete, it asserts the high priority ID, and opens the switches on the high priority ID line for each of the devices that participated in the initial arbitration. The non-participating devices therefore withdraw, and the remaining devices are allowed to arbitrate for control of the bus. In each subsequent arbitration, the device that gains control of the bus is thereafter prevented from arbitrating by closing its high priority ID data line switch until all of the other devices that participated in the initial arbitration have had access to the bus. The switch located in the data line for that device that corresponds to its own ID is also opened, to prevent it from being asserted against the other participating devices that did not yet gain access to the bus.

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First Claim