Compaq S5 manual Using the 16-Bit UltraSCSI Hub, Large Hub Fair Arbitration

3

Using the 16-Bit UltraSCSI Hub

This chapter discusses fair arbitration of the SCSI bus by the 5.25" SBB Hub. It describes addressing configurations, tells you how to use the large Hub front panel, and gives guidelines for selecting the SCSI cables.

UltraSCSI configuration guidelines are documented in EK-ULTRA-CG. These guidelines include a list of all UltraSCSI components and the last few example configurations include an UltraSCSI Hub. Refer to the configuration guidelines for bus length and SCSI bus data transmission rates.

3.1Large Hub Fair Arbitration

The DWZZH-S5 and the DWZZH-09 can be operated with Fair Arbitration enabled or disabled. Although it is recommended that both be operated with Fair Arbitration enabled, it is not required.

In order to allow four or more host SCSI IDs to participate on a single SCSI bus and to prevent lower SCSI host IDs from being starved for bus time, a fair arbitration (Fair ARB) scheme is employed. Fair ARB works by assigning SCSI ID 7, the highest priority ID to the Hub. When a SCSI arbitration phase occurs, all the arbitrating IDs are captured in a register.

ID7 is reserved and may not be assigned to any SCSI device connected to the Ultra SCSI Hub, regardless of having FairARB enabled or disabled.

CAUTION

Most manufacturers ship Host Bus Adapters with a default SCSI ID of 7. Ensure that the ID is changed before establishing a connection betwen the adapter and the Hub.

The winning ID for this group will be the highest priority ID and allowed bus privilege. After this ID has completed its transaction, the ID will be removed from the group and at the next arbitration phase, the remaining highest ID will be serviced. This will continue until all of the IDs in the group have been serviced once. All requests from IDs not contained in the register will be backed off using ID 7.