Browse Prior Art Database

SCSI Bus Fairness Through Selective Reselection

IP.com Disclosure Number: IPCOM000099513D
Original Publication Date: 1990-Jan-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 3 page(s) / 79K

Publishing Venue

IBM

Related People

Buesing, DA: AUTHOR [+2]

Abstract

When two or more SCSI devices arbitrate for the SCSI bus at the same time, the highest-priority device will always win. This is a part of the SCSI standard. In some applications, this architecture is undesirable as it may prevent the low-priority devices from being serviced in a timely fashion. A selective reconnection scheme is disclosed for these applications.

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SCSI Bus Fairness Through Selective Reselection

       When two or more SCSI devices arbitrate for the SCSI bus
at the same time, the highest-priority device will always win. This
is a part of the SCSI standard.  In some applications, this
architecture is undesirable as it may prevent the low-priority
devices from being serviced in a timely fashion.  A selective
reconnection scheme is disclosed for these applications.

      One initiator with SCSI ID 7 (highest priority) monitors the
bus and determines when an arbitration phase is started (t1 in Figs.
1 and 2).

      All participants in arbitration must wait at least an
Arbitration Delay (time A) from the assertion of their SCSI ID on the
bus before determining if they should leave the bus.  Further, a
device may not enter into arbitration for a Bus Set Delay (time E)
after the start of arbitration (busy asserted).  One may conclude
from this that all those that are participating in arbitration have
placed their SCSI ID on the bus by the end of the Bus Set Delay
(time E). This is called "Latch Arbitrators" time (t3 in Fig. 1).

      The initiator enters arbitration by placing its SCSI ID on the
bus prior to Latch Arbitrators time (t2 in Figs. 1 and 2).  At Latch
Arbitrators time, the initiator latches the SCSI IDs of the targets
participating in arbitration (t3 in Fig. 1).  This establishes a Poll
List of SCSI IDs. Since the initiator has the highest-priority SCSI
ID, it wins the arbitration (t4 in...