X3T9.2/90-080

MEMORANDUM    --    15 May 1990

TO:           John Lohmeyer, Chairman X3T9.2

FROM:         Bill Spence and Steven Justiss, Texas Instruments

SUBJECT:      Proposal for Note re Aborting Arbitration

A SCSI disk implementation has surfaced which violates the spirit if not the
law of arbitration.  Under certain circumstances, after winning arbitration
it does not go ahead and assert SEL.  Instead, it just drops its SCSI ID bit
and its claim on BSY so as to service an internal interrupt.

Since the standard recommends that SCSI devices which do not win arbitration
wait for the SEL signal to become true before releasing the BSY signal and
SCSI ID bit, the result may well be a hung bus.

To clarify the arbitration protocol in this regard, we propose that the fol-
lowing sentence be added to the third IMPLEMENTORS NOTE in 5.1.2, right at
the bottom of p 5-2 of Rev 10C:

    "Accordingly, when arbitration is won, a SCSI device should always
     proceed to assert SEL so as to force other devices from the bus and
     thus preclude the possibility of the bus being hung."