Arbitration and Data Out bus phase similarities?

Gerry_Houlder at Gerry_Houlder at
Wed Aug 11 08:16:45 PDT 1999

* From the T10 Reflector (t10 at, posted by:
* Gerry_Houlder at
The rules for starting arbitration require that the SCSI device observe BUS
FREE phase for a specified amont of time. Seeing BUSY active when it starts
looking at the bus (even for an arbitration phase) means that the SCSI
device shall not attempt to participate. This step prevents any "late"
devices from participating in arbitration or from arbitrating in the middle
of a DATA OUT phase, so the situation you describe should never happen.

>Just a quick question.  I just noticed that the Arbitration and Data Out
>phase look the same (looking at BSY being asserted and SEL, C/D, I/O,
>ATN, MSG being deasserted).  If the situation occurred where we were in
>the middle of the Data Out phase (synchronous) and a target (call it
>targ1) has REQ'd up to the max offset and is having to waiting an amount
>of time for the ACK's.  In this time frame a second target (targ2)
>decides that it is going to try and reselect the initiator.  If it looks
>at the bus at this point could it confuse the Data Out phase for an
>arbitration phase and drive BSY and it's ID onto the bus.  Additionally,
>if this can occur ,then if the Initiator starts to respond with the
>ACK's and driving data onto the bus at the same time then there could be
>an unknown on the lower 8 bits of the data bus.  I'm not even sure how
>often (if at all) this situation could present itself, but what would
>the proper way to handle this be?  Thanks.
>Art Rodgers

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