multi scsi questions

[Justin T. Gibbs]

>On 07-Jul-99 Justin T. Gibbs wrote:
>> Think of the arbitration like this:
>>         Low Byte High Byte
>>         XXXXXXXX XXXXXXXX
>>         -----------------
>>         01000000 10000000
>> 
>> This is what an arbitration for the bus might look like. The device
>> at id 1 cannot see that id 8 is also arbitrating.  So if they both
>> pop up during the same arbitration cycle, id 8 will bow out. 
>
>It seem I have no real idea how this arbitration works.
>Do the devices look from time to time if there is something? 
>I thought of it more like: The adapter sends a request, all are
>listening but only the one with the right ID is responding.

The bus is either free, in arbitration, in selection, or in an
information transfer phase (message phases, status phase, command phase, etc).
When the bus is free, devices with work to do (either an initiator
wanting to send a command to a device, or a device wanting to reconnect
to the initiator to continue a transaction) set the data line corresponding
to their ID on the bus.  If no line of a higher priority comes on within
the arbitration window, they have won arbitration and can claim the bus
by setting the BSY signal. The winner must now select the device it wishes
to talk to.  It does this by driving the data line corresponding to the ID
of the device of interest (his own line is still driven), asserts the
SEL signal and drops BSY.  The SEL signal tells other devices that they
need to pay attention to the data bus and look for their own ID.  To
accept selection, the target device will assert BSY, the initiator of
the selection will drop SEL, and we move on to the information phases
you typically hear about in SCSI (command, data, msgin, msgout, status).

>So what happens when the adapter asked for a high ID and the devices
>bows out? Has the adapter to repeat the request? Is there a delay in
>transfer? 

If you lose the arbitration, you have to try again during the next
opportunity for arbitration.

>And why doesn't the narrow device react. It sees only the Low Byte --
>so there is his own ID  number, and the Wide device left the Bus to the
>narrow device, so the Bus is not busy.

The device being selected needs to see the ID line for the selecting
device.  This isn't a requirement for arbitration.

--
Justin



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