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In article <317c0381.0@news.globalnet.co.uk>
           bds@globalnet.co.uk "Brian Sheehan" writes:

| Could someone please clarify the following point regarding SCSI
| addresses.
|
| On a chain of devices will a device with ID1 get more priority than a
| device of ID4 or are all devices treated the same.
|
| Any advice on this appreciated
|
|
| Brian Sheehan
| bds@globalnet.co.uk
|

   The priority is _only_ used when there is bus contention for a
   connection.  This happens during the arbitration phase that precedes
   a selection or re-selection phase.  This is typically a few
   micro-seconds, say one thousanth of the total time for a transfer.

   Contention will only happen if you have two initiators (host
   adaptors) on the same bus, which is rare, and they both want to use
   the bus at exactly the same time, even rarer!  Contention will also
   happen if you have only one host adaptor but two (or more)
   peripherals with _active/outstanding_ transfers.  Then the targets
   (peripherals) can also want to re-select at the same time.  The
   priority (ID) is used to resolve this clash.  Note both peripherals
   have to in use _at the same_  Simply having two perpherals connected
   or using them alternately is not enough to cause contention.

   Priority is a vital part of the SCSI protocol (it cannot have
   undefined situations) but in real life it will have very little or no
   impact.

--
Peter Hayward, Cambridge, UK.      | In a democracy the public will only
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