Bus Conditions
20 SL500: Interface Reference Manual • July 2008 Revision: F • 96122
Bus Conditions
SCSI uses bus conditions to interrupt operations and to handle errors. These
conditions cause a SCSI device to accomplish certain actions by altering the bus
phase sequence.
The library supports five types of bus conditions:
■ Attention
■ Parity Errors
■ Reset
■ Contingent Allegiance
■ Unit Attention
Example: A device has a problem executing a command or something happens to
that device during an I/O operation, such as a device reset. In this case, a
Contingent Allegiance condition is flagged to indicate to the initiator that a
problem exists with the device.
Attention
The Attention condition allows an initiator to inform a device that a message is
ready for transfer. The target obtains this message by going to the Message Out
phase.
The initiator creates the Attention condition by asserting the ATN signal on the
SCSI bus any time, except during the Arbitration or Bus Free phases.
Parity Errors
A Parity Error condition occurs if the target detects one or more parity errors in the
message bytes received. The target retries the message by asserting the REQ signal
after detecting the negated ATN signal and before changing to another bus phase.
When the initiator detects this condition, it resends the message bytes in the same
order as before.
If a parity error occurs during Command Out, Data Out, or Status In, the target
sends a “Restore Pointers” message to the initiator and retries the command, data,
or status phase.
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