Behavior of TEST UNIT READY command in idle and standby power conditions

Gerry Houlder gerry.houlder at
Tue Nov 27 07:27:12 PST 2012

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Some questions have come up about the behavior of TEST UNIT READY command
when the target is in an idle or standby power condition.
One opinion is that TUR should be treated as a command that can be
processed in idle and standby power conditions, so therefore should
complete with GOOD status.
Another opinion is that TUR should respond with CHECK CONDITION status with
sense bytes indicating the target is in a low power condition and the
target should not attempt  to change to a higher power condition. (Read or
write commands would trigger the target to change to active power
condition, so TUR action would be different). The advantage of this
behavior is that it could be used to poll for low power conditions in lieu
of using REQUEST SENSE command.
The existing description of TUR command in SPC-4 rev. 36e doesn't address
the expected behavior for this case, other than the table of "preferred TUR
responses" doesn't include any of the low power condition sense
combinations. However, folks can argue that there are a lot of other
combinations that are not included there but should be reported if the
conditions are present.
I would like to see responses on the reflector for the preferred behavior
of TUR when in a low power condition. If there is a convergence of opinion,
i would like to have that opinion reflected in description of the TEST UNIT
READY command to avoid having to readdress this issue in the future.

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