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

Matthew Jacob mj at
Tue Nov 27 09:19:47 PST 2012

* From the T10 Reflector (t10 at, posted by:
* Matthew Jacob <mj at>
On 11/27/12 07:27, Gerry Houlder wrote:
> 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.
Why would this be any different than a TUR response for a spun down disk 
which responds with a check condition that asc that says in essence 
"need spinup"?
You know the unit is there and present because it responds to the TUR 
(with any status). You know that it isn't ready (for media access in the 
case of disk) because of the specific CHECK CONDITION response.
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