Low Power Conditions

George Penokie GOP at rchvmp3.vnet.ibm.COM
Tue Jun 11 12:52:15 PDT 1996


* From the SCSI Reflector, posted by:
* "George Penokie" <GOP at RCHVMP3.VNET.IBM.COM>
*
 Jim McGraths note of 6/5/96 stated:

>The current draft SCSI specification calls for devices to return a
>CHECK CONDITION to the host if a command is issued to the device when
>the device is unable to execute the command due to being in a low
>power mode.  Thus if the host issues a READ that requires disk
>access, and the device is in STANDBY mode (motor turned off), it is
>suppose to fail the command.

 The statement above is true but ONLY if you use the START/STOP UNIT
 commands power condition mode 1, 2, and 3 to control the drives power
 conditions.  Those modes are there as a alternate to the automatic
 power control.  They should only be used by systems that want
 absolute control over what power conditions occur and when they
 occur. Under those conditions the check condition is appropriate.

 For automatic power control the power condition mode page should be
 used.  When this is used commands will execute without any extra
 intervention from a host.  The only thing the host will notice is a
 delay in the total time it takes to execute the command.  In addition
 to using the mode page parameters a host can force a drive into a
 power condition by issuing a START STOP UNIT command using modes Ah
 and Bh.  Those modes force the selected mode page timer to zero so
 the automatic power control stays in effect.

 Bye for now,
 George Penokie






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