LASER MAGNETIC STORAGE MEMO INTERNATIONAL COMPANY ______________________________________________________________________________ Optical Storage Division 7 March 1988 To: Members of X3T9.2 From: Paul Boulay (719) 593-4323 Subject: Request Sense Clarification cc: Randy Hines, LMS The Request Sense Command may receive a report of a Unit Attention condition before a report of CHECK CONDITION Status on a previous command. This behavior is not mandatory. (See 6.1.3 Unit Attention Condition.) However some implementers of SCSI see a contradiction to this in the second paragraph of the Request Sense Command description. This note proposes wording changes to that paragraph to make it clearer that reporting a Unit Attention condition is acceptable. There need to be other changes to this paragraph (Long vs. Short Busy reporting - 87-192, etc.) that are not considered here. The sense data shall be valid for a CHECK CONDITION Status returned on the prior command and may be valid if a bus free error occurred (see section 5.1.1.) in other circumstances. A REQUEST SENSE Command may report sense data due to the following conditions without a prior report of CHECK CONDITION Status: A Bus Free error (see section 5.1.1), or a Unit Attention Condition (see section 6.1.3.). Targets are not precluded from reporting other device or controller status information in response to a REQUEST SENSE Command received without a prior CHECK CONDITION report. Sense Data created when CHECK CONDITION Status is presented shall be preserved by the target until .... I believe that this is a clarification and that it does not change the definition of Request Sense. It may be that this is not consistent with the way others have read the Request Sense and Unit Attention reporting requirements. For devices that implement "hard" reset (especially when the reset action is caused by a Bus Device Reset message) reporting Unit Attention sense data in response to a Request Sense received after such a reset is better than the alternative. The other allowed response is to report No Sense and wait for a command other than Request Sense or Inquiry to send a CHECK CONDITION response before posting Unit Attention sense data. Imagine a scenario where a target reports a CHECK CONDITION due to an error encountered executing a command. Before the initiator of that command sends a Request Sense, a different SCSI bus device selects and sends Bus Device Reset. Then the first Initiator sends Request Sense. In this case, Unit Attention sense data would less confusion than a No Sense response.