LASER MAGNETIC STORAGE                            MEMO
     INTERNATIONAL
_________________________________________________________________________

    Optical Storage Division
                                                     10 October 1988

To:       SCSI Standards Committee X3T9.2

From:     Paul Boulay
          (719) 593-4323     

Subject:  Disconnect Control via Mode Select

  This proposal expands the Device Disconnect-Reconnect Parameters page (Draft 
SCSI-2, Rev. 5, 7.5.1, pg. 7-84) to allow an initiator to make explicit the 
times within a command when a disconnect is appropriate. 

  The objective is to obsolete the facility allowing multiple outbound 
IDENTIFY messages during the course of a command. (See draft SCSI-2, Rev. 5, 
5.6.8, pg. 5-21.) As currently defined, the only function performed by the 
initiator sending multiple IDENTIFY messages is to change the disconnect 
privilege bit. I don't propose to remove this facility altogether, but to 
provide a cleaner, more efficient alternative. If this option ends up being 
supported, perhaps the subsequent outbound IDENTIFY can be removed in SCSI-3.

  In the discussion on this at the Colorado Springs Plenary and at the San 
Jose Working Group meetings, the only use for multiple outbound IDENTIFY 
messages is to prohibit disconnect after the initial physical positioning 
action of a command. Not permitting disconnect during the data transfer is 
desirable in some systems to enhance performance and/or because of design 
constraints on the host adapter to system interface. Sending a subsequent 
IDENTIFY message interrupts the flow of data and many current target 
implementations are not able to handle this. Rejecting the disconnect message 
may cause additional delays due to the targets error recovery. In sum, the 
current situation is a mess and compromises the performance gains that might 
be gained by not doing the disconnect.




















  Add the following to Table 7-74, Device Disconnect-Reconnect Parameters:

==============================================================================
  Bit |   7    |   6    |   5    |   4    |   3    |   2    |   1   |   0    |
Byte  |        |        |        |        |        |        |       |        |
==============================================================================

                                   .  .  .         

------|----------------------------------------------------------------------|
 12   |                       Reserved                              |  DDTD  |
------|----------------------------------------------------------------------|
 13   |                             Reserved                                 |
------|----------------------------------------------------------------------|
 14   |                             Reserved                                 |
------|----------------------------------------------------------------------|
 15   |                             Reserved                                 |
==============================================================================


  A Disable Data Transfer Disconnect (DDTD) bit of zero indicates that 
a target may initiate a disconnect during the data transfer phase of commands 
as permitted by the other parameters on this page.

  A Disable Data Transfer Disconnect bit of one indicates that a target shall 
not initiate a disconnect once the data transfer phase of a command has 
begun.

  IMPLEMENTORS NOTE: If this bit is set, for write type commands, consider 
  allowing the initial seek to complete before starting data transfer.