From:	James McGrath				X3T9.2/91-52  Rev 0
	Quantum Corporation
	580 Cottonwood Ave
	Milpitas, CA 95035

To:	John Lohmeyer
	Chairman, X3T9.2
	3718 N. Rock Road
	Wichita, KS 67226

Date:	April 22, 1991

Subject: Cable Select Feature


Purpose:

    This is a proposal to add a feature in the ATA (AT Attachment
    Interface).  The feature has been discussed in concept at a
    couple of meetings, beginning in December 1990.  At those
    times the committee was supportive.  Unfortunately, it took
    me until last week to reach agreement on all the technical 
    elements of the proposal.

    I would like to see this proposal adopted and forwarded as part
    of revision 2.3 of the ATA standard.


Background:

    This feature allows a drive to determine whether it is a master
    or a slave by simply examining the signals on the ATA cable.  This
    eliminates the need for jumpering drives as master/slave.

    Two drives attached to the same ATA cable can have identical
    jumper settings.  This has a large practical benefit in
    configuring end user systems, since the end user can simply take
    the drive out of the box and install it in any 2 drive system
    without first adjusting the jumpers.

    A large OEM requested that we develop this feature for them.
    It would be desirable to make it available to all system designers
    and insure a compatible implementation between drive suppliers.
    The best way to accomplish this is via incorporation in the ATA
    standard.

    The Cable Select feature does not solve the Master/Slave booting
    problem (i.e.  determining whether a slave is present and responding
    appropriately to the host).  However, it does not interfere with
    any of the solutions to this problem - it is orthogonal.






    This proposal has been discussed before in the committee and the
    line chosen for this feature is line 28, Spindle Sync.  Although
    the primary reason for choosing this line was the lack of another
    possibility, pin 28 has certain advantages.  It is a drive to drive
    signal, and so should not interfere with host system operation.
    While using this pin for the Cable Select feature will preclude
    using it for spindle synchronization, a drive can have a jumper
    installed to allow selection of either feature.

    It is envisioned that drives will be shipped with the jumper set
    to support the Cable Select feature.  This will allow most end
    users to take advantage of the ease of configuration offered
    by the feature.  For those users wishing to take advantage of
    spindle synchronization, they will have to change the jumper
    setting and use another means to inform that drive whether is
    is a master of a slave (e.g. another jumper).  This requires more
    work on the part of the end user, but anyone who is installing a
    drive into a drive array will have to perform a number of
    configuration steps anyway.


Proposal - in rev 2.3 of the AT Attachment Interface:

    5.1 - Replace the second paragraph with:

      The designation as Drive 0 or Drive 1 is made by either a
      jumper plug/switch on the drive or through use of the Cable
      Select (CSEL) function.

    6.1 - Add the following as the last paragraph:

        Control signals that may be used for two mutually exclusive
        functions are named with the two functions separated by a 
        colon (:), e.g. pin 28, described as SYSYNC:CSEL, can be used
        for either the Spindle Synchronization or the Cable Select
        functions.

    Table 6-1: Interface Signals

    Replace the wording for pin 28 with the phrase "SPINDLE SYNC"
    with "SPINDLE SYNC:CABLE SELECT" and "SPSYNC" with "SPSYNC:CSEL".

    Table 6-2: Interface Signals Description

    Replace the wording for pin 28 with the phrase "Spindle Sync"
    with "Spindle Sync:Cable Select" and "SPSYNC" with "SPSYNC:CSEL".






    Replace section 6.3.15 with:

      6.3.15  SPSYNC:CSEL (Spindle Synchronization/Cable Select) (Optional) 

      Either or both functions may be implemented.  If both functions are
      implemented, then they cannot be active at the same time.  Which
      function is active depends on a vendor-defined switch.  All drives
      connected to the same cable should have the same function active
      at the same time (i.e. SPSYNC and CSEL cannot be mixed on the
      same cable).

      Prior to the introduction of this standard, this signal was defined
      as DALE (Drive Address Latch Enable), and used for an address valid
      indication from the host system. If used, the host address and chip
      selects, DAO through DA2, CS1FX-, and CS3FX- were valid at the
      negation of this signal and remained valid while DALE was negated,
      therefore, the drive did not need to latch these signals with DALE. 

    Create a new section, "6.3.15.1 SPSYNC (Spindle Synchronization)"
    with the wording of the current (rev 2.3) section 6.3.15, except for
    the last paragraph (which is instead moved to the common section above).

    Create a new section, "6.3.15.2 CSEL (Cable Select)" with the following
    wording:

      The drive is configured as either Drive 0 (Master) or Drive 1
      (Slave) depending upon the value of pin 28.  If pin 28 is 
      grounded, the drive is drive 0.  If pin 28 is open, the drive
      is drive 1.