
S T 1 2 5 5 0 W D   B A R R A C U D A 2    SEAGATE
NO MORE PRODUCED                                      Native|  Translation
                                                      ------+-----+-----+-----
Form                 3.5"/HH               Cylinders    2707|     |     |
Capacity form/unform  2139/ 2572 MB        Heads          19|     |     |
Seek time   / track   9.0/ 0.9 ms          Sector/track     |     |     |
Controller           SCSI2 DI/WI/FAST      Precompensation
Cache/Buffer           512 KB SEGMENTED    Landing Zone
Data transfer rate    6.000 MB/S int       Bytes/Sector      512
                     20.000 MB/S ext
Recording method     RLL 1/7                        operating  | non-operating
                                                  -------------+--------------
Supply voltage     5/12 V       Temperature *C         5 50    |    -40 70
Power: sleep              W     Humidity     %                 |
       standby            W     Altitude    km                 |
       idle          13.0 W     Shock        g                 |
       seek               W     Rotation   RPM      7200
       read/write         W     Acoustic   dBA
       spin-up            W     ECC        Bit
                                MTBF         h     500000
                                Warranty Month        60
Lift/Lock/Park     YES          Certificates


**********************************************************************
                        L   A   Y   O   U   T
**********************************************************************
SEAGATE  ST12550W/WD BARRACUDA 2  PRODUCT DESCRIPTION

  +---------------------------------------------------------+  1
  |                                         +J01 +--J4---+  |XX Power
  |                                         1--+ 1-------+  |XX
  |                                                         |1-+
  |                                                         || |J5
  |                                                         || |
  |                                                         |+-+
  |                                                         1XX
  |                                                         |XX
  |                                                         |XX
  |                                                         |XXI/O
  |                                                         |XX
  |                                                         |XX
  |                                                         |XX
  |                                                         |XX
  |                                                         |XX
  |                                                         |XX
  +---------------------------------------------------------+



**********************************************************************
                      J   U   M   P   E   R   S
**********************************************************************
SEAGATE  ST12550W/WD BARRACUDA 2  PRODUCT DESCRIPTION

 Jumper Setting
 ==============


                                         +------ 5V
                                         | +---- GND
     (TOP)                               | | +-- GND
      34                   1         1   | | | + 12V
      +---------------------+  +-J5---+ +-------+
  ----+|||||||||||||||||||||+--+::::::+-+O O O O+-
      +---------------------+  +------+ +-------+
      68pin I/O Cable           ||||||    Power   SCSI ID = 0 (none)
      Connection                |||||+----------- SCSI ID = 1
                                ||||+------------ SCSI ID = 2
                                |||+------------- SCSI ID = 4
                                ||+-------------- SCSI ID = 8
                                |+--------------- Remote LED
                                +---------------- Spindle Sync


 Remote LED
 ----------
 Remote LED Connector, pin 9 cathode (neg); pin 10 anode (pos)


 Spindle Sync
 ------------
 Spindle Sync Cable Connector; pin 11 SSREF; pin 12 ground


 SCSI ID
 -------
   +----------+-------------------------------+
   | SCSI ID  |       Jumper Location         |
   |          |             PINS              |
   |          |   8   |   4   |   2   |   1   |
   +----------+-------+-------+-------+-------+
   |    0     | OPEN  | OPEN  | OPEN  | OPEN  |
   +----------+-------+-------+-------+-------+
   |    1     | OPEN  | OPEN  | OPEN  | CLOSED|
   +----------+-------+-------+-------+-------+
   |    2     | OPEN  | OPEN  | CLOSED| OPEN  |
   +----------+-------+-------+-------+-------+
   |    3     | OPEN  | OPEN  | CLOSED| CLOSED|
   +----------+-------+-------+-------+-------+
   |    4     | OPEN  | CLOSED| OPEN  | OPEN  |
   +----------+-------+-------+-------+-------+
   |    5     | OPEN  | CLOSED| OPEN  | CLOSED|
   +----------+-------+-------+-------+-------+
   |    6     | OPEN  | CLOSED| CLOSED| OPEN  |
   +----------+-------+-------+-------+-------+
   |    7     | OPEN  | CLOSED| CLOSED| CLOSED|
   +----------+-------+-------+-------+-------+
   |    8     | CLOSED| OPEN  | OPEN  | OPEN  |
   +----------+-------+-------+-------+-------+
   |    9     | CLOSED| OPEN  | OPEN  | CLOSED|
   +----------+-------+-------+-------+-------+
   |   10     | CLOSED| OPEN  | CLOSED| OPEN  |
   +----------+-------+-------+-------+-------+
   |   11     | CLOSED| OPEN  | CLOSED| CLOSED|
   +----------+-------+-------+-------+-------+
   |   12     | CLOSED| CLOSED| OPEN  | OPEN  |
   +----------+-------+-------+-------+-------+
   |   13     | CLOSED| CLOSED| OPEN  | CLOSED|
   +----------+-------+-------+-------+-------+
   |   14     | CLOSED| CLOSED| CLOSED| OPEN  |
   +----------+-------+-------+-------+-------+
   |   15     | CLOSED| CLOSED| CLOSED| CLOSED|
   +----------+-------+-------+-------+-------+


 J01  Terminator Power Jumpers
 -----------------------------
 1-+-+
 |xxx| Terminator power from the drive (default) *
 +-+-+
 1-+-+
 | |X| Terminator power from pin 26 on the SCSI bus.
 +-+-+
 1-+-+
 |X| | Terminator to the SCSI bus on pin 26. *
 +-+-+
 1-+-+
 |X|X| Terminator power to the SCSI bus and drive.
 +-+-+

 * Valid for single ended drives only.

 NOTE
 ST12550WD (differential models) does not come with any provision to
 use Termination resistors. Therefore, SCSI bus needs to be externally
 terminated.


 J4 Jumpers
 ----------
 1--+  Pins 1-2     Write Protect Enable
 +--+  Pins 3-4     Delay Motor Start (10 sec * ID)
 +--+  Pins 5-6     Enable Motor Start
 +--+  Pins 7-8     Parity Disable
 +--+  Pins 9-10    Reserved (default OFF)
 +--+  Pins 11-12   Enable Drive Terminators (Valid for single-ended
 +--+               drives only.)


 Mating cable connector: AMP 786090-7, Male 68-pin
 Drive connector:        AMP 749721-7, Female 68-pin



**********************************************************************
                      G   E   N   E   R   A   L
**********************************************************************
SEAGATE   SCSI

 Interface requirements
 ----------------------
 How to use this interface manual

 This specification is designed to provide a universal detailed
 description of the SCSI interface for those disc drive products whose
 Product Manuals (Volume 1) do not contain the details of how the
 SCSI interface is implemented by that drive.

 Note: Volume 1 Product Manuals have tables in Section 11 that specify
 which SCSI-1 or SCSI-2/SCSI-3 features they implement, what the
 default parameters are for the various features they implement and
 which parameters are changeable and which are not. No method exists
 at present to inform an initiator if a target supports "SCSI-3"
 features as opposed to only SCSI-2 features. A few "SCSI-3" features
 are supported by Seagate drives, but no attempt has been made herein
 to differentiate between SCSI-2 and "SCSI-3" features. Therefore,
 when an Inquiry command reports what the ANSI approved version of the
 drive is, it reports either SCSI-1 or SCSI-2, where "SCSI-2" means
 SCSI-2 features plus some "SCSI-3" features.

 No attempt is made in this universal specification to specify which
 descriptions or tables apply to SCSI-1 and which to SCSI-2 or SCSI-3.
 The combination of this general specification with the details in
 the Section 11 tables of the individual drive Product Manual (Volume
 1) provides a description of the individual drive implementation of
 the SCSI interface. This interface manual is not intended to be
 stand-alone text on SCSI-1 or SCSI-2/SCSI-3 features.

 Reference must be made back to the individual drive Product Manuals
 to find out what are SCSI-1 and what are SCSI-2/SCSI-3 features.
 This specification is Volume 2 of a set of manuals that is made up of
 separate drive Product Manuals (Volume 1) and this manual. This
 Volume 2 Manual is referenced by other Volume 1 Product Manuals
 representing the drives listed below.

 Product Manuals for the following models reference this volume:
 ST11200N/ND/NC, ST1980N/ND/NC, ST1830N, ST1950N, ST3500N, ST3600N,
 ST3610N/ND/NC, ST12400N/ND/NC, ST12400 Wide, ST11900N/ ND/NC,
 ST31200N/ND/NC, ST31200 Wide, ST11950N/ND, ST11950W/WD,
 ST12450W/WD, ST12550N/ND, ST12550W/WD, ST15150N/ND, ST15150W/WD,
 ST3655N, ST3550W, ST3390N and ST3285N, ST32430N/ND/NC,
 ST32430W/WD/WC, ST15230N/ND/NC, ST31250N/ND, ST31250W/WD/WC,
 ST32151N, ST31051N, ST32550N/ND, ST32550W/WD/WC, ST3471N,
 ST410800N/ND, ST410800W/WD.


 General interface description
 -----------------------------
 This Product Manual describes the Seagate Technology, Inc. subset of
 the SCSI (Small Computer Systems Interface) as implemented on the
 Seagate built disc drives listed above. The interface is compatible
 with the SCSI Interface Specifications of the ANSI SCSI-1 standard,
 the ANSI SCSI-2 Standard and the common command set (CCS) document,
 Revision 4.B. The disc drives covered by this Product Manual are
 classified as "Intelligent" peripherals.

 The Seagate SCSI interface described herein consists of a 9 or 18 bit
 bidirectional bus (8 data + 1 parity or 16 data + 2 parity) plus 9
 control signals supporting multiple initiators, disconnect/
 reconnect, self configuring host software, automatic features that
 relieve the host from the necessity of knowing the physical
 architecture of the target (logical block addressing is used), and
 some other miscellaneous features.

 The SCSI physical interface uses either single ended drivers and
 receivers or differential drivers and receivers and uses asynchronous
 or synchronous communication protocols. The bus interface transfer
 rate for asynchronous or synchronous is given in individual disc
 drive Volume 1 Product Manuals. The bus protocol supports multiple
 initiators, disconnect/reconnect, additional messages plus 6 byte and
 10 byte Command Descriptor Blocks. Unless specified otherwise in the
 individual drive Product Manuals (Vol. 1), the disc drive is always a
 target, and never an initiator. For certain commands, which may or
 may not be supported by a particular drive model, the drive must act
 as an initiator, but does not otherwise do so. For purposes of this
 specification, "disc drive" may be substituted for the word "target"
 wherever "target" appears.


 GLOSSARY
 --------
 Arbitration - SCSI bus phase wherein SCSI devices try to gain control
 of the SCSI bus to operate as an initiator or target

 Byte - This term indicates an 8 bit hexadecimal construction.

 Command Descriptor Block (CDB) - The structure used to communicate
 requests from an initiator to a target.

 Connect - The function that occurs when an initiator selects a target
 to start an operation.

 Disconnect - The function that occurs when a target releases control
 of the SCSI bus, allowing it to go to the Bus Free phase.

 FRU (Field Replaceable Unit) - An assembly that is believed faulty
 based on test results. A value of 00h indicates an unknown cause or
 the end of a list of known possible causes. Nonzero values have
 product unique meanings.

 Initiator - A SCSI device (usually a host system) that requests an
 operation to be performed by another SCSI device.

 Intermediate Status - A status code sent from a target to an
 initiator upon completion of each command, except the last command,
 in a set of linked commands.

 I/O Process - An I/O process consists of one initial connection and
 zero or more reconnections, all pertaining to a single command or
 group of linked commands. More specifically, the connection(s)
 pertain to a nexus as defined below in which one or more command
 descriptor blocks are usually transferred. An I/O process begins with
 the establishment of a nexus. An I/O process normally ends with the
 BUS Free phase following successful transfer of a COMMAND COMPLETE,
 ABORT, ABORT TAG, or CLEAR QUEUE message. An I/ O process also ends
 when a hard RESET condition occurs, an unexpected BUS FREE phase
 occurs, or when the BUS FREE phase occurs following a BUS DEVICE
 RESET message.

 I T nexus - A nexus prior to the successful receipt of an IDENTIFY
 message, at which time the nexus is changed to an I T L nexus. (See
 glossary word "Nexus").

 I T L nexus - A nexus that exists between an initiator and a Logical
 Unit. This relationship replaces the prior I T nexus. (See glossary
 word "Nexus").

 I T L Q nexus - A nexus between an initiator, a Logical Unit, and a
 queue tag following the successful receipt of one of the QUEUE
 messages. This relationship replaces the prior I T L nexus. (See
 glossary word "Nexus").

 Logical Unit - A physical device or virtual device addressable
 through a target. The disc drive is a target but also a Logical Unit.

 Logical Unit Number - An encoded three bit identifier for the logical
 unit. The disc drive is considered Logical Unit number zero.

 LSB - Least significant byte

 MSB - Most significant byte

 ms -  millisecond

 LUN - Logical unit number

 mm -  Millimetre


 SCSI bus
 --------
 This manual discusses only the "logical" and timing characteristics
 of the SCSI system and interface. The SCSI bus physical
 characteristics (voltages, connector configurations, pinouts, etc.)
 are given in the individual disc drive Product Manuals (Volume 1)
 Section "Interface requirements", which covers all of the interface
 requirements and SCSI features supported by the drive described in
 the particular Product Manual being referenced.
 Communication on the SCSI Bus is allowed between only two SCSI
 devices at a time. Some Seagate drives support systems with a maximum
 of eight SCSI devices including the host computer(s) connected to the
 SCSI bus. Some Seagate drives support systems with a maximum of
 sixteen SCSI devices on the SCSI bus. Each SCSI device has a SCSI ID
 Bit.

 The SCSI ID is assigned by installing from 0 to 3 (8 device systems)
 jumper plugs or 0-4 (16 device systems) jumper plugs onto a connector
 in a binary coded configuration during system configuration. Some
 drive models have an interface that includes the SCSI bus ID lines,
 so that the host can set the drive ID over the interface. See
 individual disc drive Product Manual, Section "Option/configuration
 headers".

 When two SCSI devices communicate on the SCSI Bus one acts as an
 initiator and the other acts as a target. The initiator (typically a
 host computer) originates an operation and the target performs the
 operation. The disc drive always operates as a target, unless
 specified otherwise (i.e., certain commands are supported) in the
 individual drive Product Manual.

 The Host Adapter/Initiator must be identified by one of the eight
 SCSI Device Addresses. Make sure that none of the devices on the SCSI
 bus have duplicate addresses. Certain SCSI bus functions are assigned
 to the initiator and certain SCSI bus functions are assigned to the
 target. The initiator will select a particular target. The target
 will request the transfer of Command, Data, Status or other
 information on the data bus.

 Information transfers on the data bus are interlocked and follow a
 defined REQ/ACK Handshake protocol. One byte of information will be
 transferred with each handshake. Synchronous data transfers do not
 require a one for one interlocking of REQ/ACK signals, but the total
 number of REQ pulses in a particular data transfer event must equal
 the total number of ACK pulses.

 The disc drive supports single initiator, single target; single
 initiator, multiple target; multiple initiator, single target; or
 multiple initiator, multiple target bus configurations.


 SCSI bus signals
 ----------------
 There are ten control and eighteen data signals, as listed below:

 - BSY
 - C/D
 - MSG
 - DIFFSENS
 - SEL
 - I/O
 - REQ
 - DB(7-0, P); DB(15-8,P1)
 - ACK
 - ATN
 - RST

 Some drive models have a single 80 pin I/O connector that contains
 additional interface lines that carry drive configuration select
 signals. These are peculiar to certain drives and are not SCSI
 standard signals. These are described in the drive model's Volume 1
 Product manual, but not here. The 28 SCSI standard signals are
 described as follows:

 BSY (Busy) - An "OR-tied" signal to indicate the bus is being used.

 SEL (Select) - A signal used by an initiator to select a target, or
 by a target to reselect an initiator.

 C/D (Control/Data) - A signal driven by a target to indicate whether
 Control or Data information is on the Data Bus. Assertion indicates
 Control.

 I/O (Input/Output) - A signal driven by a target to control the
 direction of data movement on the Data Bus with respect to an
 initiator. Assertion indicates input to the initiator. This signal
 also distinguishes between Selection and Reselection phases.

 MSG (Message) - A signal driven by a target during the Message phase.

 REQ (Request) - A signal driven by a target to indicate a request for

 REQ/ACK data transfer handshake.

 ACK (Acknowledge) - A signal driven by an initiator to indicate an
 acknowledgment for a REQ/ACK data transfer handshake.

 ATN (Attention) - A signal driven by an initiator to indicate the
 Attention condition. It is used to request to send a message out to
 the target. If an initiator asserts ATN while asserting SEL it
 indicates to the target that the initiator supports messages other
 than command complete.

 RST (Reset) - An "OR-tied" signal that indicates the Reset condition.

 DIFFSENS (Differential Sense) - When the drive has differential SCSI
 I/O circuits, the DIFFSENS signal disables the drive s differential
 driver/receiver circuits if the SCSI I/O cable is plugged in upside
 down, or if a single-ended SCSI I/O cable is plugged into a
 differential I/O drive. Disabling the differential I/O drivers/
 receivers is necessary to prevent burning them out if a grounded I/O
 line is connected to any of the differential circuit outputs, which
 are at a positive voltage (+2 V or +3 V) when not disabled.

 DB(7-0,P) and DB(15-8,P1)
 (Data Bus) - Sixteen data bit signals, plus parity bit signals form a
 Data Bus. DB(7) is the most significant bit and has the highest
 priority during the Arbitration phase (on both eight and sixteen
 device systems). Bit number significance, and priority decrease
 downward to DB(0), and then from DB15 down to DB8 (DB0 is higher than
 DB15). A data bit is defined as one when the  signal is asserted and
 is defined as zero when the signal is negated.

 Data parity DB(P) and DB(P1) is odd - The use of parity is a system
 option. The disc drive always checks parity on the data bits, but has
 the capability to enable/disable parity error reporting to the host.
 See configuration selection in the applicable Product Manual. Parity
 checking is not valid during the Arbitration phase.
 Greater detail on each of the SCSI Bus signals is found in the
 following sections.


 Drive Select
 ------------
 For SCSI ID selection install drive select jumpers as shown in
 configuration selection figure in applicable Product Manual. Refer to
 section 10 of the individual drive Product Manual for the location
 of the drive select header. The disc drive using the eight bit data
 interface can have one of eight ID bits selected by installing 0 to 3
 jumpers in a binary coded configuration on the drive select header.
 Drives using the 16 bit data interface can have one of sixteen ID
 bits selected by installing 0 to 4 jumpers in a binary coded
 configuration on the drive select header.

 Signal Values
 -------------
 Signals may assume true or false values. There are two methods of
 driving these signals. In both cases, the signal shall be actively
 driven true, or asserted. In the case of OR-tied drivers, the driver
 does not drive the signal to the false state, rather the bias
 circuitry of the bus terminators pulls the signal false whenever it
 is released by the drivers at every SCSI device. If any driver is
 asserted, then the signal is true. In the case of non-OR-tied
 drivers, the signal may be negated. Negated means that the signal may
 be actively driven false, or may be simply released (in which case
 the bias circuitry pulls it false), at the option of the implementor.


 OR-Tied signals
 ---------------
 The BSY and RST signals shall be OR-tied only. In the ordinary
 operation of the bus, these signals are simultaneously driven true by
 several drivers. No signals other than BSY, RST, and DB(P) are
 simultaneously driven by two or more drivers, and any signal other
 than BSY and RST may employ OR-tied or non-OR-tied drivers. DB(P)
 shall not be driven false during the Arbitration phase. There is
 no operational problem in mixing OR-tied and non-OR-tied drivers on
 signals other than BSY and RST.


 Signal sources
 --------------
 All SCSI device drivers that are not active sources shall be in the
 passive state. Note that the RST signal may be sourced by any SCSI
 device at any time. The disc drive functions as a target.


 Nonarbitrating system
 ---------------------
 In systems with the Arbitration phase not implemented, the initiator
 shall first detect the Bus Free phase and then wait a minimum of a
 bus clear delay. Then, except in certain single initiator
 environments with initiators employing the single initiator option
 the initiator shall assert the desired target's SCSI ID and its own
 initiator SCSI ID on the Data Bus. After two deskew delays, the
 initiator shall assert SEL.


 Arbitrating systems
 -------------------
 In systems with the Arbitration phase implemented, the SCSI device
 that won the arbitration has both BSY and SEL asserted and has
 delayed at least a bus clear delay plus a bus settle delay before
 ending the Arbitration phase. The SCSI device that won the
 arbitration becomes an initiator by releasing I/O. Except in certain
 single initiator environments with initiators employing the single
 initiator option, the initiator shall set the Data Bus to a value
 which is the OR of its SCSI ID bit and the target s SCSI ID bit. The
 initiator shall then wait at least two deskew delays and release
 BSY. The initiator shall then wait at least a bus settle delay before
 looking for a response from the target.


 All systems
 -----------
 In all systems, the target shall determine that it is selected when
 SEL and its SCSI ID bit are true and the BSY and I/O signals are
 false for at least a bus settle delay. The selected target will
 examine the Data Bus in order to determine the SCSI ID of the
 selecting initiator unless the initiator employed the single
 initiator option. The selected target shall then assert BSY within a
 selection abort time of its selection; this is required for correct
 operation of the timeout procedure. In systems with parity
 implemented, the target shall not respond to a selection if bad
 parity is detected. Also, if more than two SCSI ID bits are on the
 Data Bus, the target shall not respond to selection. At least two
 deskew delays after the initiator detects BSY is asserted, it shall
 release SEL and may change the Data Bus.


 Single initiator option
 -----------------------
 Initiators that do not implement the Reselection phase, and do not
 operate in the multiple initiator environment, are allowed to set
 only the target's SCSI ID bit during the Selection phase. This makes
 it impossible for the target to determine the initiator s SCSI ID.


 Selection time out procedure
 ----------------------------
 A Selection timeout procedure is specified for clearing the SCSI bus.
 If the initiator waits a minimum of a selection timeout delay and
 there has been no BSY response from the target, the initiator shall
 continue asserting SEL and shall release the Data Bus. If the
 initiator has not detected BSY to be asserted after at least a
 selection abort time plus two deskew delays, the initiator shall
 release SEL allowing the SCSI bus to go to the Bus Free phase. SCSI
 devices shall ensure when responding to selection that the selection
 was still valid within a selection abort time of their assertion of
 BSY. Failure to comply with this requirement could result in an
 improper selection (two targets connected to the same initiator,
 wrong target connected to an initiator, or a target connected to no
 initiator). The disc drive supports systems that implement this
 procedure.


 Asynchronous information transfer
 ---------------------------------
 The target shall control the direction of information transfer by
 means of the I/O signal. When I/O is true, information shall be
 transferred from the target to the initiator. When I/O is false,
 information shall be transferred from the initiator to the target.
 If I/O is true (transfer to the initiator), the target shall first
 drive DB(7-0,P)* to their desired values, delay at least one deskew
 delay plus a cable skew delay, then assert REQ. DB(7-0,P)* shall
 remain valid until ACK is true at the target. The initiator shall
 read DB(7-0,P)* after REQ is true, then signal its acceptance of the
 data by asserting ACK. When ACK becomes true at the target, the
 target may change or release DB(7-0, P)* and shall negate REQ. After
 REQ is false the initiator shall negate ACK. After ACK is false, the
 target may continue the transfer by driving DB(7-0,P)* and asserting
 REQ, as described above.

 If I/O is false (transfer to the target) the target shall request
 information by asserting REQ. The initiator shall drive DB(7-0,P)* to
 their desired values, delay at least one deskew delay plus a cable
 skew delay and assert ACK. The initiator shall continue to drive the
 DB(7-0,P)* until REQ is false.

 When ACK becomes true at the target, the target shall read
 DB(7-0,P)*, then negate REQ. When REQ becomes false at the initiator,
 the initiator may change or release DB(7-0,P)* and shall negate
 ACK. The target may continue the transfer by asserting REQ, as
 described above.


 Synchronous data transfer
 -------------------------
 Synchronous data transfer may be used only in the data phase if
 previously agreed to by the initiator and target through the message
 system. The messages determine the use of synchronous mode by both
 SCSI devices and establish a REQ/ACK offset and a transfer period.
 The REQ/ACK offset specifies the maximum number of REQ pulses that
 can be sent by the target in advance of the number of ACK pulses
 received from the initiator, establishing a pacing mechanism. If
 the number of REQ pulses exceeds the number of ACK pulses by the
 REQ/ACK offset, the target shall not assert REQ until the next ACK
 pulse is received. A requirement for successful completion of
 the data phase is that the number of ACK and REQ pulses be equal.
 The target shall assert the REQ signal for a minimum of an assertion
 period. The target shall wait at least the greater of a transfer
 period from the last transition of REQ to true or a minimum of a
 negation period from the last transition of REQ to false before
 asserting the REQ signal.

 The initiator shall send one pulse on the ACK signal for each REQ
 pulse received. The ACK signal may be asserted as soon as the leading
 edge of the corresponding REQ pulse has been received. The initiator
 shall assert the ACK signal for a minimum of an assertion period. The
 initiator shall wait at least the greater of a transfer period from
 the last transition of ACK to true or for a minimum of a negation
 period from the last transition of ACK to false before asserting the
 ACK signal.


 Unit attention condition
 ------------------------
 The disc drive sets up the Unit Attention condition when it stores
 (within itself) a Unit Attention condition flag for each device on
 the SCSI bus having an initiator relationship with the disc drive,
 and this Unit Attention condition persists for each initiator until
 the condition is cleared (flag negated) by each initiator
 individually. The Unit Attention condition results when one of the
 following events occur:

  1. A power-on sequence occurs.
  2. A reset is generated internally by the disc drive (caused by a
     power glitch).
  3. A Bus Device Reset message causes the disc drive to reset itself.
  4. The RESET I/O line resets the disc drive.
  5. An initiator changes one or more of the Mode Select parameters in
     the disc drive (these changes could affect one or more of the
     other initiators).
  6. The inquiry data has been changed.
  7. The mode parameters in effect for an initiator have been restored
     from nonvolatile memory.
  8. An event occurs that requires the attention of the initiator.
  9. A Clear Queue message received.
 10. The Log parameters are changed. Unit Attention Condition is
     posted for all initiators in the system other than the one
     that changed the Log Parameters.

 The Unit Attention Parameters page (page 00h, bit 4 of byte 2) of the
 Mode Select Command controls whether or not a Check Condition Status
 is to be reported to affected initiators when a Unit Attention
 condition exists. The Unit Attention condition for a particular
 initiator is cleared when that initiator does one of the following:

 1. It sends a Request Sense Command.
 2. It sends any other legitimate command, with the exception
    of the Inquiry command. The Inquiry command does not clear the
    Unit Attention condition.

 When a Unit Attention condition flag is stored in the disc drive
 for an initiator, the commands that initiator issues to the disc
 drive operate as described in the following paragraphs.

 If an initiator sends an Inquiry command to the disc drive when the
 disc drive has stored a Unit Attention condition flag for that
 initiator before or after the disc drive reports Check Condition
 status), the disc drive shall perform the Inquiry command and shall
 not clear the Unit Attention condition.

 If an initiator sends a Request Sense command to the disc drive when
 a Unit Attention condition flag is stored for that initiator (before
 or after the disc drive reports Check Condition), the disc drive
 shall discard any pending sense data, report the Unit Attention Sense
 Key, and clear the Unit Attention condition (negate the flag) for
 that initiator.

 If an initiator issues a command other than Inquiry or Request Sense
 while a Unit Attention condition flag is stored for that initiator,
 the disc drive may or may not perform the command and report Check
 Condition status, depending on whether or not the Unit Attention bit
 is zero or one in the Unit Attention Mode Parameters page (Page 00h,
 bit 4 of byte 2). If a Request Sense is issued next, the Unit
 Attention condition is reported and cleared (flag negated) as noted
 in the preceding paragraph. If another command other than Request
 Sense or Inquiry is issued instead, the disc drive shall perform the
 command and return the appropriate status. The Unit Attention
 condition for the subject initiator is cleared (flag negated) and the
 sense data and flag indicating there has been a Unit Attention
 condition are lost.


