S T 1 5 1 5 0 N D B A R R A C U D A 4 SEAGATE Native| Translation ------+-----+-----+----- Form 3.5"/HH Cylinders 3711| | | Capacity form/unform 4294/ 5062 MB Heads 21| | | Seek time / track 8.0/ 0.9 ms Sector/track | | | Controller SCSI2 DI/FAST Precompensation Cache/Buffer 1024 KB SEGMENTED Landing Zone Data transfer rate 6.000 MB/S int Bytes/Sector 512 10.000 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 12.5 W Shock g | seek W Rotation RPM 7200 read/write W Acoustic dBA spin-up W ECC Bit MTBF h 800000 Warranty Month 60 Lift/Lock/Park YES Certificates ********************************************************************** L A Y O U T ********************************************************************** SEAGATE ST15150N/ND INSTALLATION GUIDE 83328870, REV. D 1/95 +---------------------------------------------+ | +-+ | | J01+-+| |J4 | +-+| | | | | | | | +-+ | +XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX-XXXXXXXX+ SCSI I/O Connector DC Power ********************************************************************** J U M P E R S ********************************************************************** SEAGATE ST15150N/ND BARRACUDA 4 INSTALLATION GUIDE 83328870, REV. D Jumper Setting ============== 1-+ J4 1-+ | | J01+-+ | | Terminating Resistors | | (TOP) +-------+ +-------+ +-+ +-------1 +-------1 +-------------------------+ +--------+ ------+oooooooooooooooooooooooo1+--+ O O O O+-- |ooooooooooooooooooooooooo| | 4 3 2 1| +-------------------------+ +--------+ 50-pin I/O Cable Connection | | | +- 12 V | | +--- GND | +----- GND +------- 5 V 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. +-+-+ NOTE ST15150ND (differential models) does not come with any provision to use Termination resistors. Therefore, SCSI bus needs to be externally terminated. J4 Jumpers ---------- 1--2 --1-2-- Spindle Sync Cable Connector (pin-1 SSREF, pin-2 GND) +--+ --3-4-- RESERVED (DEFAULT OFF) +--+ --5-6-- Remote LED Connector (pin-5 cathode (neg), pin-6 anode) +--+ --7-8-- Write Protect Enable +--+ --9-10- Delay Motor Start (10 sec * ID) +--+ -11-12- Enable Motor Start +--+ -13-14- Parity Disable +--+ -15-16- RESERVED (DEFAULT OFF) +--+ -17-18- SCSI ID = 1 +--+ -19-20- SCSI ID = 2 +--+ -21-22- SCSI ID = 4 +--22 SCSI ID = 0 (none) J4 SCSI ID ----------- +----------+-----------------------+ | SCSI ID | Jumpers | | | 21-22 | 19-20 | 17-18 | +----------+-------+-------+-------+ | 0 | OPEN | OPEN | OPEN | +----------+-------+-------+-------+ | 1 | OPEN | OPEN | CLOSED| +----------+-------+-------+-------+ | 2 | OPEN | CLOSED| OPEN | +----------+-------+-------+-------+ | 3 | OPEN | CLOSED| CLOSED| +----------+-------+-------+-------+ | 4 | CLOSED| OPEN | OPEN | +----------+-------+-------+-------+ | 5 | CLOSED| OPEN | CLOSED| +----------+-------+-------+-------+ | 6 | CLOSED| CLOSED| OPEN | +----------+-------+-------+-------+ | 7 | CLOSED| CLOSED| CLOSED| +----------+-------+-------+-------+ Already low-level formatted at the factory with 9 spare sectors per cylinder and one spare cylinder per unit. Terminating the drive --------------------- ST15150WC and ST15150DC drives do not have internal terminators or any other way of adding internal termination to the drive. You must provide single-ended external active termination (ANSI SCSI-2 Alternative 2) to these drives when termination is required. Synchronizing spindles Spindle synchronization for WC and DC drives is normally handled on the SCSI bus by the host system using connector contact 37 for the spindle synce reference signal. System designers may, optionally, install a spindle sync cable using pin 6 on the J4 connector to provide +SSREF. DC drives --------- SCSI ID jumpers Each device on the SCSI chain must have a unique SCSI ID. The host system's SCSI controller usually uses the highest- numbered ID available; therefore the lower-numbered SCSI IDs are normally used for other SCSI devices such as this Barracuda disc drive. Most Barracuda disc drives are factory set with the SCSI ID set at 0. To change the SCSI ID, refer to the appropriate drive section for your model. NOTE Most SCSI controllers (host adapters) allow you to skip a SCSI ID. For example, you can have ID0, ID1, and ID3 (skipping ID2). Other controllers do not allow this; refer to your system or controller user's manual for details about its requirements for proper SCSI device installation. After completing the installation process, if the drive's LED does not show on/off activity when the host is trying to communicate with the drive, a duplicate SCSI ID may be the problem. If this is the case, change the ID so that each device on the SCSI chain has its own unique ID. Also check your system or controller user's manual to ensure that you have not violated its SCSI ID numbering recommendations. Drive termination ----------------- If you are installing the drive in a system that has other SCSI devices installed, terminate only the end devices on the SCSI chain. A SCSI "device" is any disc drive, scanner, tape backup unit, or other piece of hardware connected to your system using the SCSI bus. Terminator power ---------------- You usually will not need to change this option and can normally leave the drive configured as it was shipped from the factory. Synchronizing spindles ---------------------- If you are installing two or more Barracuda drives, you may synchronize their spindles to reduce the latency associated with switching from one drive to another. Spindle sync cables are used to connect the drives. After connecting each drive with spindle sync cables, designate a master spindle sync reference source. This master source is usually a disc drive located on the same SCSI bus as the other drives you want to synchro-nize with. To designate a drive as the master, use SCSI interface commands. Other applicable jumper options ------------------------------- Barracuda 4 drives are designed for use in a variety of systems. Some installations may require you to change one or more of the other jumpers to meet specific system requirements; however, in most cases, you will not need to change any of these jumpers for normal drive operation. Providing adequate cooling -------------------------- The enclosure design must ensure adequate cooling for the drive. The maximum ambient temperature is 50 o C. Terminating the N drive ----------------------- To terminate the ST15150N drive, enable the permanently mounted IC active terminators by installing an Enable Drive Terminator jumper on J01 pins 1 and 2. If you install one of these models and it is not on the end of the SCSI bus, remove the jumper to disable the permanently mounted IC terminators and thereby remove internal termination from the drive. The ST15150ND drive does not have internal terminators or any way of adding internal termination. You must provide external active termination (ANSI SCSI-2 Alter-native 2) to the drive when termination is required. Terminator power N/ND --------------------- There are four possible terminator power configurations for the ST15150N drive. You will not normally need to change this option and can leave the drive configured as it was shipped from the factory. The ST15150ND drive should be configured with a jumper connecting J01 pins 1 and 3 (and no jumper on J01 pins 2 and 4). ********************************************************************** I N S T A L L ********************************************************************** SEAGATE ST15150 PRODUCT MANUAL Notes On Installation ===================== Mounting the drive and connecting cables ---------------------------------------- Note. This mounting procedure does not apply to WC or DC model drives. To mount a WC or DC drive, plug the drive into the system's singleconnector attachment (SCA) position on the system's back panel. Do not touch the connector pins or any components on the control board without observing static-discharge precautions. Always handle the drive by the frame only. The drive may be mounted in any orientation (horizontally, vertically, and any combination thereof); however, you must ensure that the drive receives adequate air flow for cooling. Mount the drive to the host system's chassis using four 6-32 UNC screws. Two mounting holes are in each side of the drive and there are four mounting holes in the bottom of the drive. The maximum length that the screws should extend into the chassis mounting holes is 0.15 inch (3.81 mm), measured from the outer surface of the chassis. Tighten the screws down evenly. Do not over tighten or force the screw if it does not seem to screw in easily, because this means the threads are not properly aligned. In this case, back the screw out and try again. ********************************************************************** F E A T U R E S ********************************************************************** SEAGATE ST15150 PRODUCT MANUAL General description ------------------- The drive is a component for installation in an enclosure designed for the drive. This is often a rack within the system or an external enclosure designed to house one or more disc drives. In either case, the disc drive must receive adequate cooling and it must be sufficiently grounded and shielded from emissions. SCSI ID ------- Most SCSI controllers (host adapters) allow you to skip a SCSI ID. For example, you can have ID0, ID1, and ID3 (skipping ID2). Other controllers do not allow this; refer to your system or controller user's manual for details about its requirements for proper SCSI device installation. After completing the installation process, if the drive's LED does not show on/off activity when the host is trying to communicate with the drive, a duplicate SCSI ID may be the problem. If this is the case, change the ID so that each device on the SCSI chain has its own unique ID. Also check your system or controller user's manual to ensure that you have not violated its SCSI ID numbering recommendations. Providing adequate cooling -------------------------- The enclosure design must ensure adequate cooling for the drive. The maximum ambient temperature is 50*C. The evaluation consists of ensuring that the case temperature of certain critical components remains within acceptable limits during drive operation. We recommend orienting the drive or directing the air flow in a way that creates the least amount of air flow resistance while providing air flow above the circuit boards and around the head and disc assembly (HDA). Also, choose the shortest possible path between the air inlet and exit. This minimizes the distance traveled by air that is heated by the drive and by other nearby heat sources. ********************************************************************** 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.