Plug and Play SCSI
`Specification
`Version 1.0
`March 30, 1994
`
`Information Solutions, Digital
`Copyright 1994 by Adaptec, AT&T Global
`Equipment Corporation, Future Domain, Maxtor, and Microsoft.
`
`Permission is granted to copy and distribute this document for purpose of review,
`comment, and development of SCSI PnP products.
`
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`Apple 1031
`U.S. Pat. 9,189,437
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`

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`Plug and Play SCSI -- Version 1.0
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`EMAIL 71053.3431@compuserve.com
`FAX
`408-957-7145
`
`EMAIL gardner@ssag.enet.dec.com
`FAX
`719-548-2451
`
`EMAIL jeffe@fdc.mhs.compuserve.com
`FAX
`714-253-0913
`
`EMAIL evan_steiner@Maxtor.com
`FAX
`408-432-4510
`
`EMAIL stevetim@microsoft.com
`FAX
`206-936-7329
`
`EMAIL john.lohmeyer@ftcollinsco.ncr.com
`FAX
`719-597-8225
`
`POINTS OF CONTACT:
`
`Vince Bastiani
`Adaptec
`
`
`
`John Lohmeyer
`AT&T GIS (NCR Microelectronics)
`
`Edward Gardner
`Digital Equipment
`
`
`Jeff Epstein
`Future Domain
`
`Evan Steiner
`Maxtor
`
`Steve Timm
`Microsoft
`
`Plug and Play Forum - login to CompuServe, enter 'go plugplay', leave messages in SCSI/IDE section.
`
`719-574-0424
`
`
`SCSI BBS:
`SCSI Reflector (to join): scsiadm@wichitaks.ncr.com
`
`
`(to post):
`scsi@wichitaks.ncr.com
`ftp site:
`
`ncrinfo.ncr.com (/pub/standards/pnpscsi directory)
`
`
`
`Microsoft does not make any representation or warranty regarding this specification or any product or item developed based
`on this specification. Microsoft disclaims all express and implied warranties, including but not limited to the implied warranties
`of merchantability, fitness for a particular purpose and freedom from infringement. Without limiting the generality of the
`foregoing, Microsoft does not make any warranty of any kind that any item developed based on this specification, or any
`portion of it, will not infringe any copyright, patent, trade secret or other intellectual property right of any person or entity in any
`country. It is your responsibility to seek licenses for such intellectual property rights where appropriate. Microsoft shall not be
`liable for any damages arising out of or in connection with the use of this specification, including liability for lost profit,
`business interruption, or any other damages whatsoever. Some states do not allow the exclusion or limitation of liability for
`consequential or incidental damages; the above limitation may not apply to you.
`
`2
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`

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`Plug and Play SCSI -- Version 1.0
`
`TABLE OF CONTENTS
`
`1. Scope ....................................................................................................................................... 7
`
`2. Reference documents ............................................................................................................ 7
`
`3. Glossary and Abbreviations ..................................................................................................... 7
`3.1. Glossary ................................................................................................................... 7
`3.2. Abbreviations ........................................................................................................... 8
`
`4. Conformance ........................................................................................................................... 8
`
`5. Hardware ................................................................................................................................. 8
`
`6. Connectors .............................................................................................................................. 9
`6.1. Cables ...................................................................................................................... 9
`6.2. Terminator requirements ......................................................................................... 9
`6.3. Device Power ........................................................................................................... 10
`6.4. SCSI ID assignment ................................................................................................ 10
`6.4.1. Automatic SCSI ID assignment ............................................................... 10
`6.4.2. Default SCSI ID's ..................................................................................... 10
`6.4.3. SCAM ID assignment algorithm .............................................................. 11
`
`7. System configurations ............................................................................................................. 11
`7.1. Add-in SCSI Design Alternative ............................................................................... 12
`7.2. Motherboard SCSI Design Alternative ..................................................................... 13
`7.3. Mixed configuration Example................................................................................... 16
`
`8. X86 Software Issues ................................................................................................................ 16
`8.1. Geometry translation ............................................................................................... 16
`
`9. Generic Software Issues .......................................................................................................... 17
`9.1. READ CAPACITY command ................................................................................... 17
`9.2. INQUIRY command ................................................................................................. 17
`9.3. Synchronous negotiation ......................................................................................... 17
`9.4. Virtual DMA services (VDS) ..................................................................................... 17
`9.5. Device driver architecture ........................................................................................ 17
`9.6. Software configuration ............................................................................................. 18
`9.7. Boot considerations ................................................................................................. 18
`
`APPENDIX A - CHS mapping ...................................................................................................... 19
`
`APPENDIX B - Checklist of compliance requirements ................................................................ 21
`
`APPENDIX C - SCSI ID Assignment Examples .......................................................................... 23
`
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`Plug and Play SCSI -- Version 1.0
`
`LIST OF TABLES
`Table 1 - As shipped default SCSI ID assignment ......................................................... 11
`
`
`LIST OF FIGURES
`Figure 1 - SCSI Icon ....................................................................................................... 9
`Figure 2 - Add-in SCSI Design ....................................................................................... 12
`Figure 3 - Motherboard SCSI Design ............................................................................. 14
`Figure 4 - Mixed Configuration Example ........................................................................ 16
`
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`Plug and Play SCSI -- Version 1.0
`
`Foreword
`
`The Plug and Play SCSI (PnP SCSI) effort was undertaken as an extension to Plug and Play ISA.
`Microsoft encouraged host adapter manufacturers to participate in this effort to develop an easy to
`use SCSI environment in the personal computer marketplace.
`
`PnP SCSI is a profile, not a standard. PnP SCSI references the SCSI-2 standard and portions of
`the draft SCSI-3 standards. PnP SCSI selects specific options and features from these
`documents.
`
`This specification is part of the PnP family and is designed to facilitate usage of the Small
`Computer System Interface (SCSI) on personal computing products. This document is an open,
`public specification supplementing existing SCSI specifications. This specification is intended to
`be platform and operating system independent.
`
`Companies which contributed to developing the initial draft specification include: Adaptec, AT&T
`Global Information Solutions (formerly NCR), Digital Equipment Corporation, Future Domain,
`Maxtor, and Microsoft.
`
`5
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`Plug and Play SCSI -- Version 1.0
`
`Introduction
`
`This document contains a complete set of specifications required for implementing Plug and Play
`SCSI 1.0 systems. Future revisions are possible but are not expected to alter baseline
`requirements defined herein.
`
`SCSI offers a cost-effective, high-performance interface for storage devices and permits
`attachment of external devices such as CD-ROMs, scanners, and printers.
`
`SCSI is designed to accommodate a broad range of applications and performance levels. This
`results in multiple choices for such items as connectors, cables, termination, etc. These choices
`can lead to incompatibilities. This specification defines a profile for personal computing
`environments that eases the integration task and promotes compatibility of conforming devices.
`PnP SCSI systems are easier to configure because the configuration choices are simplified.
`Adding a PnP SCSI device to a PnP SCSI system requires no special user skills or knowledge.
`
`PnP SCSI simplifies termination by making proper termination a side effect of configuring the
`system. Termination is not included within SCSI peripheral devices; thus the confusion over the
`presence of a terminator and the possibility of excessive terminators are eliminated.
`
`PnP SCSI eliminates the problem of setting unique SCSI IDs through use of a protocol called
`SCAM, that performs an automatic ID assignment. Legacy SCSI devices are permitted to coexist
`with newer SCAM devices by using an ID assignment convention based on the peripheral device
`class.
`
`PnP SCSI specifies the SCSI 50-position high-density shielded connector for all external
`connectors and requires that the cables meet the SPI draft standard. This eliminates the
`confusion that exists with the multiplicity of connector and cable choices.
`
`Command set profiles are under consideration for future PnP documents.
`
`Products conforming to the requirements herein are termed PnP SCSI.
`
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`Plug and Play SCSI -- Version 1.0
`
`1. Scope
`
`This specification defines requirements for Plug and Play SCSI host adapters, cables, and
`peripheral devices by establishing a profile for personal computing applications. This common
`profile simplifies integration and configuration of SCSI systems. This specification is a profile of
`the SCSI-2 standard and the SCSI-3 Parallel Interface (SPI) draft standard.
`
`2. Reference documents
`
`Common Access Method Transport and SCSI Interface Module [X3.232-199x]
`Small Computer System Interface - 2 [X3.131-1994]
`SCSI-3 Parallel Interface [X3T10/855D]
`
`The above documents are available from:
`
`
`Global Engineering Documents
`
`
`15 Inverness Way East
`
`
`Englewood, CO 80112-5704
`
`
`Phone: (800) 854-7179 Outside USA and Canada: (303) 792-2181
`
`
`FAX: (303) 792- 2192
`
`SCSI Configured AutoMagically [X3T9.2/93-109r5] (Accepted by X3T9.2 for SPI)
`
`The above document is available from the SCSI BBS 719-574-0424
`
`Advanced SCSI Programming Interface
`
`The above document is available from the Adaptec BBS 408-945-7727.
`
`Mini-Port Driver Developer Kit
`INT 13 Extensions
`Virtual DMA Services
`
`The above documents are available from Microsoft at 1-800-759-5474.
`
`PnP BIOS Specification
`PnP Option ROM specification
`PnP ISA Specification
`The above documents are available from the Plug and Play forum on CompuServe (Go
`plugplay) *.
`
`
`3. Glossary and Abbreviations
`
`3.1. Glossary
`
`External configuration - all SCSI peripheral devices are external to the host enclosure.
`
`Host - A personal computer system that contains a host adapter and zero or more SCSI
`peripheral devices.
`
`Host adapter - either an embedded or add-in SCSI device that acts as an initiator.
`
`Internal configuration - all SCSI peripheral devices are internal to the host enclosure.
`
`Legacy SCSI device - an SCSI device implemented prior to this specification that does not
`conform to PnP SCSI.
`
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`Plug and Play SCSI -- Version 1.0
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`Mixed configuration - a combination of internal and external SCSI peripheral devices.
`
`SCSI peripheral device - an SCSI device other than a host adapter that does not include an
`enclosure, power supply, or external connector.
`
`Peripheral subsystem - an enclosure that contains one or more SCSI peripheral devices.
`
`Internal terminator - the terminator that exists within the host that terminates the internal end of
`the SCSI bus.
`
`External terminator - the terminator that exists on the last peripheral subsystem that terminates
`the external end of the SCSI bus.
`
`Exit-point terminator - a terminator that may be enabled or disabled which exists at the 50-
`position high-density connector on hosts that support a mixed configuration.
`
`3.2. Abbreviations
`ASPI - Advanced SCSI Programming Interface
`BIOS - Basic Input Output System
`CAM - Common Access Method
`ISA - Industry Standard Architecture
`PnP - Plug and Play
`SCAM - SCSI Configured AutoMagically
`SCSI - either SCSI-2 or SCSI-3.
`SPI - SCSI-3 Parallel Interface
`VDS - Virtual DMA Services
`
`4. Conformance
`Products are required to meet all applicable sections of this specification to claim conformance
`with PnP SCSI. Host adapters are also required to satisfy requirements of the applicable Plug
`and Play host bus, including ISA, PCI, PCMCIA, etc. See Appendix B: Compliance Checklist for a
`summary of requirements.
`
`An exemption to section 6.3 of this specification requiring automatic switching of exit-point
`termination is allowed until July 1, 1994. Any PnP products using this exemption shall have the
`exit-point terminator configurable via a software utility.
`
`5. Hardware
`This PnP SCSI specification is intended for use with single-ended SCSI devices. All PnP SCSI
`devices are required to use drivers and receivers which meet the specifications defined in single-
`ended alternative of the SCSI-3 Parallel Interface.
`
`All PnP SCSI devices are required to implement SCSI Bus parity as defined in the SCSI-2
`standard.
`
`6. Connectors
`All hosts and SCSI peripheral subsystems are required to use the 50-position high-density
`shielded connector for external connections. See the SCSI-2 standard (section 5.3.2.1, shielded
`connector alternative 1 - A cable). All external SCSI connectors shall display the SCSI icon (figure
`1) as defined in SPI Annex F.
`
`
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`Plug and Play SCSI -- Version 1.0
`
`
`
`Figure 1 - SCSI Icon
`
`
`The internal SCSI peripheral device connector is not specified by this document.
`
`6.1. Cables
`All cables used in PnP SCSI system must meet the cable specifications defined in clause 6 (SCSI
`bus cables) of the SCSI-3 Parallel Interface draft standard.
`
`In mixed configuration systems, reliability is enhanced by the following provisions.
`
`The SCSI bus segment in the host adapter should be of minimal length and capacitance. Where
`significant length is unavoidable, the bus layout on the board should follow transmission line
`principles and SPI impedance requirements.
`
`The overall length of the internal bus segment should be minimized. The internal bus is either a
`SPI compliant backplane or a SPI compliant ribbon cable (examples: 3M 3801 .050 pitch 26 AWG
`PVC dielectric--89 ohms, or 3M 3749 .025 pitch 30 AWG TPE dielectric--85 ohms or equivalent).
`
`
`Note: Conventional ribbon cable (.050 pitch, 28 AWG, PVC dielectric--108 ohms)
`generally will work in internal configuration (only) implementations, even though it
`is not compliant with the SPI impedance requirements.
`
`
`6.2. Terminator requirements
`
`This section defines the requirements of the terminator and terminator power. Section 7 defines
`the location of terminators in system configurations.
`
`All terminators in PnP SCSI systems shall conform to the terminator requirements in the SCSI-3
`Parallel Interface document. The terminators shall operate correctly over a TERMPWR voltage
`range of 4.0-5.25 Vdc. Regulated terminators are required. These are often referred to as
`alternative 2 or active terminators.
`
`The host and the peripheral sub-system shall provide terminator power (TERMPWR) to the SCSI
`bus. All terminators must be powered from the TERMPWR line(s) in the SCSI bus.
`
`
`Note - Exceptions may be necessary for battery operated devices and PCMCIA
`PC-Cards.
`
`
`Devices providing TERMPWR must provide a self-resetting device (e.g. positive-temperature-
`coefficient or circuit breaker device) to limit the maximum amount of current sourced instead of a
`
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`Plug and Play SCSI -- Version 1.0
`
`fuse. Such a device opens the circuit when excessive current is drawn and closes the circuit
`when the device resets.
`
`6.3. Device Power
`Only terminators may draw power from TERMPWR. SCSI devices that are powered off should
`not sink current from the SCSI bus.
`
`Power management functions are outside the scope of this document.
`
`6.4. SCSI ID assignment
`Each device on the SCSI bus requires a unique SCSI identifier or ID for that bus. ID conflicts,
`indeterminate switch settings, and location of ID switches on the devices complicate the process
`of configuring the SCSI bus. An automated SCSI ID assignment technique is the required
`solution to the problem of users inadvertently assigning the same ID to multiple devices on the
`same bus.
`6.4.1. Automatic SCSI ID assignment
`The X3T9.2 (now X3T10) committee has voted to include a protocol, SCSI Configured
`AutoMagically (SCAM), into the SCSI-3 Parallel Interface draft standard. SCAM automatically
`assigns device ID's. All PnP SCSI devices are required to conform to level 1 of this document
`and level 2 conformance is recommended.
`
`The SCAM protocol is designed so that legacy devices can be detected and used. If more than
`one legacy device is present, the user must ensure that they do not have SCSI ID conflicts.
`Devices that support SCAM function correctly in legacy systems.
`6.4.2. Default SCSI ID's
`It is important for SCAM devices to have a default ID setting in case they are used on a SCSI bus
`that does not use a SCAM master host adapter. Therefore, default IDs listed in table 1 are
`recommended settings for all PnP SCSI devices as shipped from the factory. Provision may be
`given to the user to change the default ID to control configuration.
`
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`
`Plug and Play SCSI -- Version 1.0
`
`
`
`Table 1 - As shipped default SCSI ID assignment
`SCSI ID
`SCSI Default ID
`7
`Host Adapter
`6
`Magnetic Disk Drive
`5
`
`4
`Tape or R/W Optical
`3
`CD-ROM
`2
`Scanner/Printer
`1
`
`0
`
`
`
`SCSI IDs 5, 1, and 0, are used during system configuration for additional devices.
`6.4.3. SCAM ID assignment algorithm
`SCAM masters should assign the same SCSI ID to SCAM devices as previously assigned to the
`SCAM device to ensure consistent ID assignment. If the SCAM master is unable to maintain a
`non-volatile device table or there has been no previous ID assignment, then the following
`algorithm should be used (see Appendix C for examples):
`
`
`During the isolation stage, the SCAM master isolates each SCAM device one at a
`time and stores the device's Default ID. The 5-bit ID field within the Type Code
`contains the device's Default ID. The SCAM master shall assign the SCAM
`device a Soft ID after its isolation stage is completed and before proceeding to
`isolate the next SCAM device.
`
`The SCAM master shall assign Soft IDs that are available and not already used
`by any other device on the bus. The first choice shall be the device's Default ID.
`Otherwise, the choice shall be the next smaller ID that is available. If ID zero is
`not available, the SCAM master shall assign the highest available ID. If no ID is
`available, no Soft ID shall be assigned to the device.
`
`
`7. System configurations
`Plug and Play requirements exist for three possible configurations:
`
`
`
`
`
`
`
`a)
`
`b)
`
`c)
`
` Internal configuration - all SCSI peripheral devices are internal to the host
`enclosure.
` External configuration - all SCSI peripheral devices are external to the host
`enclosure.
` Mixed configuration - a combination of internal and external SCSI peripheral
`devices.
`
`
`Each of these configurations exist for two design alternatives: add-in SCSI and motherboard
`SCSI. The term host adapter is used to refer to either the SCSI add-in card for the add-in SCSI
`design alternative or the SCSI logic within the host for the motherboard design alternative.
`
`PnP SCSI host adapters that support a mixed configuration shall have an automatic switchable
`terminator at the exit point.
`
`PnP SCSI peripheral devices shall not terminate the SCSI bus.
`
`
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`
`Plug and Play SCSI -- Version 1.0
`
`PnP SCSI peripheral subsystems attached to the SCSI bus shall be terminated by installing an
`external terminator into the remaining open connector on the last peripheral subsystem on the
`bus.
`7.1. Add-in SCSI Design Alternative
`The add-in SCSI alternative for the internal, external, and mixed configurations is shown in figure
`2.
`
`
`COMPUTER CASE
`
`(ON)
`
`HOST
`ADAPTER
`
`EXTERNAL SCSI
`PERIPHERAL
`DEVICES
`
`COMPUTER CASE
`
`(ON)
`
`TERMINATOR MUST
`BE SWITCHABLE IF
`BOTH INTERNAL AND
`EXTERNAL DEVICES
`ARE SUPPORTED.
`
`HOST
`ADAPTER
`
`INTERNAL SCSI
`PERIPHERAL
`DEVICES
`
`COMPUTER CASE
`
`(OFF)
`
`HOST
`ADAPTER
`
`INTERNAL SCSI
`PERIPHERAL
`DEVICES
`
`EXTERNAL SCSI
`PERIPHERAL
`DEVICES
`
`
`
`DISABLED (OFF) SWITCHABLE TERMINATOR
`PERMANENT TERMINATOR OR ENABLED (ON) SWITCHABLE TERMINATOR
`
`
`
`Figure 2 - Add-in SCSI Design
`
`
`In the internal configuration the host adapter and internal terminator supply the necessary
`termination.
`
`
`12
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`Plug and Play SCSI -- Version 1.0
`
`The internal SCSI bus is routed as necessary within the PC, complying to the rules in SPI, to one
`or more SCSI devices. The end of the internal SCSI bus ends with an internal terminator.
`
`Keyed connectors are required on the host adapter and the cables to ensure that the cable is
`plugged in correctly (not reversed or shifted). Systems with backplane SCSI buses are required
`to meet the electrical requirements of the SCSI-3 Parallel Interface document, particularly with
`respect to characteristic impedance.
`
`In the external configuration the internal and external terminator supply the necessary termination.
`
`Peripheral subsystems are required to provide two 50-position high-density shielded connectors
`on the enclosure. The SCSI bus internal to the enclosure is routed from one external SCSI
`connector to the SCSI device(s) within the enclosure and then to the second external SCSI device
`connector.
`
`One external terminator must be installed on the last SCSI peripheral subsystem. The user may
`purchase the external terminator and cable when installing the first SCSI peripheral subsystem.
`
`In mixed configurations the exit-point terminator is automatically enabled or disabled to correctly
`terminate the SCSI bus based on detection of the configuration. This is accomplished through
`mechanical or electronic detection of the attached cable or electronic detection of attached SCSI
`device(s).
`
`
`Note - A switchable terminator may be employed to serve as either the internal
`terminator (external-only configuration) or the host adapter terminator (internal-
`only configuration). In the mixed configuration, the switchable terminator must be
`disabled whenever both internal or external cables are attached or both internal
`and external SCSI devices are present.
`
`7.2. Motherboard SCSI Design Alternative
`The motherboard SCSI alternative for the internal, external and mixed configurations is shown in
`figure 3.
`
`
`13
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`
`Plug and Play SCSI -- Version 1.0
`
`COMPUTER CASE
`
`HOST
`ADAPTER
`
`INTERNAL SCSI
`PERIPHERAL
`DEVICES
`
`COMPUTER CASE
`
`(ON)
`
`(OFF)
`
`TERMINATOR MUST
`BE SWITCHABLE
`
`HOST
`ADAPTER
`
`INTERNAL SCSI
`PERIPHERAL
`DEVICES
`
`EXTERNAL SCSI
`PERIPHERAL
`DEVICES
`
`DISABLED (OFF) SWITCHABLE TERMINATOR
`
`PERMANENT TERMINATOR OR ENABLED (ON) SWITCHABLE TERMINATOR
`Figure 3 - Motherboard SCSI Design
`
`
`
`
`In the internal configuration the host adapter and internal terminator supply the necessary
`termination.
`
`The internal SCSI bus is routed as necessary within the PC, complying to the rules in SPI, to one
`or more SCSI devices. The end of the internal SCSI bus ends with an internal terminator.
`
`If cables are employed, keyed connectors are required on the motherboard and the cables to
`ensure that the cable is plugged in correctly (not reversed or shifted). Systems with backplane
`SCSI buses are required to meet the electrical requirements of the SCSI-3 Parallel Interface
`document, particularly with respect to characteristic impedance.
`
`In the external configuration the internal and external terminator supply the necessary termination.
`
`
`14
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`

`
`Plug and Play SCSI -- Version 1.0
`
`Peripheral subsystems are required to provide two 50-position high-density shielded connectors
`on the enclosure. The SCSI bus internal to the enclosure is routed from one external SCSI
`connector to the SCSI device(s) within the enclosure and then to the second external SCSI device
`connector.
`
`One external terminator must be installed on the last SCSI peripheral subsystem. The user may
`purchase the external terminator and cable when installing the first SCSI peripheral subsystem.
`
`In systems that support mixed configurations, the exit-point terminator is automatically enabled or
`disabled to correctly terminate the SCSI bus. This is accomplished through mechanical or
`electronic detection of the attached cable or electronic detection of attached SCSI device(s).
`
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`
`Plug and Play SCSI -- Version 1.0
`
`7.3. Mixed configuration Example
`Figure 4 illustrates a mixed configuration with an add-in SCSI design alternative. This figure is
`provided as a reference and should not be construed as a specification of requirements. The
`internal terminator should be as close as possible to the last internal SCSI peripheral device; the
`distance in the figure is exaggerated for clarity. The system combines internal and external
`devices on the same SCSI bus.
`
`
`
`PERIPHERAL
`SUBSYSTEM
`
`EXTERNAL
`TERMINATOR
`
`50-POSITION
`HIGH-DENSITY
`CONNECTORS
`
`SWITCHABLE
`EXIT-POINT
`TERMINATOR
`HOST SCSI
`PROTOCOL CHIP
`
`SCSI
`PERIPHERAL
`DEVICES
`
`COMPUTER CASE
`
`SCSI EXTERNAL
`CABLE
`
`SCSI INTERNAL
`CABLES
`
`INTERNAL
`TERMINATOR
`
`
`
`Figure 4 - Mixed Configuration Example
`
`
`8. X86 Software Issues
`
`The following subsections describe software issues specific to X86 architecture DOS-based
`systems when developing a plug and play system.
`
`8.1. Geometry translation
`In a DOS environment host adapter option ROM's are required to support the logical block
`addressing as defined by the INT 13h Extensions document. For backward compatibility, SCSI
`option ROM's are required to support the current INT 13h functions as well. To determine the
`geometry for INT 13h function 08h, these option ROM's read the partition table from the disk at
`LBA 0 and if it exists as determined by the 55AAh signature at offset 1FEh, use the existing
`geometry to calculate the Cylinder-Head-Sector (CHS) values. Appendix A provides an example
`of how to determine the geometry. If the partition table does not exist, SCSI option ROM's may
`use any algorithm to set the CHS values.
`
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`
`Plug and Play SCSI -- Version 1.0
`
`
`9. Generic Software Issues
`
`There are several generic software issues related to designing a plug and play system. Some of
`these are addressed in the subsections that follow. One issue that is not addressed is command
`sets for the specific device types. Command set profiles are under consideration for future PnP
`documents to insure minimum device functionality and device driver interoperability.
`
`9.1. READ CAPACITY command
`A direct-access device is required to return the actual number of user-accessible logical blocks on
`the drive. This value does not include the sectors reserved for defect mapping.
`
`9.2. INQUIRY command
`
`All devices are required to return INQUIRY data in a timely manner without excessive delays.
`This enables a SCSI host to scan the SCSI bus and boot without significant delays.
`
`
`NOTE - Historically, some SCSI devices when first powered up performed
`operations that delayed their response to selection for lengthy mechanical actions
`and device initialization (e.g., rewinding an inserted tape, spinning up the
`medium, etc.). Excessive delays are not acceptable in a PnP SCSI environment.
`
`9.3. Synchronous negotiation
`The synchronous negotiation protocol is required to be implemented as defined in SCSI-2. If the
`device supports only asynchronous data transfers it must respond with a MESSAGE REJECT
`message to a SYNCHRONOUS DATA TRANSFER REQUEST message and continue in
`asynchronous data transfer mode.
`
`9.4. Virtual DMA services (VDS)
`PnP SCSI host adapters that support bus-mastering are required to support Virtual DMA Services
`(VDS) in the host adapter option ROM. VDS solves the problem of mapping linear addresses
`(segment:offset) into physical addresses. VDS is not applicable to host adapters that do not use
`bus-mastering.
`
`9.5. Device driver architecture
`There are two primary device driver protocols in use today. These are ASPI and CAM. Windows
`NT and future windows OS's use their own SCSI protocol which is referred to as the Miniport
`interface. It is recommended that vendors of host adapters supply drivers compatible with one or
`more of these layered driver architectures.
`
`9.6. Software configuration
`
`All PnP host adapter configuration parameters are required to be configurable with a software
`utility. The PnP ISA specification provides one example of host parameters that are configurable.
`SCSI parameters that are configurable are required to be controlled with a software utility (e.g.,
`enable/disable synchronous operation).
`
`17
`
`

`
`Plug and Play SCSI -- Version 1.0
`
`9.7. Boot considerations
`
`PnP hosts are required to assign SCSI IDs in a consistent and repeatable manner through power
`cycles or resets using the algorithm specified in section 6.4.3, so long as the SCSI bus
`configuration remains static. Non-static environments, such as physically removing or powering
`down a device prior to a boot cycle, may affect drive ID assignment, which can lead to no-boot
`situations. Resolution of this issue is beyond the scope of this specification. The issue requires
`system level solutions possibly affecting the BIOS and the operating system.
`
`The long-term solution to non-static configuration environments is PnP system extensions
`enabling the end user to select the boot device on multiple-device systems. Until such extensions
`are defined and implemented, the no-boot condition, which can occur in legacy systems today,
`may also occur in PnP systems. In both cases, the correct boot device may be recognized by
`adjusting the SCSI IDs through host adapter utilities or changing the device default ID. This is an
`interim solution which will be replaced when a broader PnP system solution is defined.
`
`
`18
`
`

`
`Plug and Play SCSI -- Version 1.0
`
`APPENDIX A - CHS mapping
`Existing operating systems use a Cylinder-Head-Sector (CHS) mapping for "talking" to hard disk
`drives via INT 13h. These CHS values limit the maximum number of blocks on a disk to 1024 *
`255 * 63 which is slightly less than eight Gigabytes. To overcome this, new operating systems
`have defined new partition table system type values. These values will be used for partition entries
`whose limits exceed the CHS mapping capabilities.
`
`The two new system types are 0Eh for primary partitions and 0Fh for extended partitions meeting
`the criteria above. These entries will have CHS values of 0Fh and will use the LBA starting field
`(offset 08h in the partition entry) and the Length field (offset 0Ch) to describe the partition. Access
`to these partitions will be by the INT 13h extensions as defined by Microsoft. The following routine
`is pseudo-code for detecting an existing partition and calculating geometry using this information.
`
`Partition table entry format
`
`Offset Description
`------ ---------------------------
`
` 00h Boot indicator
` 01h Start Head
` 02h Start Sector/Cylinder
` 03h Start Cylinder (low 8 bits)
` 04h System type
` 05h Ending Head
` 06