SCSI Controllers and Peripherals - A reference for Designing Servers and Peripherals for the Microsoft® Windows...

SCSI Controllers and Peripherals
SCSI is a flexible I/O bus that is used in the design of a variety of peripherals, including disk drives, CD‑ROM drives, tape drives, scanners, and magneto-optical drives. This section presents the requirements for Windows NT–compatible SCSI hardware, including adapters, peripherals, and any device that uses a SCSI controller.
112 System includes SCSI host controller and SCSI peripherals
The SCSI host adapter is the circuitry that serves as an interface between the system and one or more SCSI peripherals. A host adapter can be a card that plugs into the system’s expansion bus, such as a PCI card, or it can be designed directly into the system board.
113 SCSI option ROMs support virtual DMA services
Plug and Play SCSI host adapters must support virtual DMA services in the host-adapter option ROM together with the required bus-mastering support. Virtual DMA supports scatter/gather capabilities, solving the problem of mapping linear addresses (segment:offset) into physical addresses.
114 Bus type is clearly indicated on connectors for all adapters, peripherals, cables, and terminators
Connectors for each SCSI adapter, peripheral, cable, and terminator must be clearly labeled to show the bus type. All external SCSI connectors must display the appropriate SCSI icon defined in Small Computer Interface (SCSI‑3) Parallel Interface (SPI) specification, Annex F, plus any clarifying abbreviations or acronyms.
115 Differential devices support DIFFSENS as defined in SCSI‑3
Without DIFFSENS, the differential bus drivers and/or a single-ended device will burn up if a single-ended device is put on a differential bus.
The specification for DIFFSENS is defined in Section 5.4.2 of the SCSI‑3 specification.
116 Automatic termination circuit meets SCSI‑3 specification
SCSI add-on adapters and on-board controllers must use automatic termination that allows a user to add external devices without removing the server case. Terminators used in the SCSI host adapter must be regulated terminators, which are also known as active, SCSI‑3 SPI, SCSI‑2 alternative‑2, or Boulay terminators.
117 SCSI terminator built onto internal cables meets SCSI‑3 specification
For SCSI subsystem configurations, internal cables must be preconfigured with active termination at one end of the cable.
118 Terminator power is supplied to the SCSI bus, with over-current protection
For system-board implementations using PCI or another expansion bus, the host adapter must supply terminator power (TERMPWR) to the SCSI bus. All terminators on the host adapter, as well as those on the internal and external SCSI bus, must be powered from the TERMPWR lines in the SCSI bus.
Devices that provide TERMPWR must also provide some means of limiting the current, through use of a self-setting device. For example, a positive-temperature coefficient device or circuit breaker can be designed into the circuit. These devices open during an over-current condition and close after the condition ends.
119 External connector meets SCSI‑2 or later specification
If an external connector is implemented, it must meet the requirements defined in SCSI‑2 or a later specification.
120 Internal terminator is as close as possible to the last peripheral on the cable
The internal terminator should be as physically close as possible to the last peripheral on the cable. There should be some means, such as written instructions on the cable, to ensure that the user always connects internal peripherals starting with the plug closest to the terminator.
121 SCSI bus parity signal meets SCSI‑2 specification
All SCSI peripherals must implement the SCSI bus parity signal defined in the SCSI‑2 specification.
122 Cables meet SCSI‑3 SPI Clause 6 requirements
Clause 6 of the SCSI‑3 SPI specification defines the various characteristics of cables for SCSI devices.
123 User cannot incorrectly plug in cables
For an internal configuration, the internal SCSI bus cable must be plugged into shrouded and keyed connectors on the host adapter and devices. This requirement ensures that the cable is properly positioned. Pin 1 orientation must be designated on one edge of the ribbon cable and also on the keyed connector for the SCSI peripheral device.
For an external configuration, the SCSI connector must not use the same connector type as any other non-SCSI connector on the system.
124 External devices use automatic termination, an external pluggable terminator, or an accessible on‑board termination switch
The recommended implementation for an external SCSI peripheral device is to provide automatic termination. In the absence of automatic termination, an external pluggable terminator must be connected to the last open device connector on the bus. If a mechanical means is provided for setting termination, the switch must be accessible to the user without opening the server case.
125 Shielded device connector meets SCSI‑2 or higher specification
Device connectors must meet the specifications defined in the SCSI‑2 or a higher specification.
126 SCSI removable media include media status notification support
A specification has not yet been completed for implementing media event status notification. However, the projected specification will be similar to the Media Event Status Notification subsection of SFF 8090 (Mt. Fuji specification). This specification is available at ftp://fission.dt.wdc.com/pub/standards/SFF/specs/.
When a completed specification is available, support for media status notification will become a requirement.
127 Hardware supports the STOP/START UNIT command as defined in the SCSI‑2 specification
The hardware in SCSI peripherals must be able to fully recover from a software-initiated spin down without rebooting the system or cycling power. To properly support power management on SCSI drives and to ensure that the operating system responds to appropriate driver calls, be sure to correctly implement the STOP/START UNIT command as defined in the SCSI‑2 specification.
128 STOP/START UNIT command can be used to decrease power consumption
Wherever appropriate—for example, for storage disks—the STOP UNIT command can be used to decrease power consumption of the base platform. For any form of power management to work on SCSI, the device should be capable of supporting many tens of thousands of START/STOP UNIT commands over the life of the device.
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