Seagate Ultra 160 ユーザーズマニュアル

Parallel SCSI Interface Product Manual, Rev. A                                        

   11

third-party command—A SCSI command which requires a logical unit within the target device to assume the 
initiator role and send a SCSI command to a SCSI target port device.

transaction—A cooperative interaction between two objects, involving the exchange of information or the exe-
cution of some service by one object on behalf of the other.

transfer period—The negotiated time between edges of REQ or ACK that latch data. For ST, the transfer 
period is measured from assertion edge of the REQ or ACK signal to the next assertion edge of the signal. For 
DT, the transfer period is measured from a transition edge of the REQ or ACK signal to the next transition edge 
of the signal.

unconfirmed protocol service—A service available at the protocol service interface, which does not result in 
a completion confirmation.

unlinked command—A SCSI command having the link bit set to zero in the CDB control byte.

upper level protocol—An application-specific protocol executed through services provided by a lower level 
protocol.

Several keywords are used to differentiate between different levels of requirements and optionality, as follows:

vendor-specific—Specification of the referenced item is determined by the device vendor.

protocol-specific—Implementation of the referenced item is defined by a SCSI protocol standard (see Sec-
tion 1.1.1.)

expected—A keyword used to describe the behavior of the models specified by this standard.

invalid—A keyword used to describe an illegal or unsupported bit, byte, word, field, or code value. Receipt of 
an invalid bit, byte, word, field, or code value shall be reported as an error.

mandatory—A keyword indicating items required to be implemented as defined by this standard.

may—A keyword that indicates flexibility of choice with no implied preference (equivalent to “may or may not”).

may not—Keywords that indicates flexibility of choice with no implied preference (equivalent to “may or may 
not”).

obsolete—A keyword indicating items that were defined in prior SCSI standards but have been removed from 
this standard.

option, optional—Keywords that describe features which are not required to be implemented by this stan-
dard. However, if any optional feature defined by the standard is implemented, it shall be implemented as 
defined by the standard.

reserved—A key word referring to bits, bytes, words, fields, and code values that are set aside for future stan-
dardization. Their use and interpretation may be specified by future extensions to this or other standards. A 
reserved bit, byte, word, or field shall be set to zero, or in accordance with a future extension to this standard. 
Recipients are not required to check reserved bits, bytes, words, or fields for zero values. Receipt of reserved 
code values in defined fields shall be treated as an error.

shall—A keyword indicating a mandatory requirement. Designers are required to implement all such manda-
tory requirements to ensure interoperability with other standard conformant products.

should—A keyword indicating flexibility of choice with a strongly preferred alternative. Equivalent to the 
phrase “it is recommended.”