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2.10
ST DATA phase parallel transfers
The format of data transmitted during ST DATA phases consists of data and protection. Parity generation and
checking give some error detection protection in the ST phase data.
2.11
DT DATA phase parallel transfers
During DT DATA phases, communicating SCSI devices format information according to one of two protocol
options:
• Data group transfers. Data groups encapsulate all data and associated error protection.
• Information unit transfers. Information units encapsulate all nexus, task management, task attribute, com-
mand, data, and error protection. Usually these are called "SPI information units."
Sections 2.11.1 and 2.11.2 contain a brief description of how packetized information transfer and CRC protec-
tion fit into the SCSI I/O system operation.
2.11.1
Data group transfers
Data group transfers are permitted when a synchronous transfer agreement is in effect. Data group transfers
are not permitted when an asynchronous transfer agreement or a paced transfer agreement is in effect
When using data group transfers, each DT DATA IN phase and DT DATA OUT phase contains one or more
data groups. A data group consists of a non-zero length data field containing an even number of bytes, fol-
lowed by a pad field (when pad bytes are needed), and then followed by a pCRC field. The number of bytes
transferred within a data group shall always be a multiple of four.
If the number of bytes in the data field is not a multiple of four, the transmitting SCSI device shall place two pad
bytes into the pad field. If the number of bytes in the data field is a multiple of four, the transmitting SCSI device
shall omit the pad field. Regardless of the number of bytes in the data field, the pCRC field shall be the last four
bytes of the data group.
The value of the pad bytes within the pad field is vendor specific.
During DT DATA IN phase, if the number of bytes in a data field is not a multiple of two bytes, then after send-
ing the pad and pCRC fields, the target shall change to MESSAGE IN phase and send an IGNORE WIDE
RESIDUE message (see Section 4.3.4) with the NUMBER OF BYTES TO IGNORE FIELD set to 01h.
During DT DATA OUT phase, if a SCSI target port requests a pCRC field prior to the last data field of a task,
the initiator shall transmit an even number of bytes in that data field.
The pCRC shall be used to protect all data group transfers. The SCSI device transmitting data sends the nec-
essary pad field(s) and a pCRC field at a point determined by the target.
2.11.2
Information unit transfers
Information unit transfers are permitted when a synchronous transfer agreement is in effect. Information unit
transfers are mandatory when a paced transfer agreement is in effect. Information unit transfers are not permit-
ted when an asynchronous transfer agreement is in effect.
During information unit transfers, each DT DATA IN phase and DT DATA OUT phases contains one or more
SPI information units. The number of bytes transferred within a SPI information unit shall always be a multiple
of four.
If the number of bytes in the SPI information unit is not a multiple of four, the transmitting SCSI device shall
transmit one, two, or three pad bytes as is necessary to make the transfer a multiple of four bytes before trans-
mitting an iuCRC. If the number of bytes in the SPI information unit is a multiple of four, the transmitting SCSI
device shall not transmit any pad bytes. Regardless of the number of bytes in the SPI information unit, the last
four bytes of the SPI information unit shall be an iuCRC.
The value of the pad bytes is vendor-specific.
38
Parallel SCSI Interface Product Manual, Rev. A )
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