The iuCRC shall be used to protect all SPI information units. The SCSI device that originates the SPI informa- tion unit sends the necessary pad bytes and iuCRC fields.
An iuCRC interval may also be specified. The iuCRC interval specifies the number of bytes transferred before pad bytes (if any) and the iuCRC is transferred within SPI data information units and SPI data stream informa- tion units. A SPI data information unit or a SPI data stream information unit may contain zero or more iuCRC intervals depending on the values specified in the SPI
The iuCRC interval is required to be a multiple of two, however, if it is not a multiple of four, then two pad bytes shall be transmitted before the iuCRC is transmitted.
SPI data stream information units may be used to transfer data to or from a SCSI device. Support of data streaming during DT DATA OUT phases, called write streaming, is mandatory. Support of data streaming dur- ing DT DATA IN phases, called read streaming, is optional. The use of read streaming is part of the negotiated transfer agreement between two SCSI devices (i.e., the RD_STRM bit set to one). A SCSI target port is not required to use read streaming even if streaming support is enabled.
A SCSI target port, while streaming data, may give an indication that the stream of SPI data stream information nits are about to end while still sending the current SPI data stream information unit. This early warning is called flow control. Support of flow control during DT DATA OUT phases, called write flow control, is optional. Support of flow control during DT DATA IN phases, called read flow control, is mandatory if read streaming is enabled. The use of write flow control is part of the negotiated transfer agreement between two SCSI devices (i.e., the WR_FLOW bit set to one).
2.12Negotiation
PARALLEL PROTOCOL REQUEST (PPR) (see Section 4.3.12), SYNCHRONOUS DATA TRANSFER REQUEST (SDTR) (see Section 4.3.16), and WIDE DATA TRANSFER REQUEST (WDTR) (see Section 4.3.18) messages are used to alter the transfer agreement between two ports. The transfer agreement defines the protocol used during data phases (e.g., transfer period, REQ/ACK offset, transfer width) and agreement on features not affecting data phases (e.g., QAS). All other information transfer phases (i.e., COMMAND, MES- SAGE, and STATUS) use
PPR, SDTR, and WDTR messages are called negotiation messages. When a SCSI initiator port sends one of them, the message names are PPR OUT, SDTR OUT, and WDTR OUT. When a SCSI target port sends one of them, the message names are PPR IN, SDTR IN, and WDTR IN. A negotiation sequence consists of at least one matching set of negotiation messages (e.g., PPR OUT and PPR IN).
A transfer agreement is maintained by each port for each other port on the SCSI bus. Each port may be used as either a SCSI target port or a SCSI initiator port. The same transfer agreement applies whether the port is being used as a SCSI target port or as a SCSI initiator port.
2.12.1Negotiation algorithm
A SCSI initiator port and SCSI target port exchange negotiation messages to perform negotiation. The originat- ing port is the one that sends the first negotiation message and the responding port is the one that replies. Ports shall not set message fields to values they do not support. The originating port should set the fields in the originating negotiation message to the maximum values (e.g., fastest transfer period, largest REQ/ACK offset) it supports. If the responding port is able to support the requested values, it shall return the same values in the responding negotiation message. If the responding port requires different values (i.e., a subset of the originat- ing port’s request), it shall return those values in the responding negotiation message (e.g., if the originating port asks for a REQ/ACK offset of 32 and the responding port only supports a REQ/ACK offset of 16, then the responding port replies with an offset of 16).
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