Seagate Ultra 160, Ultra 320 manual Message restrictions, Negotiation message sequences

2.12.10Message restrictions

PPR may be originated by SCSI initiator ports but shall not be originated by SCSI target ports. If bus expanders are present, SCSI initiator ports should only use PPR when requesting values not attainable via WDTR and SDTR (e.g., setting any protocol option bits to one). If a SCSI target port responds to PPR only with values that are attainable via WDTR and SDTR (i.e., all protocol option bits set to zero), the SCSI initiator port should repeat negotiation with a WDTR and SDTR negotiation sequence. This ensures that bus expanders that do not support PPR are still able to handle data phases correctly.

WDTR and SDTR may be originated by either SCSI target ports or SCSI initiator ports. Since WDTR resets all the values that SDTR sets, it shall be sent first if both are needed. SCSI target ports capable of wide transfer agreements shall originate negotiation with WDTR followed with SDTR.

Note. If IU_REQ was set to one and a successful SCSI target port originated WDTR negotiation occurs, a BUS FREE phase generated because the SCSI target port detected that IU_REQ was changed is indistinguishable from a BUS FREE phase generated because the target port was detecting parity errors on the WDTR OUT. Following the WDTR negotiation with an SDTR negotiation before the BUS FREE occurs ensures that the SCSI initiator port and SCSI target port both know that IU_REQ has changed.

2.12.11Negotiation message sequences

A SCSI initiator port originated negotiation sequence contains up to four steps:

1.SCSI initiator port’s originating message;

2.SCSI target port response;

3.SCSI initiator port response; and

4.SCSI target port second response.

A SCSI target port originated negotiation sequence contains up to four steps:

1.SCSI target port’s originating message;

2.SCSI initiator port response;

3.SCSI target port response; and

4.SCSI initiator port second response.

If the negotiation fails after a vendor-specific number of retries, the SCSI port originating the negotiation sequence may discontinue communication with the other SCSI port.

For illustrations showing the various negotiation responses, refer to the ANSI SPI-5 specification Section 4.12.7.

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