Seagate Ultra 160, Ultra 320 manual Parallel Protocol Request messages implied agreement

be changed. Each SCSI device when transmitting data shall respect the negotiated limits set by the other's PARALLEL PROTOCOL REQUEST message, but it is permitted to transfer data with larger periods, smaller synchronous REQ/ACK offsets, or both. The completion of an exchange of PARALLEL PROTOCOL REQUEST messages implies an agreement as shown in Table 35.

If the target does not support the selected protocol option it shall clear as many bits as required to set the pro- tocol option field to a legal value that it does support.

Table 35: PARALLEL PROTOCOL REQUEST messages implied agreement

Target’s Parallel Protocol
Request response	Implied agreement
Non-zero REQ/ACK OFFSET	Synchronous transfer (i.e., each SCSI device transmits data with a
	period equal to or greater than and a REQ/ACK OFFSET equal to or
	less than the negotiated values received in the target’s PPR message).
REQ/ACK OFFSET equal to zero	Asynchronous transfer
Transfer width exponent equal	16-bit data
to one
Transfer width equal to zero	Eight-bit data
Protocol options equal to 0h and	Eight-bit/asynchronous data transfer with protocol options field set to 0h
transfer period factor equal to 9h
IU_REQ, DT_REQ, and QAS_REQ	ST DATA IN and ST DATA OUT phases to transfer data
equal to zero
DT_REQ equal to one	DT DATA IN and DT DATA OUT phases to transfer data
IU_REQ and DT_REQ equal to one	DT DATA IN and DT DATA OUT phases with information units
IU_REQ, DT_REQ, and QAS_REQ	DT DATA IN and DT DATA OUT phases with information units and use
equal to one	QAS for arbitration
MESSAGE REJECT message	The initiator shall set eight-bit/asynchronous data transfer with protocol
	options field set to 0h.
Parity error (on responding	Eight-bit/asynchronous data transfer with protocol options field set to
message)	0h
Unexpected bus free (as a result of	Eight-bit/asynchronous data transfer with protocol options field set to
the responding message)	0h
No response	Eight-bit/asynchronous data transfer with protocol options field set to
	0h

If there is an unrecoverable parity error on the initial PARALLEL PROTOCOL REQUEST message (see Sec- tion 4.3.12), the initiator shall retain its previous data transfer mode and protocol options. If there is an unex- pected bus free on the initial PARALLEL PROTOCOL REQUEST message, the initiator shall retain its previous data transfer mode and protocol options.

4.3.12.1PARALLEL PROTOCOL REQUEST

If the initiator recognizes that Parallel Protocol Request negotiation is required, it creates an attention condition and sends a PARALLEL PROTOCOL REQUEST message to begin the negotiating process. After successfully completing the MESSAGE OUT phase, the target shall respond with a PARALLEL PROTOCOL REQUEST message or a MESSAGE REJECT message.

Parallel SCSI Interface Product Manual, Rev. A 95