Seagate Ultra 320, Ultra 160 manual Sdtr messages implied agreements

a.after a HARD RESET;

b.after a TARGET RESET message;

c.after a power cycle; and

d.after a change in the transceiver mode (e.g., LVD mode to MSE mode).

Any condition that leaves the data transfer agreement in an indeterminate state shall cause the SCSI device to enter an asynchronous data transfer mode.

A SCSI device may initiate an SDTR message exchange whenever it is appropriate to negotiate a new data transfer agreement (either synchronous or asynchronous). SCSI devices that are capable of synchronous data transfers shall not respond to an SDTR message with a MESSAGE REJECT message.

Renegotiation after every selection is not recommended since a significant performance impact is likely.

The SDTR message exchange establishes the permissible transfer periods and the REQ/ACK offsets for all logical units on the two SCSI devices. This agreement only applies to ST DATA IN phases and ST DATA OUT phases. COMMAND, MESSAGE, and STATUS phases shall use asynchronous transfers.

The originating SCSI device (the SCSI device that sends the first of the pair of SDTR messages) sets its values according to the rules above to permit it to receive data successfully. If the responding SCSI device is able to also receive data successfully with these values (or smaller transfer periods or larger REQ/ACK offsets or both), it returns the same values in its SDTR message. If it requires a larger transfer period, a smaller REQ/ ACK offset, or both in order to receive data successfully, it substitutes values in its SDTR message as required, returning unchanged any value not required to be changed. Each SCSI device, when transmitting data, shall respect the negotiated limits set by the other's SDTR message, but it is permitted to transfer data with larger transfer periods, smaller synchronous REQ/ACK offsets, or both. The completion of an exchange of SDTR messages implies an agreement as shown in Table 37.

Table 37: SDTR messages implied agreements

Responding SCSI device Parallel
Protocol Request response	Implied agreement
Non-zero REQ/ACK offset	Synchronous transfer (i.e., each SCSI device transmits data with a
	transfer period equal to or greater than and a REQ/ACK offset equal to
	or less than the values received in the other device’s SDTR message)
	with ST DATA IN and ST DATA OUT phases. This is equivalent to pro-
	tocol options of 0h in the PARALLEL PROTOCOL REQUEST mes-
	sage.
REQ/ACK offset equal to zero	Asynchronous transfer.
MESSAGE REJECT message	The originating SCSI device shall set synchronous transfer.
Parity error (on responding	Asynchronous transfer
message)
Unexpected bus free (as a result of	Asynchronous transfer
the responding message)
No response	Asynchronous transfer

If there is an unrecoverable parity error on the initial SDTR message (see sections 3.12.2 and 3.12.3), the initi- ating SCSI device shall retain its previous data transfer mode. If there is an unexpected bus free on the initial SDTR message, the initiating SCSI device shall retain its previous data transfer mode.

98 Parallel SCSI Interface Product Manual, Rev. A