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SDLT 600 SCSI Interface Guide Chapter 2: SCSI Parallel Protocol

81-81200-02 REV A01 January 2005 2-3

• If an initiator wishes to send a message before transitioning to a new bus phase, the initiator asserts

the ATN signal, then waits at least two deskew delays before negating the ACK signal for the last

byte transferred in the current bus phase. Asserting the ATN signal later may not be honored until

a later bus phase which may result in an unexpected action.

The SDLT tape drive responds with a MESSAGE OUT phase, as described in the following table:

The initiator keeps the ATN signal asserted if more than one byte is to be transferred. The initiator may

negate the ATN signal at any time, except that it should not negate ATN while the ACK signal is

asserted during a MESSAGE OUT phase.

NOTE: Normally, the initiator negates the ATN signal while the REQ signal is

true and the ACK signal is false during the last REQ/ACK handshake of

the MESSAGE OUT phase.

Table 2-1. Drive’s MESSAGE OUT Phase Response

ATN Signal True in Phase... The Drive Enters MESSAGE OUT…

COMMAND After transferring part or all of the command descriptor block

bytes.

DATA At the drive’s earliest convenience (often on a logical block

boundary). The initiator continues REQ/ACK handshakes

until it detects the phase change.

STATUS After the initiator has acknowledged the status byte.

MESSAGE IN Before it sends another message. This permits a MESSAGE

PARITY ERROR message from the initiator to be associated

with the appropriate message.

SELECTION

Immediately after that SELECTION phase.

RESELECTION

After the drive has sent its IDENTIFY message for that

RESELECTION phase.

1. Before the initiator releases BSY, provided the initiator asserted ATN.

2. The initiator should only assert the ATN signal during a RESELECTION phase to transmit a TARGET

RESET or DISCONNECT message.

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