Information Transfer Phases - Seagate ST11200 Product Manual

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3.1.4.2 Reselection timeout procedure
This Reselection timeout procedure is specified for clearing the SCSI bus during a Reselection phase. If the
target waits a minimum of a selection timeout period and there has been no BSY response from the initiator,
the target shall continue asserting SEL and I/O and shall release all Data Bus signals. If the target has not
detected BSY to be true after at least a selection abort time plus two deskew delays, the target shall release
SEL and I/O allowing the SCSI bus to go to the Bus Free phase. SCSI devices that respond to Reselection
shall ensure that the Reselection was still valid within a selection abort time of their assertion of BSY. Failure
to comply with this requirement could result in an improper Reselection (two initiators connected to the same
target or the wrong initiator connected to a target).
If an initiator times out while waiting to be reselected, the initiator should attempt to select and issue Request
Sense to determine if the previous command is:
1. Still in process (Busy Status is returned),
2. Aborted with valid Request Sense data, or
3. Aborted without valid Request Sense data.
3.1.5

Information transfer phases

NOTE: The Command, Data, Status, and Message phases are grouped together as information transfer
phases because they are all used to transfer data or control information via the Data Bus. The actual con-
tents of the information is beyond the scope of this section.
The C/D, I/O, and MSG signals are used to distinguish between the different information transfer phases.
(See Table 3.1.5-1). The target drives these three signals and therefore controls all changes from one phase
to another. The initiator can request a Message Out phase by asserting ATN, while the target can cause the
Bus Free phase by releasing MSG, C/D, I/O, and BSY.
Signal
MSG C/D I/O
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1
Key: 0 = False, 1 = True, * = Reserved
The information transfer phases use one or more REQ/ACK handshakes to control the information transfer.
Each REQ/ACK handshake allows the transfer of one byte of information. During the information transfer
phases BSY shall remain true and SEL shall remain false. Additionally, during the information transfer phases,
the target shall continuously envelope the REQ/ACK handshake(s) with C/D, I/O, and MSG in such a manner
that these control signals are valid for a bus settle delay before the assertion of REQ of the first handshake
and remain valid until the negation of ACK at the end of the last handshake.
Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E
Table 3.1.5-1. Information transfer phases
Phase Name Direction of Transfer
DATA OUT Initiator to target
DATA IN Initiator from target
COMMAND Initiator to target
STATUS Initiator from target
*
*
MESSAGE OUT Initiator to Target
MESSAGE IN Initiator from Target
Comment
Data
Phase
Message
Phase