Information Transfer Phases; Asynchronous Information Transfer - Seagate SCSI Interface Product Manual

SCSI Interface Product Manual, Rev. J
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
contents 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 2). 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.
Table 2: Information transfer phases
Signal
MSG C/D I/O Phase name
0 0 0 Data out
0 0 1 Data In
0 1 0 Command
0 1 1 Status
1 0 0 *
1 0 1 *
1 1 0 Message Out
1 1 1 Message In
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.
3.1.5.1

Asynchronous information transfer

The target shall control the direction of information transfer by means of the I/O signal. When I/O is true, infor-
mation shall be transferred from the target to the initiator. When I/O is false, information shall be transferred
from the initiator to the target.
If I/O is true (transfer to the initiator), the target shall first drive DB(7-0,P)* to their desired values, delay at least
one deskew delay plus a cable skew delay, then assert REQ. DB(7-0,P)* shall remain valid until ACK is true at
the target. The initiator shall read DB(7-0,P)* after REQ is true, then signal its acceptance of the data by
asserting ACK. When ACK becomes true at the target, the target may change or release DB(7-0, P)* and shall
negate REQ. After REQ is false the initiator shall negate ACK. After ACK is false, the target may continue the
transfer by driving DB(7-0,P)* and asserting REQ, as described above.
If I/O is false (transfer to the target) the target shall request information by asserting REQ. The initiator shall
drive DB(7-0,P)* to their desired values, delay at least one deskew delay plus a cable skew delay and assert
ACK. The initiator shall continue to drive the DB(7-0,P)* until REQ is false. When ACK becomes true at the tar-
get, the target shall read DB(7-0,P)*, then negate REQ. When REQ becomes false at the initiator, the initiator
may change or release DB(7-0,P)* and shall negate ACK. The target may continue the transfer by asserting
REQ, as described above.
And where applicable, DB(15-8, P1).
*
Direction of transfer
Initiator to target
Initiator from target
Initiator to target
Initiator from target
Initiator to target
Initiator from target
Comment
Data
Phase
Message
Phase
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