Seagate Ultra 320 manual Information transfer phases, Signal Phase Direction of transfer Comment

3.5Information transfer phases

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 21). The target drives these three signals and therefore controls all changes from one phase to another. The initiator requests a MESSAGE OUT phase by creating an attention condition, The target causes the BUS FREE phase by releasing MSG, C/D, I/O, and BSY signals.

Table 21:		Information transfer phases
	Signal
	MS
C/D	G		I/O	Phase	Direction of transfer	Comment
0	0		0	ST DATA OUT	Initiator to target	ST DATA
0	0		1	ST DATA IN	Initiator from target	Phase	DATA phase
0	1		0	DT DATA OUT	Initiator to target	DT DATA
						phase
0	1		1	DT DATA IN	Initiator from target
1	0		0	COMMAND	Initiator to target
1	0		1	STATUS	Initiator from target
1	1		0	MESSAGE OUT	Initiator to target	MESSAGE
1	1		1	MESSAGE IN	Initiator from target	Phase
Key:	0 = False, 1 = True

The information transfer phases use one or more REQ/ACK handshakes to control the information transfer. Each REQ/ACK handshake allows the transfer of 8- or 16-bits of information depending on the negotiated transfer width (see Section 4.3.18). During the information transfer phases the BSY signal shall remain true and the SEL signal shall remain false. Additionally, during the information transfer phases, the target shall con- tinuously envelope the REQ/ACK handshakes with the C/D, I/O, and MSG signals in such a manner that these control signals are valid for one bus settle delay before the assertion of the REQ signal of the first handshake and remain valid until after the negation of the ACK signal at the end of the handshake of the last transfer of the phase.

The SCSI target port shall not transition into an information transfer phase unless the REQ/ACK signals are negated. The target shall not transition from an information transfer phase into another information transfer phase unless the REQ and ACK signals are negated.

Note. After the negation of the ACK signal of the last transfer of the phase, the target may prepare for a new phase by asserting or negating the C/D, I/O, and MSG signals. These signals may be changed together or individually. They may be changed in any order and may be changed more than once. It is desirable that each line change only once. A new phase does not begin until the REQ signal is asserted for the first byte of the new phase.

Note. A phase is defined as ending when the C/D, I/O, or MSG signals change after the negation of the ACK signal. The time between the end of a phase and the assertion of the REQ signal beginning a new phase is undefined.

There are three methods of transferring data using information transfers:

•Asynchronous transfers

•Synchronous transfers

•Paced transfers

60 Parallel SCSI Interface Product Manual, Rev. A