7.6 Bus Phases
7.6.6 COMMAND phase
The COMMAND phase is a bus phase in which the TARG requests the INIT to transfer command information (CDB) to the TARG. The TARG keeps the C/D signal true and the I/O and MSG signals false during REQ/ACK handshaking in this phase.
7.6.7 DATA phase
The DATA phase is divided into DATA IN and DATA OUT phases according to the direction of data transfer. In a DATA phase, synchronous data transfer can be performed.
(1) DATA IN phase
In a DATA IN phase, the TARG requests to transfer data from the TARG to the INIT. The TARG keeps the I/O signal true and the C/D and MSG signals false during REQ/ACK handshaking in this phase.
(2) DATA OUT phase
In a DATA OUT phase, the TARG requests to transfer data from the INIT to the TARG. The TARG keeps the C/D, I/O, and MSG signals false during REQ/ACK handshaking in this phase.
(3) Data transfer rate in asynchronous mode
Figure 7.16 shows the REQ/ACK handshake timing that the ODD observes in a DATA phase where asynchronous transfer is used. The data transfer rate on the SCSI bus depends on sum of the ACK response time in the INIT and the signal delay time of the interface cable (T1 and T2 in Figure 7.16).
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