4-15
4.3.4
INFORMATION TRANSFER phases
Note:
The COMMAND, DATA, STATUS, and MESSAGE phases are all grouped together as the information transfer phases
because they are a]l used to transfer data or control information via the DATA BUS The actual content 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 4.5). 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 the ATN signal, while the target can cause the
BUS FREE phase by releasing the MSG, C/D, I/O, and BSY signals.
Table 4-6 INFORMATION TRANSFER phase type
Phase
C/D
I/O
MSG
DB7 to 9,P
Transfer
direction
DATA OUT
0
0
0
Data
INIT
⇒
TARG
DATAIN
0
1
0
Data
⇐
COMMAND
I
0
0
Command
⇒
STATUS
I
I
0
Status
⇐
*
0
0
1
*
0
1
I
MESSAGE OUT
1
0
1
Message
⇒
MESSAGEIN
1
1
1
Message
⇐
*: Reserved for future standardization
0:
False
1:
True
INIT:
Initiator
TARG: Target
