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A.3 Bus Phases
Appendix A-15
A.3.5 INFORMATION TRANSFER phases
Note:
The COMMAND, DATA, STATUS, and MESSAGE phases are all grouped together as the
information transfer phases because they are all 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 A.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.
* : Reserved for future standardization
0: False
1 True
INIT: Initiator
TARG: Target
Phase C/D I/O MSG DB7 to 0, P Transfer direction
DATA OUT 0 0 0 Data INIT
TARG
DATA IN 0 1 0 Data INIT
TARG
COMMAND 1 0 0 Command INIT
TARG
STATUS 1 1 0 Status INIT
TARG
* 0 0 1 -
* 0 1 1 -
MESSAGE OUT 1 0 1 Message INIT
TARG
MESSAGE IN 1 1 1 Message INIT
TARG
Table A.5 INFORMATION TRANSFER phase type