4.3.5INFORMATION 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 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 4.5 INFORMATION TRANSFER phase type
Phase | C/D | I/O | MSG | DB7 to 0, P | Transfer direction | ||||||
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DATA OUT | 0 | 0 | 0 | Data | INIT |
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DATA IN | 0 | 1 | 0 | Data | INIT |
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COMMAND | 1 | 0 | 0 | Command | INIT |
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STATUS | 1 | 1 | 0 | Status | INIT |
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* | 0 | 0 | 1 |
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* | 0 | 1 | 1 |
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MESSAGE OUT | 1 | 0 | 1 | Message | INIT |
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MESSAGE IN | 1 | 1 | 1 | Message | INIT |
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* : Reserved for future standardization
0:False
1 True INIT: Initiator TARG: Target
4 ± 17