| Table 4.2 Bus phases vs. signal drive sources (2/2) |
ID: | A unique data bit (the SCSI ID) shall be driven by each SCSI device that is |
| actively arbitrating. The other seven data bits shall be released (shall not |
| driven) by this SCSI device. The parity bit (DB(P)) may be released or |
| driven to the true state, but shall never be driven to the false state during |
| this phase. |
I&T: | The initiator and target drive the signal according to the interface operating |
| sequence. The RESELECTION phase includes a sequence in which the |
| initiator and target simultaneously drive the signal. |
| The signal shall be driven by the initiator, target, or both, as specified in the |
| SELECTION phase and RESELECTION phase. |
Table 4.3 Method of driving the interface signal
| OR connection | NON>OR connection | |
|
|
| |
False | No signal is driven by any SCSI | The signal is driven false by a certain | |
| device. Signal status is made false by | SCSI device (initiator or target), or is | |
| the termination resistor circuits. | not driven by any SCSI device. | |
|
|
| |
True | A SCSI device drives the signal true. |
| |
| |||
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|
4.3Bus Phases
The SCSI architecture includes the following eight distinct phases:
fBUS FREE phase
fARBITRATION phase
fSELECTION phase
fRESELECTION phase
f COMMAND phase
fDATA phase
f | STATUS phase | INFORMATION TRANSFER phase |
fMESSAGE phase
The SCSI bus can never be in more than one phase at any given time.
The following diagram shows how each phase transits to another.
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