Parallel SCSI Bus Operation
The parallel SCSI bus goes through eight possible phases as a command is processed. Not all phases will occur in all cases:
| Phase | Comments |
|---|---|
| Bus-free | This is the state in which no device communication is in process. |
| Arbitration | One or more devices attempt to obtain exclusive control of the bus by asserting /BSY and a single bit corresponding to the device SCSI ID. For example, a device with a SCSI ID of 2 would generate the inverted bit pattern 11111011 on the bus. |
| Selection | The arbitrating device with the highest ID takes control of the bus by asserting /BSY and /SEL. "Highest" on an eight bit bus starts from 7 and works downward to zero. On a 16 bit bus, the eight bit rule applies, followed by 15 and working downward to 8, thus maintaining backward compatibility on a bus with a mix of eight and 16 bit devices. The controlling device is now the "initiator." |
| Command | The initiator sends the command descriptor block (CDB) to a "target," which is another device on the bus. The CDB tells the target what to do. |
| Reselection | During a transaction, the target device may be required to execute an operation (e.g., winding or rewinding the tape in a tape drive) that is slow in wall clock time terms relative to the speed of the bus. In such a case, the target may temporarily disconnect from the bus, causing the latter to go to the bus-free condition and allowing other unrelated operations to take place. Reselection is the phase where the target reconnects to the initiator to resume the previously suspended transaction. |
| Data | In this phase, data is transferred between initiator and target, the direction of transfer depending on the command that was issued. For example, a command to read a sector from a disk would result in a transfer from the disk to the host. Or, if an error occurred, the initiator could send a "request sense" command to the target for details, the latter which would be returned during the data phase. |
| Message | A message code is exchanged between initiator and target for the purposes of interface management. |
| Status | A status code is sent to the initiator to report the success or failure of the operation. |
The above list does not imply a specific sequence of events. Following a command to a target to send data to the initiator and a receipt of a command complete status, the initiator could send another command or even send a message.
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