Parallel SCSI - Parallel SCSI Bus Operation

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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