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.
Read more about this topic: Parallel SCSI
Famous quotes containing the words parallel, bus and/or operation:
“As I look at the human story I see two stories. They run parallel and never meet. One is of people who live, as they can or must, the events that arrive; the other is of people who live, as they intend, the events they create.”
—Margaret Anderson (1886–1973)
“I’d take the bus downtown with my mother, and the big thing was to sit at the counter and get an orange drink and a tuna sandwich on toast. I thought I was living large!... When I was at the Ritz with the publisher a few months ago, I did think, “Oh my God, I’m in the Ritz tearoom.” ... The person who was so happy to sit at the Woolworths counter is now sitting at the Ritz, listening to the harp, and wondering what tea to order.... [ellipsis in source] Am I awake?”
—Connie Porter (b. 1959)
“Waiting for the race to become official, he began to feel as if he had as much effect on the final outcome of the operation as a single piece of a jumbo jigsaw puzzle has to its predetermined final design. Only the addition of the missing fragments of the puzzle would reveal if the picture was as he guessed it would be.”
—Stanley Kubrick (b. 1928)