4.4.1RESERVE UNIT command
The following table shows the normal sequence of the RESERVE UNIT command when used with this scanner.
Step | Bus phase | Initiator operation | ¨ Æ | Target operation |
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1 | BUS FREE | Verifies bus free |
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2 | ARBITRATION | Obtains bus>usage |
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| right |
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3 | SELECTION | Selects target | Æ |
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| Drives BSY signal |
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4 | MESSAGE OUT | Selects logical unit | Æ |
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5 | COMMAND | Specifies | Æ |
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| RESERVE UNIT |
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| (CDB) |
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6 | STATUS |
| ¨ | Reports GOOD status |
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7 | MESSAGE IN |
| ¨ | Reports message (Command |
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| Complete) |
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| Releases BSY signal |
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8 | BUS FREE |
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(1)RESERVE UNIT command: COMMAND phase (initiator Æ target)
Where a logical unit can be accessed by two or more initiators, there could be interferences with command sequences, data, etc. This situation can be avoided by issuing the RESERVE UNIT command before initiating a series of operations.
Once a logical unit has properly accepted the RESERVE UNIT command, it will be occupied by the initiator that issued the RESERVE UNIT command. If the 3rd party reservation option is supported, the logical unit might be occupied by another SCSI unit % one having an initiator function % which is specified TPID. In this condition, called ™reserved, the logical unit cannot be accessed from any other initiators. The reserved condition remains effective until one of the following events take place:
—The reservation is replaced by a new RESERVE COMMAND from the same initiator that has reserved the logical unit. (Issuing another RESERVE UNIT command with the reservation still effective does not results in an error. The previously established reservation is released as a result of “, ” or ‘ described below.)
“The RELEASE UNIT command is issued from the same initiator that has reserved the logical unit.
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