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4.4.1 RESERVE 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
1 BUS FREE Verifies bus free
2 ARBITRATION Obtains bus>usage
right
3 SELECTION Selects target Æ
Drives BSY signal
4 MESSAGE OUT Selects logical unit Æ
5 COMMAND Specifies
RESERVE UNIT
(CDB)
Æ
6 STATUS ¨ Reports GOOD status
7 MESSAGE IN ¨ Reports message (Command
Complete)
Releases BSY signal
8 BUS FREE
(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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