Viper 60s Theory Of Operation And Maintenance Manual page 46

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Table 4-8. COB Fields Common to Group 0 and 1 Commands
COMMAND DESCRIPTOR
BLOCK FIELD
Group Code
1
Command Code
1
Logical Unit
Number2
Flag bit
Link bit
3
DESCRIPTION
This field indicates which of eight possible SCSI
command groups is specified. Viper drive supports
Group O.
This field indicates which of 32 possible command
codes for a particular group code is specified.
The LUN must be set to zero.
When set, these bits select vendor unique functions in
specified commands.
The Flag bit is used only in conjunction with the Link
bit and must be set to zero if the Link bit is zero. When
the Link bit is set, the value of the Flag bit determines
the appropriate message to send to the Initiator when
a linked command completes successfully; value zero
indicates the Linked Command Complete message is
required, value one indicates the Linked Command
Complete with Flag message is required. Typically,
the Flag bit is used to cause an interrupt in the Initiator
at the end of or at logical intervals in linked command
processing.
The Link bit is used to indicate that the Initiator desires
automatic linking to the next command upon success-
ful completion of the current command. When the Link
bit is one, upon successful termination of the com-
mand, Viper drive returns Intermediate Status followed
by one of the two Command Complete messages as
determined by the Flag bit. (Refer to description of
Flag bit above).
1. Together, group code and command code make up the op code.
2. If a non-zero logical unit number is received in a COB, a Check Condition status is
returned and Extended Sense Key is set to Illegal Request.
3. If the Link bit is used, all applicable commands must have the Immed bit set
a Check Condition status is returned and Extended Sense Key is set to
Request.
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