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A-61124 March 1999 5-19
Read command
Command op code: 28H
SCSI Specification: Section 14.2.5
Command block descriptor
Bit
Byte
76543210
0 Operation Code (28H)
1 Logical Unit Number Reserved RelAdr
2 Transfer Data Type
3 Reserved
4 (MSB) Transfer Identification
5 (LSB)
6 (MSB) Transfer Length
7
8 (LSB)
9 Vendor Unique Reserved Flag Link
Read data
Descriptor Data Definition
Logical Unit
Number
= 0 Only one logical unit
RelAdr = 0 Not supported
Transfer Type = 00
= 80
= 81
= 82
Image Data (raster data)
Scanner-unique command
Header
Compound Image (header, raster)
Transfer ID = 0
=
Scanner Unique
Not used when Transfer Type is 00, 81, or 82
When Transfer Type is 80, this field is used for
a scanner-unique command.
Command
Transfer Length
= length Maximum number of blocks (block size = 1
byte) to transfer.
NOTE: If Transfer Type = 80, use 128
*
If the quantity of data is less than the transfer length blocks,
Check Condition Status is returned. Incorrect Length Indicator
(ILI) will be returned to the Request Sense that follows.
When performing continuous read commands, if a Check
Condition returns an indication the buffer is empty (sense key = B,
sense code = 80, and sense qualifier = 02), delay subsequent
read commands by at least 100 milliseconds.
*
For scanner-unique commands, a transfer length of 128 is recommended. The
command string is filled with nulls to a length of 128, ensuring the host
receives the data without generating a Check Condition for incorrect length.