Emerson MVME147 manual Scsi Packets

7

SCSI PACKETS

Some SCSI controllers return a "check" status on the first command sent to them after power-up or reset. The SCSI firmware retries the command if the controller returns "unit attention" request sense information and if you put a number that was one or greater into the retry field in the attach packet. Otherwise, the request-sense data is returned to you. You can then retry the command.

The first table below shows the details of an attach/detach packet for disk. The second table shows the details of a attach packet for streaming tape. A third table details the 384-byte ($180-byte) work area specified in the attach packet and used for all subsequent commands. The work area is normally not examined by you except when the "additional status" bit is set (refer to the packet status codes in Chapter 3). On most operations involving data transfer (except scatter/gather) in which DMA is used, the "sector number in error", "transfer address", and "command error word" can be used to assist error handling.

New Packet

Even Byte \

Odd Byte \

	FC		B8		74		30
+$00		Controller LUN				Device LUN
+$02		Status Byte 0				Status Byte 1 (Note 1)
+$04		0		0		Step Rate
+$06		0		0		Number of Heads
+$08				Number of Cylinders
+$0A			Precompensation Cylinder
+$0C			Logical Sectors per Track
+$0E		0		0		0		0
+$10				SCSI Disk Attributes
+$12		Controller Type				Drive Type
+$14		0		0		Function Code (78)
+$16		Interrupt Level				Vector Number
+$18		Status Byte 2				Status Byte 3 (Note 1)
+$1A		0		0		Retry Count
+$1C			Physical Bytes per Block

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