clemmey at apollo.hp.com clemmey at apollo.hp.com
Wed Mar 23 06:54:08 PST 1994

2 Questions Concerning SCSI-2 Phase Timing.

I looked through the SCSI spec and tried to find the timings for these
two questions and could not find definitive answers. If you could 
point me to where these timings are listed in the SCSI-2 spec I would 
appreciate it. If you have the answers that would be nice also.

Thanx in advance.

Bill Clemmey
Hewlett Packard
clemmey at apollo.hp.com

Bus has one inititator at ID 7, Drive A at ID 2 and Drive B at ID 4.

	1) Initiator does selection for Drive A at ID 2.

	2) Target 2 and Target 4 start to check if they are being selected
		by checking SEL line and if its SCSI ID are True
	3) Target 2 senses selection and asserts BSY while Target 4 is still
		checking if it was selected.
	4) Target 2 continues with Information transfer Phases and finishes
	5) The Bus goes Bus Free
	6) Next command is for Drive B at ID 4.
	7) Initiator does selection for Drive B at ID 4.
	8) Target 4 still attempting to detect SEL from Step 1 so this 
		selection is ignored. 
	9) Selection Timeout after 250 ms.
Question 1: Is there a maximum time for a device to check if it has been 
		selected. Above in step 2 and 3, Target 2 determined it
		had been selected. How long should Target 4 continue to
		check if it has been selected?
Question 2: What is the minimum time between a device asserting BSY once
		it determines it was selected, Step 3 above, and the bus
		returning to Bus Free, Step 5 above? 

My issue is this. The newer disk drives have a fast turn around time due
to the state machines for selection. A slower device has a slow selection
determination process. My initiator is attempting to select the slow device
while the slow device is still considering the last selection process. 

Timings known:
	Selection Abort: MAX time of target from detection of being selected
			until asserting BSY.	200 usecs.  
			Step 2 to Step 3
	Selection Timeout: MIN time for device to wait for selection before
			starting timeout procedure.   250 ms.

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