Question for SCSI folks

Faisal Haq fhaq at
Tue Oct 31 11:54:05 PST 1995

I have a few questions about SCSI Async. REQ/ACK handshake.
I'm trying to understand what timings are typically seen out
there, since the SCSI spec itself doesn't quantify any
delays for async. protocol. (I apologize if this question is
terribly basic, but I can't seem to find one answer....)

  ------------+                    +------------------+
   REQ        |                    |                  |
              +--------------------+                  +---------

  ----------------------+                    +------------------
   ACK                  |                    |

                   a          b          c        d

There are 4 phases to a REQ-ACK async handshake. 

1) What are min/max values for (a and c) out there, today? Is
   an assumption of 55ns minimum, for each of these correct? 

2) As a target, is it my responsiblity to ensure that (a+b+c+d)
   is always a fixed number? If so, what is that number? I've heard
   175ns (5.7MB/s), 200ns (5MB/s) among others. Is there a min,max range
   to follow? In any case, this means I would have to shorten (b and d) 
   to compensate for really slow hosts.

3) Is there a minimum assertion/deassertion time for req, in
   addition to maintaining proper handshake protocol? I've heard
   25ns for assertion and 75ns for deassertion. Is this true?

4) Is there ANY data hold time requirement for targets, past the leading 
   edge of ACK.

Thanks for your help.

  F A I S A L   H A Q
  Quantum Corporation, ASIC Division
  Email: fhaq at

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