Fast-20 HVD timing

Mike Ham mham at
Mon Aug 25 17:53:49 PDT 1997

* From the T10 (formerly SCSI) Reflector (t10 at, posted by:
* mham at (Mike Ham)
I have some questions about the Fast-20 timing requirements as they apply to an
HVD subsystem.  I am going through the signal integrity and timing margin
portion of a design and am confused as to the timing requirements for REQ and
ACK asserted and negated on the HVD interface.

For data setup and hold, the HVD setup and hold was made to match the SE setup
and hold by reducing the host adapter setup and hold requirements when
operating in an HVD environment.  This compensates for the 5ns part to part
delay scew of the HVD tranceivers.

For REQ and ACK asserted or negated, it doesn't appear that this has been done.
 The HVD tranceivers have a 5ns duty cycle skew.  Since the only note
associated with the REQ and ACK asserted or negated timing refers to the single
ended threshhold requirements, it appears that if you are measuring these
timings at the HVD side of the tranceiver, you must add 5ns to the Fast-20
timing requirement to be in spec.  Is this true?  Thanks for any help you can

Mike Ham
mham at
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