Lohmeyer, John JLOHMEYE at
Wed Sep 25 16:52:00 PDT 1996

* From the SCSI Reflector (scsi at, posted by:

Bill Ham pointed out to me that I gave a rather short answer to a   
question that deserves a longer answer.  Here is Bill's answer:

  "        You have made a point here that is only half true.  The   
  flowing in the transmission line is what pulls the signal up with a
  passive negation driver and this happens exactly when the driver turns
  off -- one does not need to wait for the terminator to supply this
  current.  This current was already there (because of the assertion that   

  had to preceed the negation) and it does come from the terminators but
  the time involved only is an issue when the initial current is
  insufficient to make the voltage step (I x Zo) needed. In that case one
  has to wait for the terminator to send MORE current and that does take

          This takes a more complex explanation for complete   
  but the bottom line is that the only reason for using active negation
  drivers is that the available current (and/or the available   
  line impedance) is too low. "

So, if you have good terminators and a good cable, you may be able to use   
passive negation drivers at Fast-20 speeds.  However, you will not be   
compliant with the Fast-20 standard...


PS:  Thanks for the more complete answer, Bill!
John Lohmeyer                 E-Mail: john.lohmeyer at
Symbios Logic Inc.             Voice: 719-533-7560
4420 ArrowsWest Dr.              Fax: 719-533-7036
Colo Spgs, CO 80907-3444    SCSI BBS: 719-533-7950 300--14400 baud

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