X3T9.2/87-148
        To: X3T9.2 Committee                         August 25, 1987
        From: Jim Schuessler, National Semiconductor
        Subject: Twisted Pair Cable Specification and Fast SCSI changes
        
        National has tested and verified Fast SCSI  operation  using
        the  26AWG  twisted  pair  cable  specified  in IPI-Physical
        sec.2.3.3.  By extrapolation, the 28AWG cable  specified  in
        sec.2.3.2  will also work.   For reliable Fast SCSI systems,
        some further cable specification is needed  beyond  that  in
        the current draft.  The IPI specs. seem to have been arrived
        at from cable manufacturer input on specific cable, the SCSI
        specification need not be so restrictive.
        
        Please consider the following sections for inclusion in  the
        SCSI-2 draft. (Underlined text within paragraphs is new. 
        Text in square brackets is removed.)
        
        4.2 Cable Requirements
        
        An ideal impedance match with cable  terminators  implies  a
        cable  characteristic  impedance  of  132 ohms (single-ended
        option) or 122  ohms  (differential  option).   In  general,
        cables with this high of a characteristic impedance are hard
        to obtain; however, impedances that are somewhat  lower  are
        satisfactory.   A  characteristic impedance of 100 ohms plus
        or minus 10% is recommended for unshielded flat  or  twisted
        pair  ribbon  cable.   [delete:  A  characteristic impedance
        greater than 90 ohms is preferred for shielded cables;  how-
        ever,  most  available  cables have a somewhat lower charac-
        teristic impedance.]  See sections on differential cable for
        characteristic  impedance requirements.  To minimize discon-
        tinuities  and  cable  reflections,  cables   of   different
        impedances  should not be used in the same bus.  Implementa-
        tions may require  trade-offs  in  shielding  effectiveness,
        cable  length, the number of loads, transfer rates, and cost
        to achieve satisfactory system operation.
        
        4.2.2 Differential Cable
        
        Add after last paragraph:
        
        4.2.2.1  28AWG Twisted Pair Cable
        
        Cables A and B shall consist of conductors  of  28AWG.   The
        cable  may  have an overall shield, suitable for termination
        in a metal shielded connector.  Such overall  shield  serves
        the  purpose  of  a  signal shield.  The two wires of a pair
        shall be connected to the same signal, one to  the  positive
        and the other to the negative signal.
        
        The cable shall have the following  electrical  characteristics:
        
        CHARACTERISTIC IMPEDANCE:  100 to 120 ohms +-10%
        
        SIGNAL ATTENUATION:      .087 db max/meter at 5MHz.
        
        PAIR TO PAIR PROPAGATION DELAY DELTA:  .20 nS/meter max.
        
        4.2.2.2  26AWG Twisted Pair Cable
        
        Cables A and B shall consist of conductors  of  26AWG.   The
        cable  may  have an overall shield, suitable for termination
        in a metal shielded connector.  Such overall  shield  serves
        the  purpose  of  a  signal shield.  The two wires of a pair
        shall be connected to the same signal, one to  the  positive
        and the other to the negative signal.
        
        The cable shall have the following  electrical  characteristics:
        
        CHARACTERISTIC IMPEDANCE:  100 to 120 ohms +-10%
        
        SIGNAL ATTENUATION:      .087 db max/meter at 5MHz.
        
        PAIR TO PAIR PROPAGATION DELAY DELTA:  .20 nS/meter max.
        
        Fast SCSI
        
        Add before  first  paragraph  in  sec.  5.1.6.2  Synchronous
        Transfer:
        
        5.1.6.2 Synchronous Data Transfer (Optional)
        
        Synchronous data transfer is optional, and uses several tim-
        ing  parameters  specified  in section 4.7 which have "Fast"
        counterparts.  Up to a 200nS  transfer  period,  the  normal
        Cable  Skew Delay, Deskew Delay, Hold Time, Assertion Period
        and Negation  Period  are  used.   When  the  transfer  rate
        exceeds  200nS, a linear curve shall be followed between the
        normal timing parameter and it's Fast counterpart. For exam-
        ple  if  a 150nS transfer period is desired, and the minimum
        Deskew Delay is needed, the result would be 32.5nS.
        
        Replace first line in existing first paragraph with:
        
        Synchronous transfers may only be used in the data phase  if
        previously agreed to by the initiator and target through the
        message system.....