To:            X3T9.2 SPI                X3T9.2/92-138R1

        From:          Bill Ham (DEC)

        Subject:       Single SCSI cable for all forms of SCSI 
                       (except 32 bit)

        Date:          June 23, 1992


        PURPOSE:


                Wording that ensures that all SCSI-3 external cables are 
compatible with all forms of SCSI (except 32 bit) is proposed.


        BACKGROUND:

                Discussions at the last working group led to the 
agreement that there should be no reason why the P cable could not be 
used for all forms of SCSI, except 32 bit, if the cables were wired for 
use in a single ended application.  The main issue was whether the
placement of the REQ and ACK lines within the cable in the differential
mode was important.  It was decided that there is no need for
restrictions on the placement of specific signals within the cable for
differential usage. 

                Many of the pins have the same function in differential 
as in single ended.  Term power especially was checked to ensure that 
any special treatment for term power within the cable would be preserved 
between the single ended and differential usages.  (There is no 
requirement for special treatment for term power lines in SCSI-3).

                Many other pins have different treatment between the two 
usages.  Most notably, the ground pins for all of the signal pairs in 
the single ended usage become active data lines in differential.  Also 
the pinout for the clock signals REQ and ACK is different as are many of 
the data and control lines.

                This leads to the conclusion that ALL SCSI P cables 
should be wired as if they were to be used in a single ended 
application.  If this is done the sensitive single ended functions will 
operate properly as will the less sensitive differential functions.





        PROPOSED WORDING:

        All shielded, round, twisted pair SCSI-3 cables having a core of 
two or more pairs and surrounded by two or more layers of twisted pairs
shall be wired according to the rules below (even if the cable will be
used for differential applications).  These rules apply to both A and P 
cables.

        Rule 1:

                The single ended REQ connector pins (including 
                both the signal and ground sides) shall be connected    
                to a twisted pair in the cable core -- similarly the 
                single ended ACK connector pins (including both the
                signal and ground sides) shall be connected to a
                different twisted pair in the cable core 

        Rule 2:

                All single ended data and parity connector pins and 
                their respective ground pins shall be connected to
                twisted pairs in the outer layer of the cable (nearest
                the external shield) 

        Rule 3:

                All single ended ground connector pins shall be
                connected to the non-signal side of their respective
                twisted pairs 

        Rule 4:

                No lines (except term power) may be internally connected
                together within the connectors or cables