To:	X3T9.2 Membership				X3T9.2/93-094r0
From:	Giles Frazier
Date:	12 May 93 
Subject: Setup and hold timings in SPI

At the April SCSI meeting there was discussion on SCSI-3 (slow)
setup and hold times. According to further investigations, it appears
that a single change to the SCSI-3 (slow) hold time would be
advantageous. I believe this change is compatible with present
implementations, and would guarantee interoperability between SCSI-2
and SCSI-3 (slow) devices.

This note first gives the relevant SCSI-2 and SCSI-3 timings and then
shows that only one specification change is needed for interoperability.
If you can, please look over the calculations, and let me know if you
have any comments.                                  Thank you.

                        RELEVANT TIMING VALUES

SCSI-2 (SLOW) TIMINGS (At cable connector, units = ns)
Transmitter must supply these times:
  SETUP TIME = DESKEW + CABLE SKEW        = 45   +    10 = 55
  HOLD  TIME = HOLD + DESKEW + CABLE SKEW = 45 + 45 + 10 = 100
Receiver guaranteed these times:
  SETUP TIME = 0
  HOLD TIME  = 45

SCSI-2 (FAST) TIMINGS (At cable connector, units = ns)
Transmitter must supply these times:
  SETUP TIME = DESKEW + CABLE SKEW        = 20   +    5  = 25
  HOLD  TIME = HOLD + DESKEW + CABLE SKEW = 10 + 20 + 5  = 35
Receiver guaranteed these times:
  SETUP TIME = 0
  HOLD TIME  = 10

SCSI-3 TIMINGS (At cable connector, units = ns)
Transmitter must supply these times:
  SETUP TIME =  23
  HOLD TIME  =  33
Receiver guaranteed these times:
  SETUP TIME =  15
  HOLD TIME  =  25

************************************************************************
                        INTEROPERATABILITY
ASSUMPTION:  SCSI-3 cables with 8 ns skew are being used.
------------------------------------------------------------------------

SCSI-2(SLOW) RECEIVING FROM SCSI-3
    Setup time seen by receiver = 23 - 8 = 15 ns   (OK--0 is required)
    Hold  time seen by receiver = 33 - 8 = 25 ns   (PROBLEM--45 IS REQUIRED)
The SCSI-2 receiver cannot be guaranteed to work since it may be expecting
a hold time of 45 ns.

SCSI-2(SLOW) TRANSMITTING TO SCSI-3(SLOW)
    Setup time seen by receiver = 55 - 8 = 47 ns   (OK--15 is required)
    Hold  time seen by receiver = 100 = 8 = 93 ns  (OK--25 is required)
------------------------------------------------------------------------
SCSI-2(FAST) RECEIVING FROM SCSI-3(FAST)
    Setup time seen by receiver = 23 - 8 = 15 ns   (OK--0 is required)
    Hold  time seen by receiver = 33 - 8 = 25 ns   (OK--10 is required)

SCSI-2(FAST) TRANSMITTING TO SCSI-3(FAST)
    Setup time seen by receiver = 25 - 8  = 17 ns  (OK--15 is required)
    Hold  time seen by receiver = 35 - 8  = 27 ns  (OK--25 is required)

                        PROPOSED CHANGE
From the above, it appears that interoperability is guaranteed in all
of the above cases except one--a SCSI-2(SLOW) device receiving from a
SCSI-3 device. This could be resolved by changing the SCSI-3 (SLOW)
hold time from 33 ns to 53 ns. This would make the hold time seen by the
SCSI-2 receiver = 45 ns.
Therefore, the ONLY change I am proposing is the following:

      Change SPI (rev. 12) Table 10, colum 2 (SLOW timings), row 17
      (Transmit Hold time) from 33 ns to 53 ns.

Comments: All the implementors who I have been able to reach have told me
          that their SCSI-3 transmitters are all being designed to exceed
          the necessary 53 ns hold time which is necessary to interoperate
          with SCSI-2 devices. No one has yet objected to making this
          change to the SPI document. Making the change, therefore,
          would seem to both bring the document into reality and
          give systems with SCSI-2 adapters a specification to rely on
          when assuming that they will be able to use SCSI-3 devices.

          The reason I am putting this on the reflector is to
          solicit additional input on my proposed change or to get
          corrections to my calculations. Thank you.

                                                Giles Frazier
                                                IBM Austin
                                                gfrazier@ausvm6.vnet.ibm.com
                                                (512) 838-1802
                                                (512) 838-1852