Path: utzoo!attcan!uunet!cs.utexas.edu!sdd.hp.com!zaphod.mps.ohio-state.edu!ncar!csn!arrayb!wicklund
From: wicklund@arrayb.uucp (Tom Wicklund)
Newsgroups: comp.periphs.scsi
Subject: Re: SCSI controller chip
Keywords: MB87035
Message-ID: <1991Jan24.205407.26048@arrayb.uucp>
Date: 24 Jan 91 20:54:07 GMT
References: <340@iphase.UUCP> <2106@travis.csd.harris.com>
Distribution: comp.periphs
Organization: Intellistor
Lines: 65

In <2106@travis.csd.harris.com> garyb@SSD.CSD.HARRIS.COM (Gary Barton) writes:

>Assumption: Negotiated sync transfer period is not greater than
>            minimum requirements as described in the standard (235 ns).
>            If it is, more waiting is required, because one of the two
>            end points is to slow to keep up with the max.

>From the SCSI spec, sync xfers (target to init) must go like this:
>        1) Target drives data lines
>        2) Target waits deskew delay (45ns) + cable skew delay (10ns)
>        3) Target asserts REQ, and holds REQ high for one min
>           assertion period (90ns).  NOTE: Target can release data
>           lines 45ns after asserting REQ.
>        4) Target negates REQ, and holds low for one min negation
>           period (90ns)
>        5) if offset < negotiated offset
>               goto 1)


Where do you get 235ns?  I assume this is the sum of 55, 90, and
90 in items 2, 3, and 4 above.  However, #1 and 2 above can
overlap the target negating ACK.

The actual sequence is (note this is DATA OUT phase only):

        1) Target drives data lines
        2) Target waits deskew delay (45ns) + cable skew delay (10ns)
        3) Target waits until REQ negated for one min negation period
           (90ns) or one transfer period after previous assertion
           of REQ (200ns).
        4) Target asserts REQ, and holds REQ high for one min
           assertion period (90ns).  NOTE: Target can release data
           lines 45ns after asserting REQ.
        5) if offset < negotiated offset
               goto 1)

A synchronous transfer will include 55ns overhead at the start of
the transfer while the data lines are initially driven.  After
this the 55ns setup time for data can overlap the 90ns REQ
negation time.


SCSI requires the following timing (DATA IN example):
				  

     |--------90ns----------------|----------- 90ns -----------|
    _                              ____________________________
REQ  |____________________________|                            |_____
                                  |
                                  |
                   |              |
                   |--- 55 ns ----|----- 45 ns ---|
                   |______________________________
DATA --------------<______________________________>------------------


The minimum requirement is that REQ be asserted 90ns and
deasserted 90ns.  Technically this should allow a 180ns transfer
period (I can't find anything in SCSI-1 specifying a minimum
transfer period in a few minutes looking), though 200ns is
traditionally considered the minimum period.

It is true that some chips like the WD33C93 transfer 5MB/s using
an assertion / negation period of less than 90ns.  This is due to
poor chip design, not anything required by the SCSI standard.