Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!samsung!rex!uflorida!travis!garyb
From: garyb@SSD.CSD.HARRIS.COM (Gary Barton)
Newsgroups: comp.periphs.scsi
Subject: Re: SCSI controller chip
Keywords: MB87035
Message-ID: <2106@travis.csd.harris.com>
Date: 21 Jan 91 20:58:53 GMT
References: <340@iphase.UUCP>
Sender: news@travis.csd.harris.com
Distribution: comp.periphs
Organization: Harris Computers Systems Division, Fort Lauderdale,FL
Lines: 48

In article <340@iphase.UUCP> uunet!iphase!phan writes:
...
>
>I spoke to Fujitsu, they said that with the timings in SCSI-I spec,
>particular the deskew delay (45ns), assertion period (90ns), negation
>period (90ns), data hold time (45ns)... there is no way that one can run
>at 5MB/s and fully comply with the spec.???
>

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)

Adding these up, we see a minimum period for each transfer is 235 ns
not including any overheads required to actually drive the signals.
This works out to be approximately 4.25 Mbytes/sec, and this is an
absolute limit.  Due to the afforemention overheads, I expect that
anyone transferring above 4 Mbytes/sec is actually shortening these
minimum periods.  For example, the WD33C93 can transfer at 5
Mbytes/sec, but to acheive this data rate, the system designer must
program the chip to use assertion/negation periods of 75 ns instead of
the required 90 ns.  This is clearly documented in the WD manuals.

That said, there will usually not be a problem at the higher rates
unless you are using a long cable and lots of target devices.
However, don't expect much help from your peripheral vendor if you
want to run one of the maximal configurations at 5 Mbytes/sec.

--
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| Gary Barton                    | System Software Development      |
| Lead Engineer			 | Harris Computer Systems Division |
| garyb@hcx1.csd.ssd.harris.com  | 2101 W. Cypress Creek Rd.        |
| gbarton@ssd.harris.com         | Ft. Lauderdale. FL 33309         |
| uunet!hcx1!garyb               | (305) 974-1700                   |
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