Path: utzoo!attcan!uunet!wuarchive!brutus.cs.uiuc.edu!apple!mattd
From: mattd@Apple.COM (Matt Deatherage)
Newsgroups: comp.sys.apple
Subject: Re: Homebrew SCSI pitfalls
Summary: Not quite.
Keywords: SCSI Termination
Message-ID: <34115@apple.Apple.COM>
Date: 18 Aug 89 18:59:42 GMT
References: <1431@esunix.UUCP>
Organization: Apple Computer Inc, Cupertino, CA
Lines: 103

In article <1431@esunix.UUCP> jsnow@esunix.UUCP (John Snow) writes:

>Here is a word of warning to those of you building homebrew SCSI drives
>for the Apple II family.  APPLE VIOLATES THE SCSI SPECIFICATIONS for
>terminating the    SCSI bus!  The SCSI spec requires that the SCSI bus
>be terminated by 220/330
>resistors at BOTH ends of the bus.  The Apple II SCSI interface card does
>NOT terminate the cable.  Apple normally gets away with this violation by
>supplying a very short SCSI cable to connect the SCSI interface card to the
>first disk drive where the SCSI cable IS terminated.  This is poor design
>practice but normally works.
>
Nope.  Not quite.  Persuasive, but wrong.  Let's quote, shall we:

ANSI X3.131-1986, ("SCSI 1"), p.22, section 4.4:

"Terminator power (optional):  Single-ended SCSI devices providing terminator
power shall have the following characteristics:"

Ibid, p.23, section 4.4.3:

"This standard contains various alternatives that are mutually exclusive within
a system:

1)  Single-ended or differential drivers
2)  Termination power supplied by the cable or not
...
"
April 24, 1989 from Sun Microsystems, # X3T9.2/89-48 Rev. 0 p2,sect.3

"It is recommended that terminator power be supplied at a minimum of 4.5 volts
and that it be supplied by more than one device on the cable preferrably by a 
device on each end of the cable."

p4-21, SCSI 2 Working Draft, rev. 9:

"Any SCSI device may supply terminator power.  Interference error rates are
lower if the termination voltage is maintained at the extreme ends of the
cable."

What these things mean is this.

Terminator power is *recommended* to be added to the SCSI-2 *draft* as being
*recommended* to be placed on both ends of the cable.  The standard requires
only one terminator.

Any device may or may not supply terminator power.  At the host end of the
SCSI system (the Apple II SCSI Card in this case), it is completely optional.
Since terminator power requires 4 to 5 volts at 800 mA, which is significantly
more than a card should be drawing from the Apple II power supply, Apple chose
to terminate in the first device of the chain instead.  This is fully within
the existing and draft specifications.

Interference is lower if the terminators are at the extreme ends of the cable.
This means that if you have a long cable between the host and the terminator,
like this:

>I purchased a SCSI cable from Tulin along with a Tulin A-hive case and power
>supply.  The SCSI cable Tulin sent me was 6 feet long -- much too long to
>work when there is no termination at one end.  Sure enough, I spent a week
>trying to figure out whether my drive or my interface card was bad because
>things just weren't working.  After a really frustrating week of trying
>different computers, drives, and interface cards and hooking a SCSI bus
>analyzer up to my system, I decided that the long cable and lack of
>termination was the problem.  I hacked the 6 foot cable down to 18 inches
>and wired on a new connector and what do you know, it works!
>
that you're going to have lots of interference - enough in your case to make it
not work.

>What would it have cost Apple to terminate things properly, less than a buck
>to put in the resistor packs and sockets to allow them to be removed if the
>CPU was not at the end of the cable!
>
What it would cost to provide terminator power on the host card is a *big*
drain on the power supply, which is easily avoided by using termination on the
first device.  The devices plug into the wall and can use as much terminator
power as they need; the card draws from the IIe power supply and has to
share itself with the rest of the system (including your modems, memory cards,
accelerators, video overlay cards, and other things).  The short cable between
the card and the first device does help reduce interference, as you found out.
But Apple is *not* violating the SCSI spec and is trying to keep those folks
out there who load down their systems from blowing up the power supply. :)

I'd like to know where you got all this information about how evil Apple's been
as far as SCSI concerns, because the ANSI SCSI spec. and the draft spec.
certainly indicate quite a different story.
>
>-- 
>-----------------------------------------------------------------------------
>John F. Snow                         UUCP: {ihnp4,decvax}!decwrl!esunix!jsnow 
>Evans & Sutherland Computer Corp.      or:         uunet!utah-cs!esunix!jsnow
>Salt Lake City, Utah                 AppleLink PE: JohnSnow    GEnie: J.SNOW2

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