Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!rutgers!usc!elroy.jpl.nasa.gov!grian!alex
From: alex@grian.cps.altadena.ca.us (Alex Pournelle)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Mac SCSI Drive Tale of Woes
Message-ID: <1990Jun28.090459.9499@grian.cps.altadena.ca.us>
Date: 28 Jun 90 09:04:59 GMT
References: <8821@goofy.Apple.COM>
Organization: Workman & Associates
Lines: 44

RANDO@applelink.apple.com (Damon Rando) writes:

>Both the Portable and the Mac Plus do not supply TERMPWR from the CPU.  
>All other Macs do suppy TERMPWR.  Most drives also supply TERMPWR for the 
>external terminator.  It is best to use a drive that supplies TERMPWR as 
>the last drive in a SCSI chain due to the transmission line 
>characteristics of the SCSI chain.  Drive manufactures are now increasing 
>the capacitance at the TERMPWR supply to decrease the probability of 
>glitches on the line (ie. it's important that the drive supply its own 
>TERMPWR).  The goal in SCSI termination is to keep the TERMPWR line at +5V 
>with as little noise as possible. 

A few, like Everex in several of its guises--most notably EMAC--have new
spiffy boxes with a switch marked "Plus/Other", which optionally
supplies external termination power.

The older SuperMAC 20-110 meg drives have OMTI interface cards that can
also be switched.

But I still believe his problem this time is Unit Attention, which
afflicts especially Pluses.  Even brand-spankin' new drives need this
line cut.  (I made a previous post on this earlier today.)

>This is the 
>reason for the new black terminator in the new IIfx box.  Note (**1 is 
>better 2 is bad** NEVER USE MORE THAN ONE BLACK TERMINATOR)

And boy, it sure makes a difference in SCSI performance.  My dad's IIfx
was dead slow with the grey one, but the black one makes it really
scary.  Good job, guys!

	Alex

"It is said by many that Pournelles never offer praise.  This is not
true--it is merely that praise is only offered where it is truly
deserved."

		--Surviving in a World of Type A Personalities,
		  Volume One, Page 943 (also discussed in Volume Eleven)
-- 
		Alex Pournelle, freelance thinker
		Also: Workman & Associates, Data recovery for PCs, Macs, others
		...elroy!grian!alex; BIX: alex; voice: (818) 791-7979
		fax: (818) 794-2297    bbs: 791-1013; 8N1 24/12/3