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From: phs172m@vaxc.cc.monash.edu.au (Stephen Harker)
Newsgroups: comp.sys.apple2
Subject: Re: Re- Much about Quantums and
Message-ID: <1991Mar19.085900.86461@vaxc.cc.monash.edu.au>
Date: 18 Mar 91 22:58:59 GMT
Article-I.D.: vaxc.1991Mar19.085900.86461
References: <8067@crash.cts.com>
Lines: 63

In article <8067@crash.cts.com>, marks@pro-shop.cts.com (System Administrator) writes:
> In-Reply-To: message from toddpw@nntp-server.caltech.edu
> 
>>>SCSI drives are supposed to		Not like
>>>look like
>>>
>>>	[]--|	  |--[]				[]-----[]
>>>	    |	  |					|
>>>	    |	  |					|
>>>	    |_____|					|
>>>	       |					|
>>>	       =					=
>>
>>Aw shucks. My HDI PowerDrive (quantum 105s mech.) does it the 'bad' way.
>>
>>I understand why there's more noise in the second configuration, though.
>>Electrically, you're splitting the signal as it passes through the drive,
>>and there will be reflections and other wave effects at high transmission
>>rates, which manifest themselves in the form of noise.
> 
> Todd,
>   It is most probable that the signal doesn't pass through your drive, but
> loops out to the other drive before it gets to the first. There shouldn't be
> any noise from this setup.
> 
>    []==|___|======[]
>        |     
>        |
>        |
>        =
>  
> More like this if you can see what I mean. If you open up your drive case you
> will see that the two plugs are directly connected, and only one lead runs to
> the drive. While you've got the drive case open look at the bottom of the
> drive itself and you will see the terminating resistors. They are right above
> the molex plug, unless your drive is not a self terminator.(Most Seagates,
> and Conner are. I'm not sure about Quantums) That might be a source of line
> noise, as feedback due to a bad resistor or not enough resistance inline.

	Since the SCSI cable is functioning basically as a transmission line
this is a possible source of noise, as you will get reflection from any lead
which is taking signals off to the side.  This is effectively connecting in
parallel.  How significant a noise problem is another matter - the major area
this problem has been brought up is with people who are extending the SCSI bus
near to its limits, ie 6m (20ft) of cable.  When you reach this situation 
sub-standard cabling or connections will likely cause problems, but for most of
us it is unlikely that we would notice any problems as the bus is not running
anywhere near its limits, which is why most drive manufacturers that do this
get away with it.

	However if you are serious about wanting low noise or a long cable run
then it is best to stick with the daisy chained cabling.  Also you should
ensure that all cables are of the correct impedance, I believe that 100 ohms is
recommended.  If not you will get reflection when two cables of different
impedance meet.

	The bottom line is that it won't matter in most cases, because the
noise level will be too low to affect the system.  But if you do have problems,
especially with long cable runs, then this is an area to look at.
	
-- 
Stephen Harker				phs172m@vaxc.cc.monash.edu.au
Monash University