[SunRescue] scsi cable order

[rescue at sunhelp.org]

It can be thought of in simple physics.  When a 
wave/pulse enters a new medium with a different (for lack 
 of a better term) impedence, you get a reflection.  The 
termination is roughly the same impedence so very little 
or no reflection off the end.  Reflections can set up 
standing waves which can create voltage and current peaks 
along the cable which can trigger other devices along the 
cable.  Those of us with ham licenses know high SWR can 
cause problems.  
> James Lockwood wrote:
> 
> > SCSI is not magic, it follows well understood rules of transmission line
> > physics.
> 
> Shhhh! you'll anger the gods.
> (-:
> Actually, I wish I understood those physics better...
> 
> > If changing the order of your cables affects your setup (and you aren't
> > doing something dumb like leaving a wide SCSI device with the high bits
> > floating) then something is wrong with either your termination, cables, or
> > devices.  Usually in that order.
> 
> That is good advice, I try to keep all devices on their most appropriate
> interface. No wide to narrow adapters and all that.
> 
> > A number of SCSI cables produced _new_ do not meet spec.  Moving a bad one
> > to a different section of the chain may reduce incident reflections enough
> > to let it work, but the real solution is to cut it in half (so nobody else
> > gets stuck with trying to make it work for hours) and get a new one.
> 
> (-: I've nearly cut apart some cables here at work, but they aren't mine. I
> do put a loop of tape around them marked SUSPECT. Tell people not to use
> them unless you are in a temporary jam.
> 
> >> I can't remember if the IPX needs the internal drive terminated or not. From
> >> what I understand the ss2's drives are on the bus with 'stubs' and don't
> >> need to be terminated.
> 
> > The IPX is the same way.
> 
> Thanks!
> 
> 
> --jeff
> 
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