[conspire] (forw) Re: June 25 MP installfest upgrade RH 7.3 PC to Centos56 continuation

[Rick Moen]

----- Forwarded message from Rick Moen <rick at linuxmafia.com> -----

Date: Fri, 8 Jul 2011 09:22:12 -0700
From: Rick Moen <rick at linuxmafia.com>
To: wood eddie <ewood111 at yahoo.com>
Subject: Re: June 25 MP installfest upgrade RH 7.3 PC to Centos56
	continuation
Organization: If you lived here, you'd be $HOME already.

I wrote:

> On the basis of personal experience, do _not_ expect Fry's to be able to
> give you anything approaching competent help, advice, or components for
> SCSI.  
> 
> If the device (hard drive?) at the end of your SCSI chain is really
> unable to provide termination -- and I have my doubts about that -- then
> I'd suggest that your best course of action is to visit HSC in Sunnyvale
> (link provided previously; just off the corner of Central and Lawrence)
> and find a SCSI ribbon cable with an active terminator hard-wired onto
> the end.

Or a separate active SCSI terminator.  I should explain, I guess.  

My vague recollection of SCSI termination is that it's a means of
ensuring a voltage drop between certain pins of a SCSI chain at either
end of the SCSI chain.  The old, cheap and basically not very good
method of doing that was with a simple resistor pack.  This is dubbed a
'passive terminator' or 'passive terminator pack', and should be
scrupulously avoided.  

The desirable way of doing it is with a set of inexpensive electronics,
probably diodes.  This is termed an 'active terminator' or 'active
terminator pack'.  Often, there's a small LED on it.  But also, you know
how salesmanship works:  If the thing doesn't actually say the word
'Active' on the side of it, it's best to assume that it's cheap junk
passive termination, and avoid it.  Because the manufacturers know this,
too, you can in fact reliably assume that active terminators indeed have
the word 'Active' embossed or otherwise written on the side.

Go to http://images.google.com/ and enter 'scsi active terminator'.  You
will see a variety of slightly different active SCSI terminators shown,
designed to plug onto the ends of 50-pin (obsolete! 8-bit!) or 68-pin
(16-bit) SCSI cabling.  However, you will also notice that there's a
whole lot of variation, and it would be rather easy to visit HSC looking
for an active terminator, rummage around in their bins and find one, and
bring it home and find that it doesn't plug into your cable because it's
for some obscure variation you don't have (like 80-pin, which is
different and uses a radically different, fatter type of connector called 
'SCA'; see http://en.wikipedia.org/wiki/Single_Connector_Attachment).  So, 
you probably would want to detach your SCSI cable from your minitower
and bring it to HSC with you, so you can make _sure_ you're buying a
compatible terminator.

Or you can rummage around HSC's bins for a SCSI cable with an active
terminator already hard-wired to the end.  Here are two pictures of such
cables:

http://www.blackmoreit.com/products/hp-compaq/9511k-hp-9511k-hp9000-k-class-scsi-cable-with-terminator.html
http://img.tomshardware.com/us/2001/01/29/a_look_at_scsi/scsi-cable.jpg

To give the larger context of this, a SCSI cable with an active
terminator at either end is a convenience.  Why do I say that?  Because
using one means that you are absolutely, positively certain there is an
active terminator at either extreme end of the chain.  You look at the
cable, and it's right there, fastened permanently onto each end.
Because of that fact, though, it then (when using such a cable) becomes
vital to ensure that none of the devices attached to the cable between
the terminators at the ends provides terminator also.

Remember the rule about termination?  There must be termination at each
extreme end.  There must not be termination anywhere between the two
ends.  So (when using such a cable), you must carefully examine the
jumpers on each device that will be within the chain to turn off
termination on those devices. 

The 'convenience' comes because, once you have done the work of
jumpering off termination for all of your SCSI devices (including the
SCSI host-bus adapter), then you can move them anywhere you want around
a cable with terminators affixed to either end, and be certain that
termination has remained correct -- because you look at the cable, and
they're right there.


_However_, as I said before, I have my doubts about whether your three
devices (two SCSI hard drives and a SCSI HBA) are unable to provide
terminator power.  Every single SCSI hard drive made, and every single
SCSI HBA, as shipped, has the means to either provide termination or
not.  Or, at least, it would be a very peculiar one that didn't.  Unless
someone has blithely detached and discarded the terminator circuitry (or
resistor pack) from your hard drive or HBA, it's there.  You just have
to set the jumpers on that device correctly.  Thus, let's assume you
have a cheap 68-pin SCSI ribbon cable that omits hardwired terminators
on the ends.  Let's say it has three connectors, and looks like this:

http://homeofficebuddy.com/images/scsi-cable.jpg

Then, you're going to want to run the chain something like this:

     ----------------------------------- 
     |            |                    |
    HD1          HD2                  HBA
  terminated     not                 terminated
               terminated
 
OK?  Really rather simple.  You just apply the iron rule of SCSI
termination via the jumpers (or in the case of the HBA, maybe its 
built-in BIOS Setup) for each device.

In the scenario where you buy one of those fancy SCSI cables with
terminators hard-wired onto the extreme ends, then you end up like this:


TERM--------------------------------------------TERM
         |            |                    |
        HD1          HD2                  HBA
        not          not                  not       
     terminated   terminated           terminated 

Conceptually simpler, but you _still_ need to examine the jumpering (or 
BIOS Setup) of each device -- in this case, to ensure that they 
_don't_ provide termination.

And don't forget to set the SCSI device numbers uniquely.  E.g.:

     ----------------------------------- 
     |            |                    |
    HD1          HD2                  HBA
   SCSI ID 0   SCSI ID 1             SCSI ID 7
  terminated     not                 terminated
               terminated


----- End forwarded message -----