| Nibble:-
is it the End for SCSI Terminators?................................. |
When I first started
using SCSI devices as a systems integrator in the mid 1980s the answer to - how
many drives you could actually hang onto an inhouse made ribbon cable? -
seemed to depend on trial and error.
In those days everything had
little mechanical switches, called DIP switches, which enabled you to set up
important configuration info such as the SCSI address and whether or not the
terminators were to be connected on this device or not. The rule of thumb was
that you had to connect the first and last devices physically looped on the
chain of cables. Of course that helpful information failed to provide a simple
solution to the artistic patterns of daisy chains intermixed with star offshoots
around the back of the cabinet which our ingenious wiring people sometimes came
up with. It was hard to get them to accept that sometimes a single long loop
worked better than a lot of little limpet like add-ons.
When working on
a group of so-called "identical" systems I would sometimes be
mystified by an intermittent fault in one storage system, even when all the
indivdual parts tested as OK. On closer inspection I might discover that
some drives had the terminator resistor packs plugged in, while their
counterparts in their twin systems didn't. The culprit would often be another
project, which in the middle of the night cured their own problems and got
closer to customer shipment by "borrowing" components from our own
system, which they knew was still a long way in advance of the critical path.
A
week or so later, I might be handed a packet with a bunch of terminators and
told "These are to return the ones we borrowed." I learned to keep a
secret stash of these little suckers for this type of emergency.
Eventually
we learned of the existence of SCSI analyzers and other similar
test equipment, which was
plug and play, instead of the plug and fall off variety when we tried to see
what what going on by hooking up about 30 test clips from a logic analyzer. That
woudl produce insights only until another project pinched the analyzer, and
deleted the custom trace settings... What do engineers do for entertainment
nowadays? - I wonder.
Now things are a lot easier with almost
everything controlled by software. Smart active terminators can measure the
characteristics of the cabling and reflections on the line on reset and
dynamically adjust the termination. With serial connections like
Serial Attached SCSI
and Serial ATA, the
problems of data skew, cross talk and reflection are greatly simplified by the
simpler cabing design. But proper termination still has a role in reliable
system integration, even if its tacky details are invisible to the user. | |
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TMC-The Mate Company,
based in California, manufactures
an extensive range of cables,
adapters, converters and
terminators for fibre-channel, SCSI
SATA and
Serial Attached SCSI. | |
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Serial Attached SCSI - Delivering Flexibility to the
Data Center - article by LSI Logic and Maxtor
If you think
you already know SAS because you know SATA and traditional SCSI then think
again. Sometimes disruptive technologies wear an unassuming disguise. In
fiction, Clark Kent, Frodo Baggins and Buffy Summers at first seem harmless, but
we see them change into Superman, the Ring Bearer and the Slayer.
SAS
too comes cloaked in plain garb - with a physical layer which looks a lot like
SATA. But like the Incredible Hulk there are muscles rippling under that shirt -
and you would be wrong to dismiss SAS so lightly. There's a lot more inside this
interface than it says on the box as this informative article reveals. ...read the article,
...LSI Logic profile,
...Maxtor profile,
Serial Attached SCSI | | |
