looking for SCSI cable ..

Benjamin Scott bscott at ntisys.com
Wed Dec 29 20:41:01 EST 2004


  Warning: Technical standardese ahead.  Proceed with caution.  :-)

On Wed, 29 Dec 2004, at 2:41pm, neil at neilschelly.com wrote:
> The last time I needed a new SCSI cable CyberGuys had a good price.  This one 
> seems to match what you're looking for, and significantly cheaper. 
> http://www.cyberguys.com/templates/searchdetail.asp?T1=127+2280

  I've ordered for CyberGuys before, they're pretty good.  For something
like an old scanner, that should be fine.

  Just watch out if you're using high-speed devices (like recent hard
disks), many devices on a single bus, or going long distances.  Then the
signal characteristics of the cable become *very* important, and paying
extra for quality becomes worth it.  There are few things more aggravating
in the world then a quirky SCSI bus.

  Note that I'm not saying CompUSA is quality.  Indeed, more likely the
opposite for something like this.  For high-quality SCSI cables, I've used
http://www.cablemakers.com in the past with good luck.

> I think the HD=HPDB question you bring up is probably just a typo, or a
> misnomer anyway.

  Terminology for cables and connectors gets corrupted incredibly often, and
SCSI especially so.  Technically speaking, "SCSI-1" and "SCSI-2" refer to
standards documents, not connector types.  You even see references to
"SCSI-3" and "SCSI-4", when no such standards exist!  (After SCSI-2, the
standard was split up into multiple parts, and SPI-1 (SCSI Parallel
Interface 1) was the next relevant document.)

  The technical term, in the spec, for the 50-pin, high density, D-shaped
SCSI connector is "SCSI A-cable Alternative 1".  Of course, go to a cable
seller and most will have no idea what that is.  All sorts of "nicknames"  
for it have propagated, including SCSI-2, MD-50, HD-50, and HPDB-50.  None
of this is standard, and it can and will vary from vendor to vendor.  I go
by pin count, mainly, and prefer to see pictures before I buy.

> And as for high-byte termination, I have no idea what it is, but it sounds
> like marketing-speak more than anything.  Any SCSI cable that satisfies
> the connection needs you have should work fine, at least it always has for
> my scanner.

  Both ends, and only the ends, of a SCSI bus need to be terminated.  If you
convert a 68-pin cable (16-bit, or "wide") to a 50-pin cable (8-bit, or
"narrow"), then putting a terminator at the end of the 50-pin cable leaves
the high byte of the bus unterminated.

  If you only have narrow devices, this doesn't matter, since you're not
using the high byte anyway.  But if you have wide and narrow on the same
bus, this becomes critical.  Otherwise, all your wide devices will get nasty
signal reflections on the high half of the bus.

  To solve this, you can use a special cable that terminates just the high
half of the bus.  Some SCSI host adapters also offer the option (usually in
a BIOS/firmware setup utility of some kind) of terminating just the high
half.  Of course, that is only useful if your wide-to-narrow conversion
occurs at the SCSI HA, but that's common.

  SCSI is a very flexible, very powerful family of standards, but with that
comes a degree of complexity.  The situation isn't helped by the countless
vendors who totally ignore the specifications anyway.

> By the way, I'm new to the list.

  Welcome!  There's a lot of good people here, and a lot of knowledge.  
Don't be afraid to ask questions or contribute knowledge.  Some of us
(myself included) may sound intimidating, but don't worry, we don't bite.  
Much.  ;-)

-- 
Ben Scott <bscott at ntisys.com>
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