[rescue] SCSI Cables/Term: LVD -vs- HVD -vs- FW/SE

[Gregory Leblanc]

OK, here are a few links from my "SCSI reference" folder.  First one is
a FAQ on termination.
http://www.cablemakers.com/scsitermination.htm

Good general SCSI FAQ
http://scsifaq.paralan.com/

Connector pictures, for Josh
http://www.ramelectronics.net/html/scsi_connecters.html

As a side note, I found that Antares is manufacturing UW SBUS SCSI
cards.  Since these are in production currently, I don't think anybody
has, or will make, u2w sbus scsi.  
http://www.antares.com/sbus_cards/sbus_cards.htm

On Thu, 2002-04-18 at 10:43, Loomis, Rip wrote:
> Over the last several years, I'm discovering that I've accumulated
> a few more odd cables and terminators than I thought.  I've
> been troubleshooting some SCSI problems on two systems,
> though, and I want to make sure I understand the differences
> and effects of cables that are labeled in different ways.
> 
> Here's what I *think* the answers are (based on some testing
> and some memory):
>  - If I need a 68-pin external cable, then I can pretty much use
>    any cable as long as the pins are all connected and the
>    total bus length is okay--so a 1ft. cable labeled "differential"
>    will work just fine with FW/SE or with LVD in addition to
>    the HVD it was probably intended for.

Yes, although the minimum spacing between "stubs" is generally 12".

>  - When termination (internal or external) gets involved, then
>    *sometimes* a "perfect" terminator (or an "active" terminator
>    which somehow isn't quite the same thing) will work with
>    any type of setup--but pretty much if you're working with
>    HVD gear then you want HVD terminators.

Almost.  LVD terminators will work on either an LVD or SE bus (since
essentially all LVD devices are LVD/SE capable).  HVD or differential
terminators will only work on HVD busses.  

For SE termination, just give away all your passive terminators to
somebody who you don't know (like at a swap meet or something).  They're
not worth using for the people on this list.  Use active terminators for
everything, preferably one with at least 1 blinken-light (more is always
better, but the single light will tell you if power is getting to the
end of the bus, and if the terminator is toast or not).

Passive terminators are just a resistor network.  Active terminators are
a resistor network, plus a voltage regulator.  Forced perfect
terminators are made from diodes, resistors, and a voltage regulator. 
They can theoretically extend the length of an SE bus, but they're
expensive, and probably not worth the money.  

>  - Although "Ultra160-rated" cables might have some fancy
>    braiding or shielding to help with RFI at the higher speeds,
>    they'll work fine at lower speeds--and in general plain old
>    FW SCSI-2 SE cables will work fine too.

Right, as far as I know, SCSI cables should be SCSI cables, based on
connectors.  The SCSI specs recommend twisted pair cabling for all
cables, internal and external.  The only place that I tend to see tp
SCSI cabling is inside of "high end" machines, like the SS20, and Vaxen
and such.  I'm sure somebody sells the stuff online.  Unfortunately,
making your own SCSI cables using TP wires is rather more of a pain than
with flat ribbon cable.

> Any gurus out there want to educate me on problems with
> the above?  The biggest thing I'm trying to figure out is when
> HVD terms are *needed*, and (separately) when they are

They're needed when you're working with differential equipment (NOT
LVD).

> *acceptable*.  And what's the *real* difference between an

They're acceptable whenever they're needed.  LVD terminators, on the
other hand, are probably the way to go if you're buying new, since
they'll work on your old SE stuff, and on new LVD stuff.

> active terminator, a passive terminator, and a forced perfect
> terminator?  (If there's a good reference that tells me all this
> then just point it out--I've checked through a bunch of stuff
> online without finding clear answers...)

Yeah, see above.  HTH,
	Greg

-- 
Portland, Oregon, USA.