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

[Greg A. Woods]

[ On Thursday, April 18, 2002 at 13:43:48 (-0400), Loomis, Rip wrote: ]
> Subject: [rescue] SCSI Cables/Term:  LVD -vs- HVD -vs- FW/SE
>
>  - 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.

See the question "Is the spacing of connectors on a SCSI cable
important?" in the SCSI Quick Start Guide at http://www.scsifaq.org/

>  - 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.

s/want/need/

>  - 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.

Generally.

>  The biggest thing I'm trying to figure out is when
> HVD terms are *needed*, and (separately) when they are
> *acceptable*.

HVD and SE buses are 100% electrically _in_compatible.  You cannot
safely use the same terminators on both buses.

>  And what's the *real* difference between an
> active terminator, a passive terminator, and a forced perfect
> terminator?

You need at least "active" termination to safely use a SCSI-2 SE bus.

FPT is better though as it clamps off all data signals preventing
overshoot/undershoot.

>  (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...)

	<URL:http://www.scsifaq.org/>

Also this, with pretty pictures:

	<URL:http://www.a2zcables.com/scsi-cheat-sheet.pdf>


-- 
								Greg A. Woods

+1 416 218-0098;  <gwoods at acm.org>;  <g.a.woods at ieee.org>;  <woods at robohack.ca>
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