On Tuesday 11 January 2005 16:40, Jon LaBadie wrote:
>Also, I think that if both types of devices exist on the same bus,
>the lower performance one determines the performance of the entire
>bus.

In theory, this is *not* the case.  One of the (many) selling
points of SCSI over IDE is supposed to be that a SCSI bus can run
each device at its own speed (though perhaps later versions of
the IDE spec have caught up, as they have in some other respects;
I dunno).  Of course, the slower/narrower device will consume
more of the SCSI bus's available bandwidth to carry the same
amount of data, even if they don't directly affect performance of
the faster/wider devices.

In practice, according to the excellent SCSI FAQ, it depends on
the devices in question.  See these questions in particular:
  - "Can I connect a SCSI-3 disk to my SCSI-1 host adapter?
    [...]"  (which isn't Brian's precise situation, but the
    answer might well apply)
  - "How can I calculate the performance I'll get with mixed SCSI
    devices?"

The SCSI FAQ is dated, but still useful.  It's at
www.scsifaq.org; click on the link for "Official
comp.periphs.scsi FAQ".  Sorry, but the site uses
too-smart-for-its-own-good navigation that makes it hard to post
the actual URL.


On Tue, Jan 11, 2005 at 10:48:17PM -0500, Gene Heskett wrote:
> [A SCSI bus is]
> double handicapped because the cable is, compared to a piece of well 
> built coax, pretty much a guestimate as to its operating impedance, 
> usually quoted as being in the 120 to 130 ohm territory,

This isn't supposed to be a problem either, because cable
impedence isn't supposed to be a guesstimate; it's explicitly
specified in the SCSI specs.  But in practice, what you've said
is true; there's all manner of out-of-spec junk sold as "SCSI
cables".

For example, I've read that you can have problems if you put a
SCSI adapter in the middle of an internal/external chain, even if
all the termination is correct, because the internal ribbon cable
and the external cable might have different impedences, leading
to signal reflections between the two cables.

For a telling, if rather ancient, anecdote told by someone from
Adaptec, see question "What is the problem with the Adaptec 1542C
and external cables?" in the SCSI FAQ.


<off-topic>
> To many folks forget that a a scsi bus is indeed 
> an rf transmission line, subject to the usual rules about vswr.

Geez, you mean we've got an actual engineer in the e-room?
Awesome!  (Despite my rambling about impedences and such, I'm
sure no hardware guy.)

>From the context it's pretty clear what "vswr" means, but what
does it stand for?
</off-topic>

--

|  | /\
|-_|/  >   Eric Siegerman, Toronto, Ont.        erics AT telepres DOT com
|  |  /
In theory, there is no difference between theory and practice.
But, in practice, there is.
        - Jan van de Snepscheut