[842] in linux-scsi channel archive
Re: NCR Problems
daemon@ATHENA.MIT.EDU (John Sutton)
Wed Oct 23 13:56:24 1996
Date: Wed, 23 Oct 1996 18:29:18 +0100
To: linux-scsi@vger.rutgers.edu
From: John Sutton <john@scl.co.uk>
At 16:48 23/10/96 -0200, Matthias Urlichs <smurf@smurf.noris.de> wrote:
>That looks suspiciously like a cabling/termination/whatever problem.
I've also got what I suspect are cabling/termination problems (but no relation to NCR host adapter) so I'd like to pick your brains a little further on this topic if I may!
>Termination: start and end of SCSI cable, NEVER in the middle.
>
>Cabling: Use identical, high-quality SCSI cables. Between two connectors
OK, but can you recommend a "high-quality" brand (or two)? Presumably this is an issue only with external cables, the internal ribbons are all of a kind, yes?
>there needs to be at least 15 cm cable (internal drives). External boxes
>_need_ to have the internal connector _between_ the external plugs, should
How could the internal connector be _other_than_ between the external plugs?
>supply terminator power, and should NOT have internal terminators -- that's
You are saying that the box itself should supply termination power? Surely if at least one of the units in the box supply it, that should be sufficient?
>the job of the terminator you plug into the open end of the external bus.
>If you still have these "pass-through" terminators, take your hot glue gun
>and close the open end. Now. There's absolutely no use for it.
I've seen mention of these so-called "pass-through" terminators but can't fathom out what it can mean! Surely it is a contradiction in terms - if the bus has been passed through, it should never have been terminated?
>If in doubt, open the boxes to make sure.
And more generally, how is it that the quality of external cables is so crucial and yet the internal cables are just flat ribbons which cannot have a well defined characteristic impedance - it will vary depending on the geometry (how it happens to be folded up), how close the ribbon lies to any earthed surface, etc?
And what of the difference between passive and active terminators? It would seem that an active terminator is always to be preferred, yet when I use one on my bus absolutely nothing is recognised. With a passive one, at least I get some activity! Could this be because the active one needs more term power and is dragging it down? Should an active terminator have its own power supply at >5V? The one I have (and it says "active" on the side!) doesn't, so it must be drawing power from the term power line. But this seems odd, because I know an active terminator has a voltage regulator inside and so there must be a voltage _drop_ across this?
Thanks in advance for any comments!
John Sutton
Tel. +44 1239 710658
