[3752] in linux-scsi channel archive
Re: SCSI cable parameters
daemon@ATHENA.MIT.EDU (Harvey Fishman)
Thu Apr 23 02:04:11 1998
Date: Thu, 23 Apr 1998 02:02:08 -0400 (EDT)
From: Harvey Fishman <fishman@panix.com>
To: Brendan Miller <brmiller@wco.com>
cc: linux-scsi@vger.rutgers.edu
In-Reply-To: <19980422223224.35679@wco.com>
On Wed, 22 Apr 1998, Brendan Miller wrote:
> > Width and length are disjoint. Recommended maximum cable lengths are 6m
> > for 5MHz., 3m for 10 MHz., and 1.5m for 20 MHz. if there are 4 or more
> > devices on the bus, 3m if there are less. I have never heard of a 40 MHz.
> > bus clock, so I do not know the recommendations for that.
>
> Well, maybe I misinterpreted what my Mylex (BusLogic) BT-958 manual said
> about cable lengths. Without permission, I reproduce this table:
>
>
> SCSI Type Data Transfer Maximum # Maximum Cable
> Rate (MB/sec) of devices Length
> ---------------------------------------------------------------------
> SCSI-2 5 or less 8 6 meters (19.7 feet)
>
> SCSI-2 10/20 8/16 3 meters (9.8 feet)
> Fast/Wide
>
> Fast-20/ 20/40 4 3 meters (9.8 feet)
> Fast-40
>
> Fast-20/ 20/40 8//16 1.5 meters (4.9 feet)
> Fast-40
Where I differ from that table is that it does not recognize "Slow/Wide". ;-)
i.e. 16 bit bus at 5 MHz. clock. Other than that and the fact that I still
do not know very much about Fast 40, we agree. It is the bus clock that
determines the sensitivity to the cable length. The faster it is, the more
noise sensitive and thus the shorter the allowed cable. The bus width just
says that you can pass data twice as fast with the same clock if you use a
16 bit wide bus rather than 8 bit. Perhaps Fast-40 means a 16 bit bus and
a 20 MHz. clock, which I think would agree with what I originally said.
> So what I gathered from this table was that you could have 6 meters of
> cable if using regular 5MB/s narrow devices, 3 meters of cable if you were
> using either Fast SCSI-2 or Wide SCSI-2 devices, 3 meters of cable if you
> had 4 or fewer Fast Ultra devices or Fast Wide devices, and only 1.5
> meters of cable if you had more than 4 of such devices.
The only thing wrong there is that three meters applies to Fast and Fast/Wide.
If you have "Slow/Wide", it is still six meters allowed.
> I don't really know what Fast-20 or Fast-40 SCSI is. I (erroneously?)
> assumed that Fast-20 was Fast/Ultra (or Ultra/Fast) SCSI-2 going at 20MB/s
> and that Fast-40 was Ultra/Wide SCSI-2 going at 40MB/s. I still don't
> understand the difference between the 20MB/sec on the second line
> (SCSI-2 Fast/Wide) and the 20MB/s on the third and fourth lines
> (Fast-20/Fast-40).
Ultra is a proprietary Adaptec name for what the proposed SCSI-3 spec calls
Fast-20 (and perhaps Fast-40). Think in terms of bus clocks and data rates
rather than names, proprietary or otherwise. It is the bus clock that is
thing that governs recommended cable lengths. Bus width determines data rate
for a given clock.
> NOR, do I understand how this relates to bus speed. as Harvey talked about.
I hope that what I said above makes it clearer. If I am still being obtuse,
ask again..
> > > 1) How close together on the bus (ribbon cable) can two drives be?
> >
> > The recommended minimum separation is 30cm. (~12 inches). That is rarely
> > obeyed, and I have not heard of many problems due to ignoring this. But
> > most SCSI buses are run with a single master, so that makes things simpler.
>
> The reason I asked is because you don't need the extra 8 inches of cable
> when you are cabling drives that are one on top of another in half-height
> drive bays.
Right. And it DOES get in the way. That is why that particular spec is
usually honored in the breach. The cable itself is cheap, but the place to
stow it may be dear.
> > > 2) How close to the adapter can one drive be?
> >
> > But the separation between the host adapter and the other devices
> > could be more critical. For the practical reasons that the host adapter
> > is generally in the card rack and separated from the drive bays,
>
> I have a case where there is a drive "hangar" right next to the first
> PCI slot. It is the only drive in the system, and I have it connected
> to an Adaptec 2940 in the first slot with a 6-8" SCSI cable!
6-8" is probably quite reasonable, but if you do see funnies (probably a
hung bus), this would be the first thing that I would change. Remember that
all of these cable lengths and termination items are NOT absolute. They are
merely the recommended things that offer the highest probability of getting
a robust system on the first try. Generally speaking, the costs of most of
them are miniscule in the usual application and so it is best to try to
adhere to them as much as possible. Cable and termination is (hopefully)
less costly/valuable than your data. And the data is what is put at risk
when you try to cut corners. But for one reason or another there is always
the exceptional case where you really cannot stick to the recommendations.
Harvey
----------------------------------------------------------------------------
Harvey Fishman |
fishman@panix.com | A little heresy is good for the soul.
718-258-7276 |
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