[3743] in linux-scsi channel archive
Re: SCSI cable parameters
daemon@ATHENA.MIT.EDU (Larry M. Augustin)
Wed Apr 22 15:13:54 1998
Date: Wed, 22 Apr 1998 12:02:45 -0700
From: "Larry M. Augustin" <lma@varesearch.com>
To: Harvey Fishman <fishman@panix.com>
Cc: Brendan Miller <brmiller@wco.com>, linux-scsi@vger.rutgers.edu
In-Reply-To: <Pine.SUN.3.95.980422142506.2487A-100000@panix3.panix.com>
With Ultra2 40MHz Low Voltage Differential (LVD) you can have up to a
12.5m cable with 16 devices at a spacing of 19cm between devices.
With shorter cables and fewer devices, you can use closer spacing.
LVD is great.
Larry
Harvey Fishman writes:
> On Wed, 22 Apr 1998, Brendan Miller wrote:
>
> >
> > Quick question. I know there are maximum recommended cable lengths
> > for SCSI depending on the speed of drives on the bus. Some things I've
> > read refer to purely MB/s--5, 10, 20, 40, etc.--and some try to refer
> > to the SCSI terminolgy--Fast, Wide, Fast-Wide, Ultra, Ultra-Wide, etc.
> >
> > I apologize if this is in the FAQ, but can someone tell me what the
> > maximum recommended cable lengths are for different speeds? I guess
> > if you have to go by the speed of your fastest drive, i.e., if I have
> > an ultra-wide 40MB/s drive and a bunch of narrow 10MB/s, I'm constricted
> > to the restraints of ultra-wide cabling.
>
> 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.
>
> > Also, two other related questions:
> >
> > 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.
>
> > 2) How close to the adapter can one drive be?
>
> Same as above; the specifications make no distinction between a host
> adapter and any other type of device that may be on the bus. 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 drave bays, I have never seen a case
> where the actual separation would have severely mangled the spec numbers.
>
> Harvey
>
> ----------------------------------------------------------------------------
> Harvey Fishman |
> fishman@panix.com | A little heresy is good for the soul.
> 718-258-7276 |
>
>
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