[3742] in linux-scsi channel archive
Re: SCSI cable parameters
daemon@ATHENA.MIT.EDU (Harvey Fishman)
Wed Apr 22 14:41:33 1998
Date: Wed, 22 Apr 1998 14:37:28 -0400 (EDT)
From: Harvey Fishman <fishman@panix.com>
To: Brendan Miller <brmiller@wco.com>
cc: linux-scsi@vger.rutgers.edu
In-Reply-To: <19980422102405.20754@wco.com>
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
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Harvey Fishman |
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