[3751] in linux-scsi channel archive
Re: SCSI cable parameters
daemon@ATHENA.MIT.EDU (Brendan Miller)
Thu Apr 23 01:32:58 1998
Date: Wed, 22 Apr 1998 22:32:24 -0700
From: Brendan Miller <brmiller@wco.com>
To: Harvey Fishman <fishman@panix.com>
Cc: linux-scsi@vger.rutgers.edu
Reply-To: Brendan Miller <brmiller@wco.com>
In-Reply-To: <Pine.SUN.3.95.980422142506.2487A-100000@panix3.panix.com>; from Harvey Fishman on Wed, Apr 22, 1998 at 02:37:28PM -0400
> 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
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.
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).
NOR, do I understand how this relates to bus speed. as Harvey talked about.
> > 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.
> > 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!
Brendan
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