Date: Mon, 01 Dec 1997 11:13:11 +0000 From: Damian Hamill <damian@cablenet.net> To: "Justin T. Gibbs" <gibbs@narnia.plutotech.com> Cc: freebsd-isp@freebsd.org Subject: Re: Adaptec 2940 and FreeBSD 2.2.2-RELEASE Message-ID: <34829BC7.41C67EA6@cablenet.net> References: <199711281446.JAA28643@bilver.magicnet.net> <199711282008.NAA11788@narnia.plutotech.com>
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Justin T. Gibbs wrote: > > Good cables are a must. However .5m cables (if you have multiple > > targets) is definately not SCSI spec, which is .3m between targets > > and .1m stub. > > People missunderstand the SCSI spec all of the time. In part this is > because this section is poorly written. The main thing to come out > of the stub/device spacing requirement is the minimum 3 to 1 ratio. > For internal cabling configurations, your stub length is determined > by the PCB tracings on the drive's electronic board which is typically > less than .05m. This means you can actually cluster devices fairly > close together. Even more important than device spacing length is > spacing consistency, having devices and cables with similar loads, > termination, and overall cable length. I'd like to understand this better so in future I can make sure I get the cables just right. We have a ribbon cable in an external drive array with 4 drive connectors spaced ~15cm apart and 50 pin centronic connectors on either end. The long end connects to the external cable which goes to the external port of the SCSI adaptor and the short end has a terminator, so it looks like this. [3940]--- | | .5m external cable | | [50 pin centronics connectors] | | .3m SCSI ribbon cable | | = drive connector 1 | .15m cable | = drive connector 2 | ...... | = drive connector 4 | .07m | [50 pin centronics connector] [terminator] It's working fine at Ultra SCSI speed but I'd like to know whether I can make these cables any better by changing any of the cable section lengths. We used one of those 8 connector SCSI cables as a starting point and chopped it in half so thats how we came to have the ~15cm spacing with the short end at the terminator. The only section I can make shorter is the external cable (down to about .35m). regards damian -- * Damian Hamill M.D. damian@cablenet.net * CableNet & The Landscape Channel * http://www.cablenet.net/ http://www.landscapetv.com/
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