Date: Sun, 8 Jun 1997 22:56:32 +0200 (MET DST) From: Wilko Bulte <wilko@yedi.iaf.nl> To: rgrimes@GndRsh.aac.dev.com (Rodney W. Grimes) Cc: gibbs@plutotech.com, asami@CS.Berkeley.EDU, scsi@FreeBSD.ORG Subject: Re: 3940uw with long cables Message-ID: <199706082056.WAA00434@yedi.iaf.nl> In-Reply-To: <199706081828.LAA24046@GndRsh.aac.dev.com> from "Rodney W. Grimes" at Jun 8, 97 11:28:39 am
next in thread | previous in thread | raw e-mail | index | archive | help
As Rodney W. Grimes wrote... > > As Justin T. Gibbs wrote... > > > > > Perhaps. There are a few important things to keep in mind when > > > tuning the bus length. > I haven't seen much problem with that as long as you use consitent > impedence cables. Ie, as long as the whole bus is run with 90 Ohm > cable it works, but you have to spend the bucks for high quality > teflon or twisted pair internal cables to get any place close to 90 ohms. > And of cource you have to buy double sheilded twisted pair 90 ohm > external cables with gold contacts. The Adaptec 1742[A] comes to mind as a very picky card when external and internal cables were used at the same time. It is of course very dependent on both card and cable characteristics. > One thing about doing what you state above is that this means you > are pretty much stuck with what ever type of termination is on the > SCSI controller. I don't know of any controllers that have FPT > on them, and for long busses FPT is a _must_. I agree. > > > 6) Honor the 6m and 3m bus length limits. 6m is the longest for 10MHz > > > and 3m is the longest for 20MHz single ended SCSI. This assumes > > > proper stub/gap matching, so real life buses may need to be shorter. > > > > Hm. The company I work for (==DEC) uses the rule that a single ended > > F10 (10MHz) bus can be a max of 3 meters. > > Justin, can you double check these numbers, I'm seeming to recall that > it is 6 meters for async upto 5Mhz, 3 meters for sync upto 10Mhz and Right, those are match my numbers. > 1 meter for sync upto 20Mhz, unless your running differential, which We assume 1.5m for 20MHz (half of what F10 uses). Be *very* conservative with stubs in case of F20 though. > is a whole other set of numbers. Yep, something like 20 or 25 meters for F10 diff. Wilko _ ____________________________________________________________________ | / o / / _ Bulte email: wilko@yedi.iaf.nl - Arnhem, The Netherlands |/|/ / / /( (_) Do, or do not. There is no 'try' - Yoda --------------------------------------------------------------------------
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?199706082056.WAA00434>