Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!hsdndev!husc6!encore!wcarroll From: wcarroll@encore.com (Mr. New Dad) Newsgroups: comp.periphs.scsi Subject: Re: SCSI max length Message-ID: <130302@jake.encore.com> Date: 7 Jan 91 17:10:42 GMT References: Organization: Encore Computer Corp, Fort Lauderdale, FL Lines: 51 From urlichs@smurf.sub.org (Matthias Urlichs): > src@scuzzy.in-berlin.de (Heiko Blume) writes: > < > < Host<-SingleEnded->Adapter<-Differential->Adapter<-Single-Ended->Device(s) > > NB: The adapters should not have to be told which IDs are on what side of the > bus. They can find out for themselves by watching the signals at runtime. The adapters shouldn't need to know which IDs are on what side of the bus. They don't need to "know" anything, since they don't need onboard intelligence. They just convert the physical interface. > < the optimum cables are: > < 132 ohms imp. 50 cond. flat or 25 signal twisted pair cable for Single Ended > < 122 ohms imp. 50 cond. or 25 signal twisted pair cable for Differential > Don't bother calling your local cable vendor and asking for 132 ohm 50 cond cable. They won't have it. And even if they did, you'd never find connectors with a 132 ohm impedance. Just use standard 100 ohm cable. > < both should be shielded. > < > On the other hand, if radio&TV antenna and any heavy-duty applicances are > sufficiently far away, you may be able to forget about the shielding. As long as you don't want to watch TV while computing and you don't have technically-astute neighbors, it shouldn't be too hard to get away with it. If it screws up their reception and they call the station to complain, the station will likely just tell them to buy a bigger antenna or a better receiver. I only know one person who has ever proved to the FCC that a neighbor was above legal RFI emmision levels, and it took MONTHS of that person's time to do it. Please note that I am not advocating the breaking of laws, or encouraging you to break any laws. I am just stating personal opinion and experience. And to add a note to the descriptions of 100 foot buses: There are timings written into the SCSI spec for things like bus delays, skew delays, etc. Many SCSI interface chips hardware time these delays. If the bus is sufficiently long to cause these times to exceed the spec values, those interface chips may run into problems. I have never experienced this personally (having never run a bus longer than ~50 ft), but I accept it as a fact. -- William R. Carroll (Encore Computer Corp., Ft. Lauderdale FL) wcarroll@encore.com uunet!gould!wcarroll "This comment without commentary is not the view of the staff or management of WKPX, the Broward County School Board, ... or even me." - KZ