Date: Fri, 29 Nov 1996 09:42:13 -0800 From: obrien@cs.ucdavis.edu (David E. O'Brien) To: rls@mail.id.net (Robert Shady) Cc: freebsd-chat@freebsd.org Subject: Re: Differential vs. Standard Message-ID: <Mutt.19961129094213.obrien@kongur.cs.ucdavis.edu> In-Reply-To: <199611291659.LAA24814@server.id.net>; from Robert Shady on Nov 29, 1996 11:59:57 -0500 References: <199611291659.LAA24814@server.id.net>
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Robert Shady writes: > Basically, I have the opportunity to grab a couple unused spare Seagate > ST-410800WD hard drives, 9GB, Fast/Wide, Differential, 68 pin connectors. > I will be able to get these drives at about 25% less than my cost through > Ingram Micro, TechData, Mersiel, etc... (Which are all out of stock BTW). > > 2. What exactly does "Differential" mean? Does it help, or hurt? Differential means you have two wires for every standard SCSI signal. In standard SCSI you measure a signal by looking at its voltage level w/respect to ground. In differential you measure a SCSI signal by taking the voltage difference between two the wires associated with that signal. The advantage is: with long cables or in an electrically noisy environment, the signals in your cables are affected, but ground isn't. Thus you don't read the signals correctly. W/differential, the signal in each of the two wires are *both* equally affected. Thus you get a correct reading. Differential drives allow you to use *much* longer SCSI cables. This is often an issue with large external backup tape silos, optical jukeboxes and other terrabyte storage. -- -- David (obrien@cs.ucdavis.edu)
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