Date: Wed, 5 Jan 2000 12:20:26 +1100 From: Peter Jeremy <peter.jeremy@alcatel.com.au> To: Mitch Collinsworth <mkc@Graphics.Cornell.EDU> Cc: hardware@FreeBSD.ORG Subject: Re: differences between SCSI and EIDE [was: wanna buy an EIDE harddisk ... 5400 or 7200 for home use (noise)] Message-ID: <00Jan5.122026est.40333@border.alcanet.com.au> In-Reply-To: <200001041610.LAA15549@benge.graphics.cornell.edu>; from mkc@Graphics.Cornell.EDU on Wed, Jan 05, 2000 at 03:06:49AM %2B1100 References: <jon@welearn.com.au> <200001041610.LAA15549@benge.graphics.cornell.edu>
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On 2000-Jan-05 03:06:49 +1100, Mitch Collinsworth <mkc@Graphics.Cornell.EDU> wrote: > My salesman says the >HDAs are the same in SCSI and EIDE drives, so reliability-wise there >should be no difference. I would also assume that to be the case. A common HDA minimises the development effort - only the controller PBA needs to be specially designed (and in the case of SCSI, there are likey to be a number of different controllers for the different SCSI interfaces). > I'd >like to know of any significant down-sides to using EIDE other than the >number of devices per controller. By definition, the EIDE bus is electrically more fragile than a SCSI bus. The SCSI specs have always required that a SCSI bus be properly terminated, use controlled-impedance cables and placed limits on the lengths of stubs. By default, SCSI uses single-ended logic levels (roughly TTL, but I'm not sure that the voltage levels are exactly TTL), but a differential version is also defined - which increases the bus robustness at high speeds. (People might not have compiled with the SCSI specs in the past, but they are forced to when they crank us the speeds - and the necessary cables and terminators are readily available). An EIDE "bus" is a random piece of ribbon cable with TTL logic levels, no termination and (from memory) insufficient ground wires. This was not a problem at low speeds, but as the speeds get higher, the cable looks more like an unterminated transmission line and the reflections eat into the noise immunity. Peter To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hardware" in the body of the message
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