Date: Wed, 13 Jun 2007 21:42:47 -0600 From: Scott Long <scottl@samsco.org> To: docaia <docaia@hotmail.com> Cc: freebsd-scsi@freebsd.org Subject: Re: SCSI termination Message-ID: <4670B937.50607@samsco.org> In-Reply-To: <11111442.post@talk.nabble.com> References: <11111442.post@talk.nabble.com>
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docaia wrote: > can somebody please explain to me a simple way what is a single ended > terminator and what is a differential terminator and what is the damage > which may be caused by putting Single Ended devices on a Differential bus? > I am a geophysicsit so I don't have a strong electronic backgroud, so > consider me as a very beginner, and please explain this to me thoroughly. > thank you The big issue to watch out for is High Voltage Differential (HVD) vs Low Voltage Differential (LVD). They operate at dangerously different voltage levels, and mixing them together will often cause smoke, fires, and permanent equipment (and personnel) damage. Single Ended buses operate at the same voltage as LVD buses, so there is no danger in mixing them. The only caveat is that an LVD bus will automatically downshift to single ended if any single-ended devices are directly attached to it. Some adapters will provide both LVD and SE connectors for the same bus, with an internal converter chip that prevents the downshift. Connecting an LVD device to an SE bus poses no problem. Putting an SE terminator on an LVD bus will downshift the bus to SE. I somewhat recall a conversation about it being unsafe to put an LVD terminator on an SE bus, but I don't recall the specific reasons. For best results just try to keep SE and LVD devices and buses matched and segregated appropriately, and don't mix HVD with anything non-HVD. Scott
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