Date: Thu, 31 Oct 1996 12:03:07 -0600 (CST) From: Joe Greco <jgreco@brasil.moneng.mei.com> To: imp@village.org (Warner Losh) Cc: rgrimes@gndrsh.aac.dev.com, lada@ws2301.gud.siemens.co.at, scsi@FreeBSD.org Subject: Re: FreeBSD support for Adaptec/Buslogic drivers Message-ID: <199610311803.MAA28103@brasil.moneng.mei.com> In-Reply-To: <E0vJ0WE-0001Qi-00@rover.village.org> from "Warner Losh" at Oct 31, 96 10:03:02 am
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> In message <199610311654.IAA04790@GndRsh.aac.dev.com> "Rodney > W. Grimes" writes: > : They are not all equal, the ``cheap'' ones use passive termination, > : the good ones (ASUS SC-200 and many others) use active termination. > > That brings up a question that I've been meaning to ask for a long > time. Can someone tell me the difference between active and passive > termination, or point me at something that does? In addition, why > would I want one over the other? A passive terminator connects each data line via a 220 ohm resistor to TERMPWR (+5v) and via a 330 ohm resistor to ground. This prevents signal "bounce" at the end of the wire, much like Ethernet 10base2 terminators or many other signals that work at high frequencies. The disadvantage is that TERMPWR may not be a real good +5v source... and this can cause some fluctuation of ALL your SCSI bus signals! An active terminator uses a regulator to provide a "better" 2.85 volt regulated source and then wires each signal to this stable voltage source. This is OK as long as TERMPWR isn't total garbage (but at that point you are in other kinds of pain anyways). A FPT (Forced Perfect Termination) terminator uses multiple regulators and some diodes to perform some clamping tricks to help minimize overshoot and undershoot. Don't ask me for more of an explanation as I only understand the general idea of what it is trying to accomplish. A SLICK terminator is some sort of modified FPT terminator that I do not know much about (but I've seen 'em). Anybody know of any sources for SLICK terminators? ... Joe ------------------------------------------------------------------------------- Joe Greco - Systems Software Engineer, UNIX/Network Hacker, Etc. 414/362-3308 Marquette Electronics, Inc. - R&D - Milwaukee, WI jgreco@mei.com
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