From owner-freebsd-scsi@FreeBSD.ORG Thu Jun 14 15:27:18 2007 Return-Path: X-Original-To: freebsd-scsi@freebsd.org Delivered-To: freebsd-scsi@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id A480416A46F for ; Thu, 14 Jun 2007 15:27:18 +0000 (UTC) (envelope-from jhs@berklix.org) Received: from thin.berklix.org (thin.berklix.org [194.246.123.68]) by mx1.freebsd.org (Postfix) with ESMTP id 2AE6C13C4B0 for ; Thu, 14 Jun 2007 15:27:17 +0000 (UTC) (envelope-from jhs@berklix.org) Received: from js.berklix.org (p549A450A.dip.t-dialin.net [84.154.69.10]) (authenticated bits=128) by thin.berklix.org (8.12.11/8.12.11) with ESMTP id l5EFDDkQ009924; Thu, 14 Jun 2007 17:13:14 +0200 (CEST) (envelope-from jhs@berklix.org) Received: from fire.jhs.private (fire.jhs.private [192.168.91.41]) by js.berklix.org (8.13.6/8.13.6) with ESMTP id l5EFD8fs080499; Thu, 14 Jun 2007 17:13:09 +0200 (CEST) (envelope-from jhs@berklix.org) Received: from fire.jhs.private (localhost.jhs.private [127.0.0.1]) by fire.jhs.private (8.13.6/8.13.6) with ESMTP id l5EFD8ht007655; Thu, 14 Jun 2007 17:13:08 +0200 (CEST) (envelope-from jhs@fire.jhs.private) Message-Id: <200706141513.l5EFD8ht007655@fire.jhs.private> To: docaia From: "Julian Stacey" Organization: http://berklix.com BSD Unix C Net Consultancy, Munich Germany. User-agent: EXMH on FreeBSD http://www.berklix.com/free/ X-URL: http://berklix.com In-reply-to: Your message "Wed, 13 Jun 2007 21:42:47 MDT." <4670B937.50607@samsco.org> Date: Thu, 14 Jun 2007 17:13:08 +0200 Sender: jhs@berklix.org Cc: freebsd-scsi@freebsd.org Subject: Re: SCSI termination X-BeenThere: freebsd-scsi@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: SCSI subsystem List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 14 Jun 2007 15:27:18 -0000 Scott Long wrote: > 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. To add to Scot's top down look, Here's a bottom up look: http://www.google.com/search?hl=en&q=scsi+terminator+electronics+theory&btnG=Google+Search --> http://home.comcast.net/~SCSIguy/SCSI_FAQ/scsi_quick_start.html#_Hlk413567563 QUESTION: What is a SCSI terminator? Why do I need them? ANSWER From: Gary Field (scsifaq@bigfoot.com) Updated: May, 1999 extract: ------ * Passive terminators consist of pairs of resistors. A 220 Ohm pulling each signal up to TERMPWR and a 330 Ohm pulling each signal down to GROUND. Passive terminators were considered adequate in SCSI-1 when the bus only ran at 5 MHz. In SCSI-2, passive terminators were given the name "Alternative 1". * Active terminators consist of 110 Ohm resistors connected from each signal line to a common 2.85 Volt regulated power supply. Active terminators both terminate the bus better (less reflection), and supply cleaner pullup current (due to their Voltage regulation). They were first defined in SCSI-2 and were given the name "Alternative 2" to distinguish them from passive terminators. ------ I see you ask about "differential terminator" too, perhaps googling will give a better answer, but: "Differential signalling costs more (not just scsi, but way back to telegraphy, modems long V24 serial cables etc), differential measures difference in voltage between _2_ adjacent usually twisted wires. Works further/ faster/ less errors, than comparing a single voltage on a single wire to a reference ground. PPS I've had 50 wire flat old scsi-1 melt through plastic & hot wire hurt fingers & droop way out of ribbon threatening more damage, (I'd got pins offset or wrong way on a board with bare 50 pins & no socket for alignement... It worked again with new cable (was a scsi-1 on a PC532 http://berklix.org/~jhs/txt/pc532.html ) -- Julian Stacey. Munich Computer Consultant, BSD Unix C Linux. http://berklix.com HTML mail unseen. Ihr Rauch=mein allergischer Kopfschmerz. Dump cigs 4 snuff.