From mboxrd@z Thu Jan 1 00:00:00 1970 From: Doug Ledford Subject: Re: Whenever two drives plugged in, second drive fails? Date: Thu, 20 Jun 2002 14:36:12 -0400 Sender: linux-scsi-owner@vger.kernel.org Message-ID: <20020620143612.C9181@redhat.com> References: <5.1.0.14.0.20020620131337.023d8d40@205.233.215.4> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <5.1.0.14.0.20020620131337.023d8d40@205.233.215.4>; from jhill@hrpost.com on Thu, Jun 20, 2002 at 02:15:36PM -0400 List-Id: linux-scsi@vger.kernel.org To: Jeff Hill Cc: linux-scsi@vger.kernel.org On Thu, Jun 20, 2002 at 02:15:36PM -0400, Jeff Hill wrote: > When I try to add a recently repaired drive to my SCSI chain, the Adaptec > 2940U2W controller times out trying to find it, even reports occasionally > a "cable or termination error, please fix." > > Sounds simple, but I don't think it's termination on the drives, since > they're an identical pair of Seagate Cheetah LVD drives. [ snipped description ] Hmmm...from what you've given I almost wonder if there isn't a termination power issue with too many devices providing term power. I would check the drives to make sure they don't have term power enabled since it shouldn't be needed. The other thing that it could be is actually an underpowering of term power with the two drives pulling more power from the reset line than the term power is capable of providing, resulting in the bus seeing infinite resets when both drives are plugged in. (Note: it's been a long time since I read the electrical specs of the LVD SPI bus. Don't flame me too hard on innaccuracies in this bit. It may not be technically correct in all places, but the basic information provided is correct. The reset line on the SCSI cable works by having something on the bus provide +2v (I think, it may be +1v on LVD, been a while since I read the electrical specs on LVD) to the terminator which then connects that power source to the reset line on the bus, pulling the voltage on the reset line up high. When the controller or any device on the bus wants to cause a bus reset, it temporarily connects the reset line to ground, pulling the voltage low and triggering all the devices to reset. However, reading the voltage on the line requires consuming a small amount of the power provided by the terminator. So, the more devices you have on the bus, the more trickle power the terminator has to supply in order to keep the voltage on the reset line up. The trickle power is limited though, or else devices wouldn't be able to pull it low at will. So, if the terminator is providing too small of an amount of power on the line, then two drives can force it low by just reading the line, resulting in what you are seeing. On the other hand, it could be that the terminator is in fact providing all the power it is suppossed to, but each of the drives you have are pulling way too much power when just sitting there and as it may work out, each device will work on a bus by itself but when they are put on a bus together the combined power pull becomes too much for the terminator and the bus goes into a permanent reset state. This would be my actual guess for what's wrong on your bus.) -- Doug Ledford 919-754-3700 x44233 Red Hat, Inc. 1801 Varsity Dr. Raleigh, NC 27606