Date: Fri, 06 Jun 1997 19:45:54 -0600 From: "Justin T. Gibbs" <gibbs@plutotech.com> To: asami@cs.berkeley.edu (Satoshi Asami) Cc: scsi@FreeBSD.ORG Subject: Re: 3940uw with long cables Message-ID: <199706070047.SAA07204@pluto.plutotech.com> In-Reply-To: Your message of "Fri, 06 Jun 1997 16:37:54 PDT." <199706062337.QAA19597@vader.cs.berkeley.edu>
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>Is this something that can be only cured by somehow shortening the >cables? Perhaps. There are a few important things to keep in mind when tuning the bus length. 1) Having equal distances between all devices is sometimes more important than total bus length since the varying RC constants can cause strange signal skew conditions if the lengths aren't matched. 2) The ratio of the stub length (the length of the PCB traces going into a drive) to the distance between devices should be 1:3. If you are plugging a ribbon cable into a drive, the trace is usually about 3.3cm, so you should have 10cm of cable between devices. 3) If the bus is going to be really long, you'll have a better chance of making it work if each device places a similar capacitive load on the bus. So, an enclosure fitted with identical drive models will often work better than if you mix and match. Put devices like tape and cdrom devices on a separate bus if possible. 4) SCSI cables can range from 90ohm to 140ohms of impedance. Favor the 90ohm part of the spectrum. You may have to go with twisted pair SCSI ribon cables to get near the 90ohm mark. 5) Use FPT terminators instead of the active terminator on the last device in the chain or an active external terminator. For narrow SCSI, you'll want an FPT-18 terminator (18 lines including the data lines are regulated by the FPT logic, the rest are actively terminated) or FPT-27 terminators for wide buses. FPT-3 terminators are also availible (REQ, ACK, and SEL are the only signals protected), so buyer beware. 6) Honor the 6m and 3m bus length limits. 6m is the longest for 10MHz and 3m is the longest for 20MHz single ended SCSI. This assumes proper stub/gap matching, so real life buses may need to be shorter. 7) If you still can't get your bus to work after tweaking all of these other things, go differential. One company that specializes in SCSI cabling and termination is CS Electronics. You can find their product offerings at their web site http://www.scsi-cables.com/. -- Justin T. Gibbs =========================================== FreeBSD: Turning PCs into workstations ===========================================
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