Path: utzoo!attcan!uunet!ncrlnk!ncrwic!encad!entec!jlohmeye From: jlohmeye@entec.Wichita.NCR.COM (John Lohmeyer) Newsgroups: comp.periphs.scsi Subject: Re: SCSI bus termination Summary: Use exactly two terminators at the ends of the bus Keywords: SCSI, Termination Message-ID: <626@entec.Wichita.NCR.COM> Date: 6 Aug 90 14:06:44 GMT Reply-To: jlohmeye@entec.UUCP (John Lohmeyer) Organization: NCR Corporation, Wichita, KS Lines: 66 SCSI bus termination always seems to generate a lot of traffic in this newsgroup. There is usually a lot of well-intentioned wrong advise in this traffic. Let me try to clear up a few points. As correctly pointed out in one response (sorry, I didn't get the name), SCSI requires one termination at each end of the bus. The characteristic impedance (Engineer talk: loosely, this means the resistance) of the terminator was selected as a compromise betweeen two factors: 1) the characteristic impedance of the cable (typically 100 ohms or less) 2) the current which SCSI drivers are required to drive (48mA at 0.5 volt) Unfortunately, it is not possible to exactly match the 100 ohm impedance without exceeding the 48 mA drive current. SCSI-1 settled for a simple 220/330 ohm termination which has a characteristic impedance of 132 ohms. This is not ideal and some of the energy from signals is reflected from the terminator which decreases the SCSI noise margin. Fortunately, most SCSI-1 systems functioned perfectly well with this decreased noise margin, mostly because they did not push the extremes of cable length, transfer rate, and number of devices. This 220/330 ohm terminator does not work as well with some SCSI-2 applications, especially when people use the new high-density connector. This is not a connector problem, but since it is physically smaller, one has to use smaller cables as well. Smaller cables tend to have lower impedance. Now we are seeing 80 to 90 ohm cables. Also, as SCSI is being used for bigger systems, we are seeing maximum length cables with 7 or 8 devices. Thanks to a lot of hard work from several people including Kurt Chan of HP and Paul Boulay of LMSI, a new, improved terminator was designed for SCSI-2 which can still be driven by 48 mA drivers, but has a characteristic impedance of 110 ohms. It also cures a nagging problem of low noise margin from a sagging terminator power line (TERMPWR). The new terminator is recommended for SCSI-2, but the old one is still acceptable if your system is not pushing all the other limits. I may have digressed a bit, but I wanted to point out that SCSI termination is important and there are consequences to ignoring it. I personally lost two weeks of work when I backed up over a SCSI cable with only one terminator. I do know of people who have damaged SCSI drivers by having too many terminators present -- this is rare. Far more often, they just get intermittent data errors (I hate intermittent errors!). The important thing to remember is that SCSI is a transmission line any time you exceed about 1 foot of cable. Transmission lines must be terminated at both ends or you will get reflections which screw up signal quality (that is, cause errors). Just because some people have gotten away with breaking these rules sometimes doesn't mean you will. Hope this helps... t impedance (Engineer talk: loosely, this means the resistance) of the terminator was selected as a compromise between matching the characteristic impedance of the cable and what the SCSI drivers can safely drive. -- John Lohmeyer J.Lohmeyer@Wichita.NCR.COM NCR Corp. uunet!ncrlnk!ncrwic!entec!jlohmeye 3718 N. Rock Rd. Voice: 316-636-8703 Wichita, KS 67226 SCSI BBS 316-636-8700 300/1200/2400 24 hours