> I'm wondering what the deal is with SCSI termination. I've never quite > understood it. My now inactive Mac Plus has/had a 40MB external hard > disk. That machine, HD included, was given to me by my grandfather over > 10 years ago. He gave it to me without a terminator on it. I ran it for > those 10+ years that way with no problems, as that's the way he apparently > ran it. When the power supply on the Plus died, I moved the drive over to > my Classic II. After less than a day on the Classic II, I got some sort > of disk error while trying to access a file on the external HD. So I > rebooted, and ever since the external HD won't show up on the desktop of > the Classic II.
Here are a few facts to consider. The SCSI termination resistors actually serve two purposes, only one of which is commonly mentioned. One is simply to provide a "pullup" to a voltage that represents one of the logical states (let's call it "1"). Ground (zero) is the other. Without a termination resistor and termination power to go with it, there's nothing to provide the "1" state. So, the fact that it once worked tells us automatically that the drive already has a terminator on it, whether you realized it or not. Otherwise it could *not* work. That drive almost certainly has termination resistors installed directly on the drive itself, which is why you didn't notice it. The other function of the terminator is a bit more mysterious, and that is to provide the absorption of reflections that the other posters talked about. Basically, if the time it takes for a signal to travel along a wire is short relative to the time it takes to go from 0 to 1, then multiple reflections can happen, and screw things up. Thus, terminations are essential for long buses, but not strictly necessary (other than the pullup issue) for short ones. Now, when you put your drive on the Classic, it's quite possible that you either had a SCSI ID conflict, or you had too many terminators on the system. Or the drive actually has a problem. To make SCSI a little less mysterious (I disagree that it's a black art; maybe a gray art), here are rules that will serve you well for compact macs: 1) The scsi devices must all have unique IDs. You cannot use 7 (that is reserved for the computer). All devices must be connected in a single continuous chain (i.e., you can't branch out in a "Y" and have two parallel chains, even with unique IDs). 2) The first and last devices, and only the first and last devices, must be terminated. No other devices may have terminations. 3) If the mac does not supply termination power (remember the pullup issue), then it must be provided externally (e.g., from an external drive's own power supply). Many drives have jumpers to arrange for this to happen. The Mac Plus does not supply termination power. 4) For best chance of success, power up the external drives first. Some Plus ROMs, for example, have a bug that prevents bootup until the external devices are switched on. Violation of these rules doesn't always guarantee failure, but following them pretty much guarantees success. --Cheers, Tom -- Prof. Thomas H. Lee Center for Integrated Systems, CIS-205 420 Via Palou Mall Stanford University Stanford, CA 94305-4070 http://www-smirc.stanford.edu 650-725-3709 voice, -3383 fax -- Compact Macs is sponsored by <http://lowendmac.com/>. Support Low End Mac <http://lowendmac.com/lists/support.html> Compact Macs list info: <http://lowendmac.com/lists/compact.shtml> --> AOL users, remove "mailto:"; Send list messages to: <mailto:[EMAIL PROTECTED]> To unsubscribe, email: <mailto:[EMAIL PROTECTED]> For digest mode, email: <mailto:[EMAIL PROTECTED]> Subscription questions: <mailto:[EMAIL PROTECTED]> Archive:<http://www.mail-archive.com/compact.macs%40mail.maclaunch.com/> --------------------------------------------------------------- >The Think Different Store http://www.ThinkDifferentStore.com ---------------------------------------------------------------
