I'll take a crack at some of your questions, but not all of them: SCSI is a black art which takes years of practice to master ...
On Tue, Feb 24, 2004 at 09:45:30PM -0800, [EMAIL PROTECTED] wrote: > I'm wondering what the deal is with SCSI termination. It has to do with noise in the SCSI bus. Noise is simply garbage data which the computer or peripheral cannot distinguish from real data. How does noise get into the bus? It may be from external sources, since all wires behave like antennas; it may also be from data bouncing around the bus. An unterminated bus is somewhat like a brick wall: if you throw something at it, it will bounce straight back at you. Unfortunately, if the SCSI bus is fast enough, you will have two strong signals in the wires at the same time (the old signal is the garbage data, the new signal is the real data). A terminator softens that proverbial wall so that old data doesn't bounce back into the bus. It is all governed by physics. If it is all governed by physics, why is SCSI so much more fussy? Well, SCSI is much more complex. A parallel printer cable is terminated where it is plugged into the computer, which is at both ends. SCSI isn't automatically terminated at both ends because devices tap into the wire. (If you pull the hard drive enclosure open, you will discover that this is the case.) SCSI also tries to transmit data very quickly, which leaves a greater probability of signals colliding. (This may be the difference between your Plus and Classic II: the Plus sends data on the SCSI bus more slowly.) SCSI also tries to do this in parallel. Remember how I mentioned that those wires act like antennas? Well, they can both transmit and receive. That leaves you with cross-talk, which are signals jumping between wires -- in a limited sort of way. (SCSI differential tries to get around the cross-talk problem.) Then there is a lot of other physics, which I'm just plain leaving out. SCSI has also had management problems: every company has had a great idea on how to make it better. Apple was one of those companies, because they decided to strip 50 pins down to 25 (think of the big end of the cable vs. the little end). Bad idea. Other companies decided to add internal terminators. That is another bad idea because you only want one terminator at each end -- which would be two in total. The whole thing is a mess in that respect to. (You would probably have more luck rewriting the laws of physics than rewriting the laws of engineering and management. <cynical sigh>) I know this doesn't solve your problems, but perhaps it helps to explain them (and why something which works on one computer wouldn't work on another). While I'm slowly figuring out how SCSI doesn't work, I'll be darned if I ever figure out how to make it work! Byron. -- 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 ---------------------------------------------------------------
