At 14:11 -0400 07/11/2002, Nancy Haitz wrote:
>hard drive should have terminated the chain at the end. However, there >were/are no jumpers on the original 4 GB drive. "Ignorance is bliss" >must have come with the machine ;-) > >About two years ago, I replaced the original 4GB SCSI drive with an 18 >GB SCSI drive. Since the 4 did not have any jumpers on it, I did not >put any on the 18. The drive and everything in the computer worked fine >like that for a good two years. This is what causes "SCSI Voodoo". It isn't that SCSI doesn't work when it should--that situation is extremely rare. It's that SCSI often works when it shouldn't. This creates all kinds of confusion. :-) >1. The SCSI hard drive in the 9600 "should" have termination enabled, >and should be connected to the last cable connector on the internal SCSI >chain. Yes. Actually, more generally, there should be some SCSI device on the last cable connector on the internal SCSI chain and that last device must have termination enabled. It doesn't matter if it is the CDROM drive or the hard drive or something else, as long as you have some device on the last connector and that device has termination enabled. >2. The SCSI hard drive in the c600 "should not" have termination >enabled and can be connected to any convenient cable connector on the >SCSI chain. The same rules apply to the C600. There should be some device at the end of its SCSI cable and that device should have termination enabled. If there is already a device on the end of the cable and it is supplying termination, then yes, you can put the SCSI hard drive on any convenient intermediate connector on the SCSI chain and termination should be disabled on this drive going in the middle. >On the c600, if the motherboard terminates the beginning of the chain, >does it "sense" an absence of external devices, if there are none >connected to the external port, and automatically terminate the combo >internal/external SCSI chain? If not, who is doing the terminating, if >there are no external devices with a terminator on the last device? Yes, the C600 also auto-senses whether it is at the end of the chain (no external devices connected). All the Apple (and clone) motherboards going back quite a long ways have this auto-sensing/termination feature. I think the Mac Plus may be the only machine (other than a few PowerBooks) that lacks it. This isn't too surprising. The early Macs had only one SCSI bus so that bus always had to be both internal and external. There had to be some way to handle termination if there were no external devices installed. Of course, Apple could have just shipped an external DB25 terminator with every machine and told the user to install it on the SCSI port if there were no external devices connected. :-) I think the first machine with a second internal only SCSI bus was the Quadra 900/950 though that may have been an add in card. The next machine with a built-in second SCSI bus was the PowerMac 8100. So everything predating the PowerMacs (except perhaps the Q900/950) had only a single SCSI bus that really needed this auto-sensing feature in order to be user friendly. Of course, even way back then, there were add-in SCSI cards, though they were NuBus rather than PCI. The NuBus JackHammer (an ancestor of our E100?) didn't have auto-termination sensing. It had actual resistor packs on the board which you installed for termination at the card and removed to disable termination. This was a pain if you sometimes operated with external devices connected and other times operated without external devices. Who wants to open the machine to pull resistor packs when all you really want to do is plug in an external SCSI cable? Jeff Walther -- SuperMacs is sponsored by <http://lowendmac.com/> and... Small Dog Electronics http://www.smalldog.com | Refurbished Drives | Service & Replacement Parts [EMAIL PROTECTED] | & CDRWs on Sale! | Support Low End Mac <http://lowendmac.com/lists/support.html> SuperMacs list info: <http://lowendmac.com/supermacs/list.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/supermacs%40mail.maclaunch.com/> Using a Mac? Free email & more at Applelinks! http://www.applelinks.com
