I hate to be the bearer of bad news, but a lot of your SCSI information is incorrect. Only the last physical device of a SCSI chain, (internal or external) is to be terminated. Both ends of each chain need termination, but since the one end is usually either a motherboard or a SCSI interface, and they normally have termination built in, the user only needs to concern him/herself with terminating the end device.
No SCSI device defaults to a particular ID. All SCSI devices have to be assigned an ID, usually with jumpers on the drive itself, or in the case of external units, a selector switch. However, every hard drive I've found installed in a computer from the factory was assigned 0 (no jumpers, and the CD-Rom assigned 3. The factory could have just as easily assigned the hard drive 3 and the CD-rom 6. Phyiscal location of any drive, CD-rom, scanner, etc. on an internal or external chain makes no difference, as long as the last physical device has the termination, and no other device does. There is one little twist to this. There are "passive termination" devices that may be located along the SCSI chain. (These are also called "pass through") By the way. Internal and external SCSI chains work exactly the same way, except internal ones normally use ribbon cable and external ones are usually molded cables. External SCSI is not one big loop. I think you're confusing the grounding return lines in external cable for being a loop. Internally, the case of the device and the computer act as the return lines. There is much more to this, like LVD, SCSI I, II, III, narrow, fast, wide, ultra, etc. I've heard lots of people speak of SCSI Voodoo, but in 21 years of playing with SCSI on Atari's and Macs, I've never had a problem getting SCSI to work like the info I've read. Most of what I've learned has been from magazines, web sites, and the hard drive manufacturers web sites. Link: http://www.scsita.org/ STeve << (snip) Now the SCSI termination issue. This is getting humorous, and I think there is a great deal of confusion going on here with this one. If my memory serves me correctly, ALL INTERNAL "SCSI" Hard Drives need to be terminated (or "should" be). Actual physical postion of a SCSI drive on the INTERNAL ribbon doesn't make a difference for INTERNAL SCSI. All SCSI devices internal or external need to be properly addressed. Only the main INTERNAL SCSI HD can get away without jumpers for addressing, since it will default to id#0. The factory default has always been; (main) HD #0, CD #3,. EXTERNAL SCSI devices are different. Only the last device on an EXTERNAL SCSI chain CAN and MUST be terminated if you want everything on the EXTERNAL chain to operate properly. ALL external devices must be addressed properly also. the address # does not necessarily have to corspond to the devices actual physical location on the chain, they all just need different address #'s. (snip) The BIG why? on the SCSI: ie; BASIC ELECTRICAL 101. INTERNAL SCSI Chains are in effect what is known as a "series/parallel" circuit. that is, one big loop with a bunch of little branches leading off the main loop. Each individual branch need a return point for the current, hence the "termination". EXTERNAL SCSI chains are series circuits. One big loop only. Hence, only the last device can and does needs to be terminated or else electrical current will stop flowing. Because of the way external devices are made, if the device is shut off, current mearly flows through the connectors on the back and onto the next device on the chain. If the last device is not terminated, current stops flowing. If any other device is terminated before the last device, current will turn around and return to it's source from that point of termination. Anything else on the chain after termination will fail to be seen. Most EXTERNAL devices can be terminated either externally or internally. As far a proper jumper locations for termination and addressing, always consult the manufacturer's website if in doubt. Although, most scsi drives do have a little diagram on them. Sure beats having to open the case and remount that drive a dozen times. Of course, I've been known to be a glutton for punishment myself..... Ok, I'm pretty sure that was way more than anyone cared for.... *****SCSI**************************SMALL***COMPUTER***SYSTEMS***INTERFACE*** >> -- PCI-PowerMacs is sponsored by <http://lowendmac.com/> and... Small Dog Electronics http://www.smalldog.com | Refurbished Drives | -- Sonnet & PowerLogix Upgrades - start at $169 | & CDRWs on Sale! | Support Low End Mac <http://lowendmac.com/lists/support.html> PCI-PowerMacs list info: <http://lowendmac.com/lists/pci-powermacs.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/pci-powermacs%40mail.maclaunch.com/> Using a Mac? Free email & more at Applelinks! http://www.applelinks.com
