At 16:22 -0400 07/10/2002, Nancy Haitz wrote: >I have a small question about using the Internal/External SCSI Bus. At >one point I was looking at various PCI SCSI cards. A number of them had >an internal and an external connection. But, if you hooked up something >to the internal SCSI slot on the PCI card, it disabled the external SCSI >connector. Is this also true on the system's SCSI Bus 1? Or, can I >connect to both the internal and external ports of SCSI Bus 1?
You should be able to use both connectors. You should also be able to use both connectors on your PCI card. However, using both connectors makes termination slightly more complex and you may be running into termination issues with your PCI card when you use both connectors. A SCSI bus must be terminated at both ends. When you use only one of the two connectors, the motherboard (or PCI card) is at one end of the SCSI chain and will automatically take care of termination (some older SCSI cards must be set manually) at its end of the chain. This situation would look like this: T:MB==D==D==D:T Where == is SCSI cable, MB is motherboard or PCI card, D is a SCSI device such as a hard drive, CDROM drive, scanner, etc. and T: or :T indicates that the device next to the : is providing the termination. So in the above situation, you have a MB or SCSI card providing termination, and three SCSI devices on a cable or chain of cables. The last device on the chain has termination enabled. Now in the situation where you use both connectors, it looks more like this: T:D==D==D==MB---D---D:T Notice that the motherboard or PCI card is now in the center of the chain. The devices at the two ends have termination enabled. I've used ---- to also represent SCSI cable to help the illustration. The above case might be one in which there are three external devices and two internal devices. So the == would each represent an external SCSI cable. The first (closest to the MB) would be between the motherboard's external connector and one of the SCSI plugs on the first device. The next goes between the first and second external devices. And the last == goes between the second external device and the third external device. The third external device either has an external terminator connected, or termination is enabled. The --- represents internal ribbon cable, so in this case there is a three position ribbon cable being used. One end connector is plugged into the motherboard, the middle connector is plugged into an internal device, and the other end connector on the ribbon cable is plugged into a device which has termination enabled. The motherboard automatically disables its own termination when it detects that both connectors are being used. Apple has a proud note of this in a few of their hardware description PDFs. I think they're in the Apple Developer portion of their web site. Now, to simplify matters greatly from all the above explanation... **If you have a ribbon cable on an internal connector, make sure that whatever device is at the opposite end of the cable from the motherboard has termination enabled. Make sure that is the only device on that ribbon cable which has termination enabled. Make sure that the terminated device is on the last connector on the ribbon cable. Do not leave additional empty connectors dangling after the terminated device. Any empty connnectors should be in the "middle" of the cable, not dangling at the end after the termination. **If you have an external SCSI chain connected (external cabling is called a chain because it's usually a daisy chain of several cables with devices at the "joints") make sure that the last external device on the external chain either has termination enabled on the device itself, or that the empty SCSI connector on the last device (external devices should ahve two connectors, though some break the rules with only one, and the last one will only have one cable connected) has an external terminator installed. Do not both install an external terminator and enable termination on the last device. Double termination is bad. Make sure that only the last device has termination enabled. If you follow the two rule sets above and the motherboard or PCI card has automatic termination, then your termination will be correct. Note that termination for LVD SCSI chains is slightly different in that LVD devices do not have any built-in termination that may be enabled, so separate terminators must always be used, even on the internal ribbon cable. So the terminator must occupy the last connector on an internal ribbon cable on an LVD SCSI bus. Now, if you have an old PCI card that doesn't have automatic termination, you just need to make sure that termination is enabled in the case where you have only an external SCSI chain connected, and in the case where you have only an internal SCSI chain connected. If you have both an internal and an external SCSI chain connected, then you disable termination on the PCI card so that you can create the situation in the second diagram above. Adaptec's web site used to have some really good explanations of all this with diagrams. I don't know if they still do, but it might be worth hunting around for it. It's the same rules whether you're using SCSI on a PC or a Mac. If you are using wide devices on one chain and narrow devices on the other chain, then some PCI cards have a termination option called "High Termination Only" which you enable to catch the 18 wires that end at the PCI card. 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
