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SCSI ID Numbers and Multiple Bus Systems May 25, 1998

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Regarding an recent item we posted concerning SCSI devices that did not work properly if two devices had the same ID #, even though they were on different buses, Videographics Corporation Technical Support sent us the following explanation and advice: The SCSI ID conflicts which have been discussed recently can be described in a bit more detail. The main problem on the SCSI buses is that you are trying to use a driver that is simply not aware of multiple SCSI buses. This compatibility issue usually coincides with SCSI Manager 4.3 support. If an "older driver" only expects there to be one bus then it will likely step on other devices with the same ID even if they're on a different bus. Most, if not all, SCSI drive formatters are aware of multiple buses but many peripheral drivers are not "up to date". You should be able to use most SCSI disk formatters to format almost all of your removables which will help as long as you don't have a disk containing a foreign driver in the drive on bootup. This would cause the old driver to load and potentially cause conflicts. It's generally best not to boot your machine with someone else's removable disk in the drive. Insert it later. Scanner drivers are a different issue and while many scanner makers have updated their drivers you could get stuck even though Apple changed their API to support multiple buses over back in the Quadra days over 5 years ago. Most major peripheral makers do support multiple buses today so update those drivers. When most systems only had one or 2 buses it was not a big deal but today three and four buses are not uncommon. We have actually seen some situations where the system appears to have over 12 buses due to some fiber channel idiosyncrasies. Yes, 12 separate devices using "SCSI" ID-3! With over 150 "SCSI" IDs on your system you should be using peripherals with driver software that supports multiple buses. Work-arounds: Place all of your "fixed" ID peripherals on the buses first and be sure they don't conflict with each other. Then, take all of the peripherals whose drivers do not support multiple buses and make sure they have their own IDs. If they conflict with a fixed ID device you're stuck. Then add those peripherals which are removable devices where foreign driver could get loaded if the system was booted with one in the drive. Keep them on their own IDs if possible. If it's not possible, be careful not to boot with a foreign disk in the drive. Then add those peripherals which do recognize multiple buses and fill in the gaps. Remember that newer SCSI-2 and later support up to ID-15 and using those upper IDs can help avoid conflicts with older single bus drivers. Also note: This should only be an issue for users with more than 7 SCSI devices. You may have to get in there and change some jumpers on the disk drives but these kinds of conflicts are not usually so mysterious.

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