Troubleshooting SCSI: ID conflict

When the operating system recognizes the host adapter, but the adapter can't see the SCSI devices, you need to check several possibilities. One of the most common sources of trouble on system setup or when adding a device is ID conflict.

Normally you should have all SCSI devices set to their proper ID numbers and connected before you power up the system. The computer only scans for ID numbers on startup; changing the settings later will confuse it. Under Windows (all versions since 95), you can force an ID scan of the SCSI bus using Device Manager. If that doesn't work, turn off the system and reboot to let the automatic scan take effect.

Each SCSI device in a chain (e.g., connected to the same SCSI port) is assigned an ID number between 0 and 15 (on modern SCSI busses) and all the numbers must be different. The SCSI adapter on the computer will typically be assigned number 7. The device manufacturers will have set their devices to defaults that may or may not need to be changed to make the system work.

Conceptually setting ID numbers is a straightforward process, but there are few things to bear in mind. Potential complication #1 is that your computer may have additional ID numbers assigned for other devices. In this instance, check the documentation to find out what numbers your equipment uses and how to change them.

Potential complication #2 is that not all SCSI devices can be set to all 16 ID numbers. For instance, it's

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not uncommon for devices to be limited to 0 to 7. The key is to know the ID numbers for all SCSI devices that are connected to the SCSI adapter. You may have to change the ID number of several devices to give everything a unique ID. It's a good idea to make a permanent record of the ID assignments on each SCSI adapter as part of the hard copy documentation.

Fast Guide: Troubleshooting SCSI

 Troubleshooting SCSI: Solving connectivity problems
 Troubleshooting SCSI: OS doesn't see adapter
 Troubleshooting SCSI: ID conflict
 Troubleshooting SCSI: Termination issues


Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last 20 years he has been a freelance writer specializing in storage and other computer issues.


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This was first published in December 2005

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