Daisy-chaining SCSI devices

With SCSI, you can daisy-chain (physically wire) multiple devices together. However, if you're considering daisy-chaining multiple SCSI devices together, you should know the SCSI bus will only support a certain number of devices.

Multiple devices can be attached to a SCSI bus. How many devices depends on the type of SCSI.

SCSI devices communicate across a SCSI bus, which is often implemented in the form of a SCSI adapter card or a SCSI port on the system board. In most cases, the SCSI adapter supports both internal and external SCSI devices.

These internal and external SCSI devices communicate across a common SCSI bus (unless the adapter's manufacturer states otherwise). This is important to know because SCSI allows you to daisy-chain multiple devices together. While the hardware itself will not stop you from daisy-chaining together an infinite number of SCSI devices, the SCSI bus will only support a certain number of devices.

Knowing how to set SCSI IDs is essential to making a chain of SCSI devices work correctly.

There are several different flavors of SCSI, and the actual number of devices that can be attached to a SCSI bus depends on the SCSI type. Most modern SCSI buses support a total of 16 devices, while older SCSI implementations usually support only eight devices.

The chart below shows the speed and number of devices supported by various SCSI implementations. The chart is by no means comprehensive; there are many more SCSI varieties available.

SCSI Bus Type Bus Speed Number of Devices Supported
SCSI-1 5 MBps 8
Fast SCSI 10 MBps 8
Ultra SCSI 20 MBps 8
Ultra 2 SCSI 40 MBps 8
Fast Wide SCSI 10 MBps 16
Wide Ultra SCSI 40 MBps 4, 8, or 16 depending on the implementation
Wide Ultra2 SCSI 80 MBps 16
Ultra 3 SCSI 80 MBps 16
Ultra 160 160 MBps 16
Ultra 320 320 MBps 16

As you can see, the various flavors of SCSI that I have listed all support either eight or 16 devices on the SCSI bus. But this is misleading; the SCSI adapter itself counts as a device. So the actual number of SCSI devices you can attach to a SCSI adapter is either 7 or 15.

In many SCSI implementations, the hardware itself does not prevent you from creating a chain of SCSI devices of an infinite length. So how does the SCSI bus know how many devices are actually on the SCSI bus? Answer: Because each device is assigned a SCSI ID. A SCSI ID is simply a number between zero and 7 (or between zero and 15) used to differentiate one device from another on the SCSI bus.

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The only firm rule regarding the use of SCSI IDs is that two devices cannot use the same SCSI ID. Of course, it's possible to install multiple SCSI adapters in a computer and create multiple SCSI busses. SCSI IDs cannot be duplicated on a bus, but it is okay to have devices with the same SCSI ID on different buses.

On a SCSI bus, the number 7 (on a bus supporting eight devices) or 15 (on a bus supporting 16 devices) is usually reserved for the SCSI adapter. However, even this rule is not set in stone; it is possible to change an adapter's SCSI ID.

There really are no rules for assigning SCSI IDs to the other devices on the bus. SCSI IDs do not have to be used in order, and skipping numbers is fine too.

So how do you set the SCSI ID on a device? I can't give you a straight answer to that question, because there's a good deal of inconsistency from one device to the next. Many SCSI devices (particularly internal devices) require you to use jumpers to set the SCSI ID. Others allow you to set the SCSI ID by simply turning a tiny dial. Still others require you to use dip switches. Regardless of the method, setting the device ID is usually a simple task.

About the author:
Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Windows 2000 Server, Exchange Server and IIS. He has served as CIO for a nationwide chain of hospitals and was once in charge of IT security for Fort Knox. He writes regularly for SearchWinComputing.com and other TechTarget sites.

This was first published in May 2007

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