Troubleshooting SCSI: Termination issues

Among the common sources of problems with SCSI storage devices are terminators, the networks of resistors needed to prevent signal reflections. The reason termination causes so many problems is that there are many kinds of SCSI busses, and different SCSI devices handle termination in different ways.

One symptom of improper termination is flaky performance. Data will be dropped, and some SCSI devices won't be visible to the system. If you have changed your SCSI setup – if you are installing a SCSI system or adding a SCSI device to a system – and all the devices in the SCSI chain have power but the system is not working, then termination should be one of your first suspects.

In many installations, a terminator is a purely passive device. However, SCSI devices working in Low Voltage Differential (LVD) mode -- that is, Ultra2 SCSI and beyond -- use active termination, which includes a voltage regulator in the terminator to hold the voltage constant. Many other SCSI setups also use active termination -- powered over the SCSI bus from either the server or the backplane – to increase reliability.

The rule about terminators is this: All SCSI busses must be terminated at both ends. . .and nowhere else. A terminator left in place in the middle of a SCSI chain will prevent the computer from seeing the devices beyond the termination.

SCSI devices used to come with an internal terminator that could be activated by setting a jumper. But

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in order to lower the load on the bus, this is no longer the case with LVD devices. This means that a separate terminator must be attached to the bus after the last device on it. Terminators are usually plugged into the bus connector, crimped to the bus or connected in series with a SCSI device, depending on the type.

When you're terminating SCSI systems, the documentation can be your best friend. It should provide information on what kind of terminators to use and how to terminate the system. Study that section of the documentation carefully before you install SCSI equipment.

Finally, here is a good SCSI termination tutorial.


Fast Guide: Troubleshooting SCSI

 Introduction
 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 80K 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 issues related to storage and storage management.

 

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This was first published in December 2005

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