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Linux Tutorial - The Computer Itself - The Expansion Bus - Termination
  The Small Computer Systems Interface SCSI ---- PCI  


With Ultra SCSI, termination plays a more important role. A steeper edge means that the reflection has a stronger effect than with Fast SCSI. Moreover, a faster cycle means that the bus is more sensitive to interference. In principle, SCSI termination, even with Ultra SCSI, is simple: both ends of the bus (that is, the physical ends of the bus) must be terminated.

If you have fewer devices than connectors on your SCSI cable, I advise you to connect devices at both ends of the cable, terminating both of them. Loose ends can definitely lead to problems with reflection. By having devices at the physical ends of the cable, there is no question which device is at the end of the bus. Keep in mind that the order of the devices on the bus is independent of this.

You run into problems when the device has no possibility of being terminated or functions only with passive termination. Although no termination is rarely found, many (especially older) devices support only passive termination. Such devices include a lot of CD-ROMS and tape drives. Read the hardware documentation to find out what type of termination your drive supports or contact the manufacturer before you purchase the drive.

You need to be careful with some hard disks. There is often a jumper labeled TERM, which does not enable/disable the termination, but rather enables/disables the power for the active termination.

If you do have a device with active termination, this device belongs at one end of the SCSI cable. The other end is usually the host adapter. PCI host adapters are almost exclusively produced with active termination.

If both external and internal devices are present, the host adapter must not be terminated because it is now in the middle of the bus and no longer at the end. The termination is now on the device at the end of the other cable. Note that older, 50-pin Centronics connectors are almost exclusively passive terminators. Therefore, if you replace your existing host adapter with an Ultra-SCSI adaptor, you really should change the termination to active.

Wide SCSI presents its own termination and cabling problems. On most Wide-SCSI host adapters, you'll find an 8-bit and 16-bit connector, both of which you can use. However, keep in mind that both must be terminated.

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Copyright 2002-2009 by James Mohr. Licensed under modified GNU Free Documentation License (Portions of this material originally published by Prentice Hall, Pearson Education, Inc). See here for details. All rights reserved.
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