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.
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