At 21:57 01/10/03, Dave Jones wrote:
>I don't think BOTH drives need to be terminated.. Only the
>one on the end of the chain...


Hi Dave

Both drives ARE at the ends of the chain. (The start of
the SCSI chain and the end of the chain are both ends of
the chain.) The SCSI bus as Ray described it starts at
the end of the wide cable (the WD hard drive), passes
through the controller and then continues on through the
narrow cable until it ends at the IOMEGA ZIP drive.

      (I chose to consider the hard drive to be the
      start of the bus. You could just as easily
      consider the ZIP drive to be the start and the
      WD drive the end.)

Both ends of the SCSI bus must be terminated. The two disk
drives (WD hard disk and Iomega ZIP drive) are at the ends
of the bus. If you used only one SCSI cable, then the
controller would be at one end of the bus. But here the
2940UW controller is in the middle of the chain. One would
not normally terminate something in the middle of the SCSI
bus, but the wide part of the bus ends at the controller.
So you have to have the 2940UW controller terminate the
upper byte (the wide part) of the bus...but not the lower
byte (the narrow part) of the bus.

      (Here I'm using the terms SCSI bus and SCSI chain
      interchangeably.)

The wide hard drive terminates both the upper and lower
bytes of the bus...which you might call the narrow and the
wide parts of the bus.

If anyone is interested: In the old days (of wide SCSI),
two separate cables (called the P and the Q cables) were
used to connect wide SCSI devices to each other. One cable
carried the upper byte of data and another cable carried
the lower byte. Now days, a single cable carries both bytes
of data. (Only 34 of the 68 wires in a wide cable carry
information. The other 34 wires, every other wire, are at
ground...to cut down on cross talk. Of the 34 wires that
carry information, 16 carry the two bytes of data and rest
carry control signals.)

Regards,
Bill

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