Hello Anne,

  AB> Well, Adaptec's web site had pretty much zilch on the AHA-2940U,
    > and the appearance of that part number in lists also containing
    > "AHA-2940", "AHA-2940UW", and "AHA-2940 Ultra" suggests they
    > they are not the same.

My guess would be the following...

               Host Adaptor               MHz Bits Mb/S*
               AHA-2940    'Fast SCSI-II'  10    8   10
               AHA-2940U   'Ultra SCSI'    20    8   20
               AHA-2940UW  'Ultra, Wide'   20   16   40

                           *theoretical peak throughput

- Andy Ball.