The vendors made a damn mess of a fine little protocol back in the 1990s.
We ended up with 10s of different standards, some not even electrically
compatible with each other (HVD, arghh!). Anyways, here is my little 
list that should disambiguate what Fast/wide actually means... and why
fast-wide and ultra-narrow are the same damn speed. 


SCSI Type	   Bus   Bus    theory   ID Signaling  Maximum cbl  Connec-
                   Spd.  Width  speed       voltage    length       or
------------------+-----+------+--------+--+----------+------------+------+
Narrow SCSI       |5MHz |8-bit | 5MB/s  |7 |SE        |6m          |50-pin|
Fast-Narrow SCSI  |10MHz|8-bit | 10MB/s |7 |SE/HVD    |3m/25m      |50-pin|
Fast-Wide SCSI    |10MHz|16-bit| 20MB/s |15|SE/HVD    |3m/25m      |68-pin|
Ultra-Narrow SCSI |20MHz|8-bit | 20MB/s |7 |SE/LVD/HVD|1.5m/12m/25m|50-pin|
Ultra-Wide SCSI   |20MHz|16-bit| 40MB/s |15|SE/LVD/HVD|1.5m/12m/25m|68-pin|
Ultra2-Wide SCSI  |40MHz|16-bit| 80MB/s |15|LVD       |12m         |68-pin|
Ultra160 SCSI	  |40MHz|16-bit| 160MB/s|15|LVD       |12m         |68-pin|
Ultra320 SCSI	  |80MHz|16-bit| 320MB/s|15|LVD       |12m         |68-pin|
------------------+-----+------+--------+--+----------+------------+------+