[rescue] SCSI Cables/Term: LVD -vs- HVD -vs- FW/SE

[George Adkins]

Here's some quick info, 

1.  anything with any kind of common grounding in it is SE Only.

2. SE and LVD are semi-compatible only that LVD will downgrade and function 
as SE in the presence of any other SE Equipment (Drives, terminators, 
external cables with common ground lines...)

3. HVD or LVD both require 68 pin cables with fully separate signal lines, 
meaning that LVD External cables will work OK for HVD, the reverse may not be 
true as LVD devices work at higher bus frequencies.

4. HVD actually has different signal pinouts from SE/LVD and is completely 
incompatible on the same bus.  it's terminators are incompatible as well, 
using different resistive values.  

5. HVD signals operate in the 12V range (-12V to +12V) as opposed to SE's 5 
Volt range (GND to +5v) or LVD's (-5v to +5v)  obviously the 24V of one is 
completely incompatible with the 5 or 10 V of the others.  Part of the pinout 
variation is the DIFFSENS signal line which *should* shut down a DIFF bus if 
it sees SE device(s) on it, but specs are not always implemented as written. 

6. HVD bus length limits stretch out to about 84 feet, as opposed to the 
lengths more practically measured in inches for SE or LVD.  It allows you to 
put the drives in another row of racks, or in another room, or potentially 
even on another floor.

George